CH-30 Homeostasis: Osmotic Regulation, Excretion, and Temperature Regulation

This document contains Chapters 30 to 31 CHAPTER 30 HOMEOSTASIS: OSMOTIC REGULATION, EXCRETION, AND TEMPERATURE REGULATION CHAPTER OUTLINE 30.1. Water and Osmotic Regulation (Figures 30.1, 30.2) A. How Marine Invertebrates Meet Problems of Salt and Water Balance 1. Most marine invertebrates are in osmotic equilibrium with their seawater environment. 2. With body surfaces permeable to water and salts, the internal and external concentrations are equal. 3. Such animals that cannot regulate osmotic pressure of their body fluids are called osmotic conformers. 4. This functions for open ocean organisms because the open ocean is stable. 5. Animals that must live within a narrow salinity range are stenohaline. 6. Organisms that can tolerate the wide variations found in estuaries are euryhaline. 7. A hyperosmotic regulator maintains body fluids in higher concentration than surrounding water. 8. Kidneys, or antennal glands in a crab, can maintain a higher concentration by excreting excess water. 9. Salt-secreting cells in the gills that remove ions from seawater counter loss of salt ions. 10. Any process that requires an expenditure of energy is an active transport process. 11. Any process that works against the diffusion gradient will require active transport. 12. Systems within an organism function in an integrated way to maintain a constant internal environment around a setpoint. B. Salt and Water Balance in Freshwater (Figures 30.3, 30.4) 1. During Silurian and Devonian periods, jawed fishes began to penetrate brackish and freshwater rivers. 2. The unexploited habitat with abundant food presented a physiological evolutionary challenge. 3. Freshwater fishes must prevent salt loss and unload excess water. 4. The scaled and mucus-covered surface of a fish is nearly waterproof. 5. Water that enters by osmosis across the gills is pumped out by the kidney as very dilute urine. 6. Salt-absorbing cells in the gills move sodium and chloride ions from water to blood. 7. Salt present in the fish’s food also replaces any salt that is lost by diffusion. 8. Clams, crayfishes and aquatic insect larvae are also hyperosmotic regulators with similar mechanisms. 9. Amphibians that live in water use their skin to transport sodium and chloride; these tissues are standard laboratory models for ion transport. C. Salt and Water Balance in Marine Fishes 1. Modern oceanic bony fishes are descendants of freshwater fishes, returning during the Triassic period. 2. Their ionic body concentration of about one-third that of seawater is related to their marine heritage. 3. Freshwater fish returning to the sea in the Triassic period lost water and gained salt—it “dried out.” 4. Marine fish therefore drink seawater and it is absorbed in the intestine; salt is carried by blood to gills. 5. Special salt-secreting cells in the gills transport the salt back to the sea. 6. Ions that remain in the intestine as residue (e.g. magnesium, sulfate, calcium) are voided with feces. 7. Marine bony fishes maintain salt concentration of body fluids at about one-third that of seawater. 8. They are therefore hypoosmotic regulators, maintaining their body fluids at lower concentration. 9. A marine fish consumes only enough water to replace water loss. 10. Elasmobranchs (i.e., sharks and other elasmobranchs) achieve the same osmotic balance by a different mechanism; the urea compounds accumulate in the blood until there is no osmotic difference with seawater. D. Salt and Water Balance in Terrestrial Animals (Table 30.1) 1. Animals carried their watery composition with them as they evolved within a terrestrial existence. 2. They continued to adapt to the threats of desiccation and became abundant in arid areas. 3. Animals lose water across respiratory and body surfaces, excretion of urine and elimination of feces. 4. Water is gained from water in food, drinking water and metabolic water. 5. Some desert arthropods can absorb water vapor from the air. 6. In desert rodents, metabolic water constitutes most of the animal’s water uptake. 7. Water balance in a human and a desert rodent is quite different. 8. Dilution of Wastes a. The primary end product of protein breakdown is highly toxic urea. b. Fishes excrete urea across the gills and the abundance of water keeps it dilute. c. Terrestrial insects, nonavian reptiles and birds convert urea to nontoxic uric acid. d. Uric acid is insoluble and is excreted with little water loss. e. Uric acid can be stored in harmless crystalline form within an egg until hatching. 9. Marine birds and turtles have a salt gland that secretes concentrated sodium chloride; these are important accessory glands to the kidneys that only produce very diluted urine. 30.2. Invertebrate Excretory Structures A. Contractile Vacuole 1. These are small, spherical, intracellular vacuoles of protozoa, freshwater sponges, and radiate animals. 2. They are not truly excretory; ammonia and other nitrogenous wastes diffuse across the cell membrane. 3. The contractile vacuole is an organ of water balance; it expels excess water that enters by osmosis. 4. A network of channels populated with proton pumps probably surrounds them. 5. Contractile vacuoles are absent in marine forms that are isosmotic with seawater. B. Nephridium (Figures 30.5, 30.6) 1. This tubular structure is the most common design to maintain osmotic balance. 2. The flame cells system or protonephridium is the simplest arrangement. 3. Planaria and other flatworms have a highly branched duct system to all parts of the body. 4. Fluid enters the system through specialized “flame cells” and passes through tubules to exit the body. 5. Rhythmical beating of a flagellar tuft creates a negative pressure that draws fluid into the tubes. 6. In the tubule, water and metabolites are recovered by reabsorption; wastes are left to be expelled. 7. Nitrogenous wastes, mainly ammonia, diffuse across the surface of the body. 8. Flatworms have no circulatory system so the flame cell system must branch throughout the animal. 9. This system is a closed system since the fluid must pass across flame cells. 10. A metanephridium is an open system found in molluscs and annelids. 11. The tubule is open at both ends; fluid is swept into the tubule through a ciliated funnel-like opening. 12. A network of blood vessels to reclaim water and valuable solutes surrounds a metanephridium. 13. The basic process of urine formation in the tubule remains the same: withdraw useful solutes and add waste solutes. C. Arthropod Kidneys (Figures 30.7, 30.8) 1. The Paired Antennal Glands of Crustaceans a. Located in the ventral part of the head, they are an advanced design of nephridia. b. They lack open nephrostomes. c. Hydrostatic pressure of the blood forms a protein-free filtrate in the end sac. d. In the tubular portion, certain salts are selectively reabsorbed or actively secreted. e. This crustacean system is similar to the vertebrate system in sequence of urine formation. 2. Malpighian Tubules a. Insects and spiders use this system in conjunction with rectal glands. b. The thin, elastic, blind Malpighian tubules are closed and lack an arterial supply. c. Urine is produced by tubular secretion mechanisms by the cells lining the Malpighian tubular lumen. 1) This process is initiated by active transport of hydrogen ions into the Malpighian lumen. 2) These ions are then transported via protein carriers back into the cells and are exchanged for sodium or potassium ions, and chloride ions follow passively. 3) The secretion of ions creates osmotic pressure that draws out water, solutes, and nitrogenous wastes (uric acid) into the tubule. 4) Once the urine drains into the rectum, water and salts may be reabsorbed by specialized rectal glands, leaving behind uric acid, excess water, salts, and other wastes. d. This is an especially efficient system for dry environments. 30.3. Vertebrate Kidney Function (Figure 30.9) A. Ancestry and Embryology 1. The earliest vertebrate kidney had segmentally arranged tubules similar to an invertebrate nephridium. 2. Each tubule opened into the coelom at a nephrostome; the other end led into a common archinephric duct. 3. This ancient kidney is called an archinephros; a similar segmented kidney is in embryos of hagfishes. 4. From the earliest time, the reproductive system from the same mesoderm used the nephric ducts. 5. Three developmental stages occur in the embryonic development of vertebrate kidneys. 6. The pronephros is observed in vertebrate embryos; it usually degenerates except in hagfish and a few bony fish species. 7. The mesonephros and metanephros replace the pronephros and forms the adult kidney of fishes and amphibians. 8. The Metanephros a. This is found in adult amniotes. b. It is more caudally located and is much larger and compact. c. It contains a large number of nephric tubules. d. The ureter is a new duct that drains the system; the archinephric duct has shifted to sperm transport. 9. The three stages succeed each other embryologically and phylogenetically in amniotes. B. Vertebrate Kidney Function (Figures 30.10, 30.11) 1. The vertebrate kidney is the principal organ that regulates volume and composition of internal fluids. 2. The removal of metabolic wastes is incidental to its regulatory function. 3. Urine is formed in the nephron by filtration, reabsorption and secretion. 4. Structure of Human Kidney a. Kidneys comprise less than one percent of body weight but filter 20–25% of blood output. b. Blood flows through nearly a million nephrons in each kidney. c. The nephron begins in the renal corpuscle that contains a tuft of capillaries called the glomerulus. d. Blood pressure in glomerular capillaries forces protein-free filtrate into a renal tubule. e. Filtrate passes into a proximal convoluted tubule, the loop of Henle and the distal convoluted tubule. f. Collecting ducts join to form the renal pelvis and carry urine through a ureter to the urinary bladder. g. Throughout the passage, some solutes are reabsorbed and some are concentrated. h. The blood from the dorsal aorta enters each kidney through the renal artery. i. A renal artery branches to an afferent arteriole; they leave the renal corpuscle as efferent arterioles. j. Efferent arterioles travel through extensive capillary networks around the proximal and distal convoluted tubules and the loop of Henle. k. The capillary network collects to form the renal vein that returns blood to the posterior vena cava. C. Glomerular Filtration (Figure 30.12) 1. The glomerulus is a specialized mechanical filter. 2. Blood pressure drives a protein-free filtrate across capillary walls into the fluid-filled renal corpuscle. 3. Smaller solute particles are also carried across if they can fit through the slits of capillary walls. 4. Red blood cells and plasma proteins are too large to pass across. 5. The filtrate will undergo extensive modification before becoming urine. 6. About 180 liters (50 gallons) of filtrate form each day; most is reabsorbed and 1.2 liters of urine form. D. Tubular Reabsorption (Figure 30.13) 1. About 60% of filtrate volume and nearly all glucose, amino acids, and vitamins are reabsorbed in the proximal convoluted tubule. 2. Most reabsorption is by active transport; unique ion pumps retrieve sodium, calcium, potassium, etc. 3. Water passively follows the osmotic gradient with active reabsorption of solutes. 4. For most substances, there is an upper limit to reabsorption (transport maximum or renal threshold). 5. Normally there is no glucose in urine because the transport maximum is well above the glucose level. 6. A kidney filters 600 grams of sodium a day and retrieves 596 grams, excreting 4 grams. 7. If human intake of sodium is higher than 4 grams, excess sodium may build in tissues and cause problems. 8. The distal convoluted tubule carries out final adjustment of filtrate composition. 9. About 85% of sodium absorbed by the proximal convoluted tubule is obligatory or set. 10. In the distal convoluted tubule, sodium reabsorption is controlled by aldosterone. 11. Aldosterone increases active reabsorption of sodium in distal tubules and decreases loss of sodium. 12. Secretion of aldosterone is regulated by the enzyme renin, produced by the juxtaglomerular apparatus, a complex of cells located in the afferent arteriole at its junction with the glomerulus. 13. Renin is released in response to low blood sodium level or low blood pressure. 14. Renin initiates a series of enzymes that result in production of angiotensin, a blood protein. 15. Angiotensin stimulates release of aldosterone, which increases sodium reabsorption in distal tubule. 16. Angiotensin also increases secretion of antidiuretic hormone (vasopressin) that promotes water conservation by the kidney. 17. Angiotensin increases blood pressure and stimulates thirst. 18. These actions reverse the circumstances that triggered the secretion of renin: sodium and water are conserved, and blood volume and pressure return to normal. 19. Selective pressure has restricted sodium levels in humans while sodium range is broad in rodents. E. Tubular Secretion 1. The nephron also secretes materials into the filtrate, the reverse of tubular reabsorption. 2. Carrier proteins in tubular epithelial cells selectively transport substances from blood to tubule. 3. This allows a build up of urine concentrations of hydrogen and potassium ions, drugs, etc. 4. Most tubular secretion is at the distal convoluted tubule. 5. Marine fishes, nonavian reptiles and birds use tubular secretion far more than mammals. 6. Marine bony fishes actively secrete large amounts of magnesium and sulfate from seawater salts. 7. Uric acid is actively secreted by the tubular epithelium. F. Water Excretion (Figures 30.14, 30.15) 1. The kidney closely regulates the osmotic pressure of the blood. 2. When fluid intake is high, the kidney excretes dilute urine and saves salts. 3. When fluid intake is low, the kidney conserves water and forms concentrated urine. 4. A dehydrated person can concentrate urine to approximately four times blood osmotic concentration. 5. Countercurrent Multiplication a. Mammal and bird kidneys produce concentrated urine by interaction between the loop of Henle and the collecting ducts. b. This forms an osmotic gradient in the kidney. c. In the cortex, the interstitial fluid is isosmotic with the blood. d. Deep in the medulla, the osmotic concentration is four times greater than that of the blood. e. High osmotic concentrations in medulla are produced by an exchange of ions in loop of Henle. f. “Countercurrent” refers to the opposite directions of the loop of Henle, down the descending limb and up the ascending limb. g. “Multiplication” describes the increasing osmotic concentration in the medulla from ion exchange between the two limbs. h. The descending limb of the loop of Henle is permeable to water but impermeable to solutes. i. The ascending limb of the loop of Henle is impermeable to both water and solutes. j. Sodium chloride is actively transported out of the thick portion of the ascending limb and into surrounding tissue. k. As the interstitial area surrounding the loop becomes concentrated, water is withdrawn from the descending limb by osmosis. l. Tubular fluid at the base of the loop is more concentrated and moves up the ascending limb where more sodium is pumped out. m. The effect of active ion transport in the ascending limb is multiplied as more water is withdrawn from the descending limb and more concentrated fluid is presented to the ascending limb ion pump. 6. Because of high concentration of solutes surrounding a collecting duct, water is withdrawn from urine. 7. As urine becomes concentrated, urea diffuses out, adding to a high osmotic pressure in the kidney medulla. 8. The amount of water reabsorbed depends on the permeability of the walls of the distal convoluted tubule. 9. This is controlled by antidiuretic hormone (ADH, or vasopressin) released by the posterior pituitary. 10. ADH increases the permeability of the collecting duct and water diffuses outward. 11. In overhydration, the pituitary stops releasing ADH, pores in duct walls close, and more urine is excreted. 30.4. Temperature Regulation A. Chemical Environment 1. Biochemical activities are sensitive to temperature because enzymes have an optimum temperature. 2. Temperature constraints on animals are due to their need to maintain biochemical stability. 3. At colder temperatures, metabolic reactions may be too slow to maintain activity and reproduction. 4. At too high temperatures, enzyme activity is impaired or destroyed. 5. Generally animals function between 0o and 40o C. 6. Animals may locate such habitats or develop means to stabilize their metabolism. B. Ectothermy and Endothermy 1. “Cold-blooded” and “warm-blooded” are not well-defined terms; fishes, insects, and reptiles basking in the sun may be warmer than mammals. 2. Many “warm-blooded” mammals hibernate and approach very cold temperatures. 3. “Poikilothermic” refers to a variable body temperature, and “homeothermic” refers to maintaining a constant body temperature; these terms likewise pose definition problems. 4. All animals produce some heat from cellular metabolism. 5. In ectotherms, the heat is conducted away as fast as it is produced. 6. Many ectotherms may behaviorally select areas of more favorable temperature, such as basking in the sun. 7. Endotherms are able to generate enough heat to elevate their own temperature to a high and stable level. 8. Birds and mammals are the commonly recognized endotherms. 9. Some reptiles and fast-swimming fishes, and at times certain insects, may also be endotherms. 10. Endothermy allows birds and mammals to stabilize internal biochemical processes and nervous system functions. 11. Endotherms can also remain active in winter and exploit habitats denied to ectotherms. 12. Endotherms that are small tend toward decreased activity and hibernation in colder climates due to the high heat loss and/or limited food supply. C. How Ectotherms Achieve Temperature Independence (Figure 30.16) 1. Some ectotherms behaviorally regulate body temperature. 2. Desert lizards exploit hour-to-hour changes in solar radiation. 3. In cool mornings, they bask with bodies flattened; in the hottest daytime, they retreat to burrows. 4. Such lizards hold body temperature between 36o and 39o C while the environment varies from 29o to 44o C. 5. Metabolic Adjustments a. Within limits, most ectotherms can adjust metabolic rate to the prevailing temperature. b. Temperature compensation involves complex biochemical and cellular adjustments. c. This process allows a fish or salamander to sustain the same level of activity in warm or cold water. D. Temperature Regulation in Endotherms (Figure 30.17) 1. Most mammals have body temperatures between 36o and 38o C; most birds range from 40o to 42o C. 2. Much of an endotherm’s daily caloric intake goes to generate heat; it must eat more than an ectotherm. 3. Heat is lost by radiation, conduction and convection to a cooler environment, and by evaporation of water. 4. If an animal becomes too cool, it can generate heat by exercise or shivering, and decrease heat loss by increasing insulation. 5. Adaptations for Hot Environments (Figure 30.18) a. Small desert animals are often fossorial, living in the ground, or nocturnal, active at night. b. Lower temperatures and higher humidity of burrows also reduces water loss from evaporation. c. Desert animals may also drink no water, but derive all water from food and produce dry feces. d. The desert eland has many adaptations for desert living: glossy, pallid fur; fur insulation; fat tissue isolated on the back; and dropping body temperature at night. e. When the body temperature reaches 41o C, it uses evaporative cooling by sweating and panting. f. The desert camel has all of these adaptations perfected for desert living. 6. Adaptations for Cold Environments (Figure 30.19) a. Mammals and birds use decreased conductance and increased heat production to survive the cold. b. In winter, fur may increase in thickness by 50%. c. Countercurrent Heat Exchange 1) A well-insulated body can lose substantial heat through blood flowing along exposed limbs. 2) Arterial blood in the leg of an arctic mammal or bird passes in contact with returning cold blood. 3) The heat exchange all along the opposite vessels transfers nearly all body heat to the returning venous blood that returns to the body core. 4) Similar countercurrent exchange systems keep aquatic mammal flippers from losing body heat. 5) Footpads and hooves must be able to operate at near-freezing temperatures. d. Augmented muscular activity increases heat by exercise or shivering. e. Nonshivering thermogenesis uses increased oxidation of stores of brown fat. f. Small mammals live in the milder climate under the snow, a subnivean environment. E. Adaptive Hypothermia in Birds and Mammals (Figures 30.20, 30.21) 1. The endotherm must always have an energy supply to support its high metabolic rate. 2. Small birds and mammals have an intense metabolism that is difficult to support on cold nights. 3. Daily torpor is dropping body temperature when asleep or inactive; it prevents energy loss. 4. Hummingbirds may drop body temperature at night when food supplies are low. 5. Hibernation is prolonged and controlled dormancy. 6. True hibernators prepare for winter by storing body fat. 7. Entry into hibernation is gradual; the animal eventually cools to near ambient temperature. 8. Respiration may drop from 200 breaths per minute to 4–5 per minute; heart rate from 150 to 5 beats per minute. 9. Arousal from hibernation may require shivering and nonshivering thermogenesis. 10. Bears, badgers, raccoons and opossums enter a prolonged sleep with little or no decrease in body temperature. 11. This prolonged sleep is not true hibernation; the animal can be awakened if disturbed. 12. Some invertebrates and vertebrates enter a state of summer dormancy called estivation. Lecture Enrichment 1. The human systems are explained in this and the following chapters since the human system is the most familiar to students and therefore is more “meaningful”; however, care must be given to prevent viewing the human as “typical” or “representative” of animals in general, since we are neither. 2. The human kidney was one of the first major organs to be transplanted. When describing the gross anatomy of the kidney, note the clear encapsulation and the limited blood vessels and ureters that make this a simple organ to transplant relative to the liver, for instance. 3. The complex of filtrations and absorptions involved in kidney function require both visuals and a logical sequence of concept descriptions. The relationship of kidney function to blood pressure is difficult to explain since students have not yet studied the circulatory system and blood pressure. 4. Careful and more-refined use of terms is encountered in both the discussion of regulation of body temperatures and hibernation. It is important for the instructor to be consistently correct. Commentary/Lesson Plan Background: Students may have family members who take blood pressure medicine that is diuretic; this chapter’s concepts should clarify why blood volume and viscosity are important factors on blood pressure, the extent the heart must work to circulate the blood and why changing kidney function can relieve the heart. Misconceptions: We culture children to think of urine as “yucky” and associated with feces as a source of disease; the fact that this is a system that filters blood and that urine is sterile unless there is an infection present, will not counteract this social attitude unless directly discussed. Schedule: HOUR 1 30.1. Water and Osmotic Regulation A. Marine Invertebrates Meet Problems of Salt and Water Balance B. Freshwater Invasion C. Fishes Return to the Sea D. Terrestrial Animals Maintain Salt and Water Balance 30.2. Invertebrate Excretory Structures A. Contractile Vacuole B. Protonephridium C. Metanephridium D. Arthropod Kidneys HOUR 2 30.3. Vertebrate Kidney A. Ancestry and Embryology B. Vertebrate Kidney Function C. Glomerular Filtration D. Tubular Reabsorption E. Tubular Secretion F. Water Excretion 30.4. Temperature Regulation A. Chemical Environment B. Ectothermy and Endothermy C. Ectotherms Achieve Temperature Independence D. Temperature Regulation in Endotherms E. Adaptive Hypothermia in Birds and Mammals ADVANCED CLASS QUESTIONS: 1. Why can the brackish-water crab withstand a wide variation in salinity while the marine spider crab is intolerant of any change in salinity? Answer:The difference in salinity tolerance between the brackish-water crab and the marine spider crab stems from their respective evolutionary adaptations to their habitats. 1. Brackish-water Crab: - Adapted to Fluctuating Salinity: Brackish-water crabs inhabit estuarine environments where freshwater from rivers mixes with seawater. These habitats naturally experience fluctuations in salinity due to tidal changes and freshwater inflow. Brackish-water crabs have evolved physiological mechanisms to regulate their internal salt levels in response to these fluctuations. - Osmoregulation: They possess specialized organs such as excretory glands and salt-secreting glands that help maintain the balance of salt and water in their bodies. These adaptations allow them to tolerate a wide range of salinity levels, from freshwater to seawater, by adjusting their internal osmotic concentrations. 2. Marine Spider Crab: - Adapted to Stable Salinity: Marine spider crabs primarily inhabit marine environments with relatively stable salinity levels. Unlike estuarine environments, marine habitats typically have consistent salinity due to the absence of freshwater input. Consequently, marine spider crabs may not have developed the same level of physiological tolerance to salinity fluctuations as brackish-water crabs. - Limited Osmoregulatory Capacity: Their osmoregulatory systems may not be as robust as those of brackish-water crabs since they haven't needed to adapt to varying salinity levels. As a result, even small changes in salinity could disrupt their internal osmotic balance, making them intolerant of fluctuations in their environment. In summary, the brackish-water crab's ability to withstand wide variations in salinity is a result of evolutionary adaptations to estuarine environments, including sophisticated osmoregulatory mechanisms. In contrast, the marine spider crab's intolerance to changes in salinity is due to its adaptation to more stable marine habitats and potentially less developed osmoregulatory systems. 2. Why would a marine fish that needs to hold onto freshwater, be so careful to only take in as much water as necessary to reach its normal body salinity, and absolutely no more? Answer: Marine fish that need to retain freshwater must carefully regulate the amount of water they intake to maintain their normal body salinity for several reasons: 1. Osmoregulation: Marine fish live in a hypertonic environment, meaning that their body fluids have a higher salt concentration compared to the surrounding seawater. To prevent dehydration, they continuously lose water to the surrounding environment through osmosis. To counteract this loss, they need to actively take in water. However, they must do so cautiously to avoid over-hydration, which could lead to a decrease in their body salinity. 2. Maintaining Internal Balance: The internal environment of marine fish is finely balanced to support essential physiological processes. Any significant deviation from their optimal body salinity can disrupt this balance and compromise their health and survival. Therefore, they regulate water intake precisely to avoid upsetting this delicate equilibrium. 3. Avoiding Hyponatremia: Over-hydration can dilute the salt concentration in a fish's body fluids, leading to a condition known as hyponatremia, where the sodium levels become dangerously low. Hyponatremia can disrupt nerve and muscle function and lead to various health problems, including neurological disorders and even death. Therefore, marine fish must be cautious not to take in more water than necessary to maintain their electrolyte balance. 4. Energy Conservation: Regulating water intake consumes energy, as it often involves active transport mechanisms across cell membranes. By conserving energy and only taking in as much water as needed, marine fish can allocate resources more efficiently for other essential physiological functions, such as growth, reproduction, and predator avoidance. In summary, marine fish carefully control their water intake to maintain their body salinity within a narrow range to support osmoregulation, maintain internal balance, avoid electrolyte imbalances, and conserve energy for essential biological processes. 3. If 180 liters of filtrate is produced but only 1.2 liters are excreted as urine, what percent of filtrate is excreted? Answer: To find the percentage of filtrate that is excreted as urine, we can use the following formula: Given: • Volume of filtrate produced = 180 liters • Volume of urine excreted = 1.2 liters Substituting these values into the formula: So, approximately 0.67% of the filtrate is excreted as urine. 4. Why would a healthy person sometimes have concentrated urine and at other times have dilute urine? Answer: The concentration of urine in a healthy person can vary depending on several factors, including hydration status, fluid intake, physical activity level, environmental conditions, and hormonal regulation. Here's why: 1. Hydration Status: When a person is adequately hydrated, their body retains water, and the kidneys produce dilute urine to remove excess water and maintain fluid balance. Conversely, when a person is dehydrated, the body conserves water by producing concentrated urine to minimize water loss through urination. 2. Fluid Intake: The volume and composition of urine are directly influenced by fluid intake. Drinking large amounts of water will generally result in more dilute urine, while limited fluid intake or increased fluid loss (e.g., through sweating) can lead to more concentrated urine. 3. Physical Activity: Physical activity and sweating can cause fluid loss, leading to temporary dehydration and more concentrated urine. Conversely, during periods of rest or low physical activity, the body may produce more dilute urine as it retains water. 4. Environmental Conditions: Hot and humid environments can increase fluid loss through sweating, potentially leading to dehydration and more concentrated urine. In contrast, cooler environments may result in less fluid loss and more dilute urine. 5. Hormonal Regulation: Hormones such as antidiuretic hormone (ADH), aldosterone, and atrial natriuretic peptide (ANP) play crucial roles in regulating fluid balance and urine concentration. ADH, for example, controls the reabsorption of water in the kidneys, influencing urine concentration based on the body's hydration status. 6. Health Status: Certain health conditions, such as diabetes insipidus or syndrome of inappropriate antidiuretic hormone (SIADH), can affect urine concentration by disrupting the body's ability to regulate water balance. In summary, variations in urine concentration in a healthy person are normal and can occur in response to changes in hydration status, fluid intake, physical activity, environmental factors, hormonal regulation, and overall health. These fluctuations help the body maintain fluid balance and eliminate waste products effectively. 5. How does the length of the loop of Henle correlate with the ability of animals to concentrate urine? Answer: The length of the loop of Henle plays a crucial role in the kidney's ability to concentrate urine, particularly in animals. This correlation is rooted in the mechanism of countercurrent multiplication, which is facilitated by the anatomical structure of the loop of Henle. 1. Countercurrent Multiplication: The loop of Henle creates a concentration gradient within the interstitium of the kidney medulla. This gradient allows for the passive reabsorption of water from the renal tubules, concentrating the urine. The longer the loop of Henle, the greater the opportunity for countercurrent multiplication to occur, resulting in a more concentrated urine. 2. Hypertonic Medulla: The loop of Henle extends deep into the medulla of the kidney, where the interstitium becomes increasingly hypertonic (having a higher solute concentration). This hypertonic environment facilitates water reabsorption by osmosis from the descending limb of the loop of Henle, leading to the concentration of urine. 3. Maximizing Water Reabsorption: In animals living in arid environments or those with limited access to water, such as desert rodents or certain species of birds, having a longer loop of Henle allows for more efficient water reabsorption. This adaptation enables these animals to produce highly concentrated urine, conserving water and maintaining proper hydration levels. 4. Evolutionary Adaptation: Species that inhabit environments with fluctuating water availability or high water demands have evolved longer loops of Henle to optimize their urinary concentrating ability. This adaptation enhances their survival by enabling them to effectively regulate their water balance in response to environmental conditions. In summary, the length of the loop of Henle correlates positively with the ability of animals to concentrate urine. A longer loop allows for more effective countercurrent multiplication, creating a hypertonic medullary environment that maximizes water reabsorption and enables the production of highly concentrated urine, particularly in species facing water conservation challenges. CHAPTER 31 HOMEOSTASIS: INTERNAL FLUIDS AND RESPIRATION CHAPTER OUTLINE 31.1. Internal Fluid Environment (Figures 31.1, 31.2) A. Fluids 1. Body fluid of a single-celled organism is cellular cytoplasm. 2. In multicellular organisms, body fluids are intracellular and extracellular. 3. Intracellular fluids are the collective fluids inside all the body’s cells. 4. Extracellular fluids are outside and surrounding the cells. 5. Extracellular fluid buffers cells from harsh physical and chemical changes outside the body. 6. In vertebrates, annelids and a few others, extracellular fluid is further divided into blood plasma and interstitial fluid. 7. Blood vessels of a closed circulatory system contain the plasma while interstitial fluid is between the cells and organs of the body. 8. Nutrients and gases passing between vascular plasma and cells must traverse this fluid separation. 9. Interstitial fluid is constantly formed from plasma by movement of fluid from microscopic vessels in close proximity to cells. B. Composition of the Body Fluids 1. Plasma, interstitial and intracellular fluids are mostly water. 2. Animals range from 70% to 90% water. 3. Humans are 70% water by weight; 50% is cell water, 15% is interstitial and 5% is blood plasma. 4. Plasma is the pathway of exchange between cells and the kidney, lung or gill, and alimentary canal. 5. Body fluids contain many inorganic and organic substances in solution. a. Sodium, chloride and bicarbonate are the chief extracellular electrolytes. b. Potassium, magnesium, and phosphate ions and proteins are major intracellular electrolytes. c. Concentrations are maintained despite continuous flow of materials into and out of cells. 6. Plasma and interstitial fluids have similar composition except that plasma has more large proteins. 31.2. Composition of the Blood (Figures 31.3, 31.4) A. Elements 1. Flatworms and cnidarians lack a circulatory system and do not have a true “blood.” 2. Invertebrates with an open circulatory system have a more complex “hemolymph.” 3. Closed circulatory systems keep blood contained in blood vessels separate from tissue fluids. 4. In vertebrates, blood is a complex liquid tissue of formed elements suspended in plasma. 5. When separated by centrifugation, blood is 55% plasma and 45% formed elements. 6. Plasma a. Water constitutes 90%. b. Dissolved solids include plasma proteins (e.g. albumin, globulins, fibrinogen), glucose, amino acids, electrolytes, various enzymes, antibodies, hormones, metabolic wastes, etc. c. Dissolved gases include oxygen, carbon dioxide and nitrogen. 7. Cellular Components a. Red blood cells contain hemoglobin and transport oxygen and carbon dioxide. b. White blood cells are scavengers and defend the body against foreign material. c. Cell fragments function in blood coagulation. 8. Plasma proteins are a diverse group with many functions. a. Albumins are 60% of plasma proteins and help maintain osmotic equilibrium. b. Globulins are high-molecular weight proteins and include immunoglobulins. c. Fibrinogen is a very large protein that is involved in clot formation. d. Blood serum is plasma minus the proteins. 9. Red Blood Cells (Erythrocytes) a. Red blood cells occur in enormous numbers in the blood. b. In mammals and birds, they form from large, nucleated erythroblasts in red bone marrow. c. In other vertebrates, kidneys and spleen are the major sites of red blood cell production. d. In mammals, the nucleus shrinks and disappears during development. e. Human red blood cells also lose ribosomes, mitochondria and most enzyme systems. f. The human cell is biconcave in shape; this provides the greatest surface area for gas diffusion. g. Each cell holds about 280 million molecules of hemoglobin. h. About 33% of the weight is hemoglobin. i. In non-mammal vertebrates, red blood cells have a nucleus and are ellipsoidal. j. Erythrocytes have an average life of four months and may travel 11,000 kilometers. k. When it is worn out and fragments, it is engulfed by macrophages in the liver. l. Iron from hemoglobin is salvaged and used again. m. The rest of the heme molecule is converted to bilirubin, a bile pigment. n. About 10 million erythrocytes are destroyed every second, and that number must be replaced. 10. White Blood Cells (Leukocytes) a. White blood cells form a part of the immune system. b. In human adults, they number about 7.5 million per milliliter of blood, about one per 700 RBCs. c. Varieties include: granulocytes (neutrophils, basophils, and eosinophils) and agranulocytes (lymphocytes and monocytes). B. Hemostasis; Prevention of Blood Loss (Figures 31.5, 31.6) 1. Blood flows under considerable hydrostatic pressure; it is important to prevent blood loss after injury. 2. When a vessel is damaged, smooth muscle in the wall of the vessel contracts and the lumen narrows. 3. In both vertebrates and invertebrates, this constriction may totally prevent blood loss. 4. Vertebrates and larger, active invertebrates have special cellular elements to form clots. 5. Blood coagulation is the dominant hemostatic defense in vertebrates. 6. Blood clots form as a tangled network of fibers from one of the plasma proteins, fibrinogen. 7. Transformation of fibrinogen into a fibrin meshwork is catalyzed by the enzyme thrombin. 8. Thrombin is present in the blood in the inactive form prothrombin. 9. Platelets and damaged cells of blood vessels play a vital role in clotting. 10. Platelets a. Platelets form in red bone marrow from large cells that pinch off bits of cytoplasm. b. Platelets are fragments of cells; ~150,000 to 300,000 per cubic millimeter of blood. c. Platelets adhere to any disruption in the normally smooth inner surface of a blood vessel. d. They release thromboplastin and other clotting factors. e. These factors and calcium ions initiate conversion of prothrombin to active thrombin. f. This involves a long and complex catalytic sequence; each reactant cascades into release of much more of the next reactant. g. 13 different plasma coagulation factors are known; a deficiency of one factor can stop the process. h. This provides a balance between providing emergency clotting and avoiding unnecessary clots. 11. Hemophilia is one of several clotting abnormalities; it is caused by a mutation on the X chromosome. 31.3. Circulation A. General Design 1. Sponges and ciliates utilize the water medium around them for transport. 2. Flattened animals can utilize diffusion across their thin surfaces, but only to a limit. 3. Larger animals cannot rely on diffusion to support respiratory and metabolic needs. 4. A full circulatory system has a propulsive organ, arteries, capillaries and a venous reservoir. 5. An earthworm demonstrates this basic system with a distributed pumping system (Figure 31.7). B. Open and Closed Circulations (Figures 31.8, 31.9) 1. A closed circulatory system confines blood to a journey through the vascular system. 2. An open circulation system lacks connecting blood vessels and capillaries. 3. In arthropods, molluscs and some other invertebrates, sinuses collectively form the hemocoel. 4. Open System a. During development, the blastoderm is not filled by mesoderm but becomes the hemocoel. b. The blood or hemolymph washes through this primary body cavity or hemocoel. c. There is no distinction between blood plasma and lymph, as is the case in closed circulation. d. Hemolymph is 20–40% of body volume in open systems; blood is 5–10% in closed systems. e. In arthropods, the heart and all organs lie in the hemocoel bathed by blood. f. Blood enters the heart through valved openings to the side or ostia. g. Forward-moving waves propel blood forward to the head where it washes into the hemocoel. h. It is routed through the body by baffles and membranes before returning back into the heart ostia. i. Blood pressure is very low in open systems, rarely over 4–10 mm Hg. j. Therefore, arthropods have auxiliary hearts or contractile vessels to boost blood flow. k. Insects and other terrestrial arthropods do not use their circulatory system for respiratory gas transport, rather a separate respiratory system has evolved for this purpose. 5. Closed Systems a. In embryonic development of animals with closed systems, the coelom increases to obliterate the blastocoel and forms a second body cavity. b. The system of continuously connected blood vessels develops within the mesoderm. c. The heart pumps blood into arteries that branch into arterioles that enter a vast capillary system. d. Blood leaves the capillaries in converging venules and larger veins to return to the heart. e. Capillary walls are thin to allow transfer of materials between blood and tissues. f. Such a closed system allows large animals to shunt blood to tissues needing it. g. However, blood pressure is much higher in closed systems; fluid is pushed across capillary walls. h. Fluid lost into tissues and interstitial spaces is returned by osmosis and the lymphatic system. C. Plan of Vertebrate Circulatory Systems (Figures 31.10–31.11) 1. Comparative Anatomy a. The principal difference in vertebrate systems is the separation of the heart into two pumps. b. These changes occurred as vertebrates converted from gill breathing to lungs. c. The Fish Heart 1) The heart has two main chambers in series: the atrium and ventricle. 2) The atrium is preceded by an enlarged sinus venosus that collects blood and smooths delivery. 3) Elasmobranchs have a fourth heart chamber, the conus arteriosus and teleost fish have a bulbous arteriosus that dampens blood pressure oscillations before blood flows into capillaries. 4) Blood makes one circuit, flowing first to gills and then on to the aorta and body. 5) Oxygenated blood is provided to the body organs before the veins return to the heart. 6) However, gill capillaries offer much resistance, and blood pressure to the body tissues is low. d. Double Circulation 1) Terrestrial animals evolved lungs instead of gills between heart and aorta. 2) This provided a high pressure system that provided oxygenated blood to capillary beds and a pulmonary circuit to serve the lungs. 3) This change is seen in lungfishes and amphibians. 4) Modern amphibians have separate atria. 5) The right atrium receives venous blood from the body. 6) The left receives oxygenated blood from the lungs. 7) The ventricle is undivided but venous and arterial blood do not heavily mix. 8) Ventricles are nearly separate in nonavian reptiles and completely separate in crocodilians, birds and mammals. 9) Systemic and pulmonary circulations are served by one half of a dual heart. 2. Mammalian Heart (Figures 31.12, 31.13) a. The mammalian heart is located in the thorax and enclosed in the pericardial sac. b. Blood returning from the lungs collects in the left atrium and passes to the left ventricle. c. The left ventricle pumps the blood to the body in the systemic circuit. d. Blood returns from the body into the right atrium and passes to the right ventricle. e. The right ventricle pumps the blood to the lungs in the pulmonary circuit. f. The bicuspid valves are between the left atrium and ventricle to prevent backflow of blood. g. The tricuspid valves separate the right atrium and right ventricle to prevent backflow of blood. h. Semilunar valves stop backflow from the pulmonary to right ventricle and aorta to left ventricle. i. Contraction of the heart is systole. j. Relaxation of the heart is diastole. k. When the atria contract, the ventricles relax; ventricular systole is accompanied by atrial diastole. l. Cardiac output is the amount of blood moved through the heart; exercise can increase it fivefold. m. Heart rates can vary from an ectothermic codfish at 30 beats per minute to a rabbit at 200. n. Smaller animals have a faster heart rate than larger animals, reflecting the increase in metabolic rate that occurs with decreased body size. o. An elephant has a heart rate of about 25 beats per minutes, humans around 70, cats around 125, a mouse has 400 and a tiny shrew has 800 beats per minute. 3. Excitation of the Heart (Figure 31.14) a. The vertebrate heart is a muscular pump composed of cardiac muscle. b. Cardiac muscle fibers are branched and striated, but do not depend on nerve activity to contract. c. Specialized pacemaker cells initiate nerve contractions. d. In a nonavian reptile, bird, or mammal heart, the pacemaker is the sinus node, a remnant of the ancestral sinus venosus still found in fish. e. Electrical activity of the pacemaker spreads over the muscle of the two atria and then the muscle of the ventricles. f. Electrical activity is conducted through the atrioventricular bundle to the apex of the ventricle and then continues through the specialized Purkinje fibers to the apex of ventricles. g. This causes the contraction to begin at the tip and pushes blood out efficiently at the same time. h. A cardiac center in the medulla sends out two sets of nerves. 1) The vagus nerves brake the heart rate. 2) The accelerator nerves speed up the heart rate. 3) Both terminate at the sinus node for direct guidance of the pacemaker. i. The cardiac center receives sensory information from pressure and chemical receptors. j. Myogenic hearts have heartbeat initiated in specialized muscle cells. k. In a neurogenic heart, as is found in decapods, a cardiac ganglion serves as a pacemaker and the heart stops beating without this stimulation. l. Isolated myogenic hearts continue to beat for hours; neurogenic hearts do not. 4. Coronary Circulation a. A constantly active heart needs a generous blood supply. b. Small fish and frog hearts are heavily channeled; the heart’s pumping action suffices to provide oxygen. c. Larger fish, frogs, and nonavian reptile hearts are thicker and need a dedicated vascular supply (coronary circulation). d. Coronary arteries divide into an extensive capillary network. e. Heart muscle has a high oxygen demand and uses 70% of the oxygen from the blood. f. When the heart is working hard during exercise, the blood supply must increase up to nine times. g. Partial or complete blockage of circulation will cause heart cells to die from lack of oxygen. 5. Coronary artery disease (CAD) a. CAD is currently the #1 killer in the U.S. b. Risk factors can be divided into those that cannot be modified and those that can. c. Risk factors that cannot be modified include family history, being a male or postmenopausal female, or age. d. Modifiable risk factors include smoking, high blood cholesterol levels, high blood pressure, uncontrolled diabetes, and others. D. Arteries (Figure 31.15) 1. All vessels leaving the heart are arteries. 2. Arteries must withstand high, pounding pressures and have thick, elastic walls. 3. The wall bulges during systole and compresses the fluid column during ventricular diastole. 4. The next heartbeat surges the blood pressure before it drops to zero. 5. Blood pressure varies between systole and diastole: 120 mm Hg over 80 mm Hg in humans, or 120/80. 6. Arteries branch into narrower arterioles with smooth muscle walls. 7. Arterioles can dilate or constrict diverting blood flow to body organs where it is most needed. 8. Blood pressure is measured as the force required to support a column of mercury. 9. A sphygmomanometer compresses arteries in the upper arm; pressure is released until blood spurts through under systolic pressure; when pressure drops below diastolic, blood flow is no longer heard. E. Capillaries (Figures 31.16, 31.17) 1. Structure a. Marcello Malpighi confirmed capillaries existed in 1661 by inspecting living frog lung tissue. b. Huge numbers of capillaries infuse most tissues; muscle has over a million per square inch. c. At rest, fewer than one percent are open; during exercise, all capillaries may be open. d. Capillaries are extremely narrow, averaging about 8 micrometers in diameter in mammals. e. Red blood cells are almost this wide and must pass through single-file. f. Capillary walls are composed of a single layer of endothelial cells held together by a basement membrane. 2. Capillary Exchange a. Blood pressure forces fluids out through the permeable capillary walls into interstitial spaces. b. Fluid may pass between the endothelial cells via water-filled clefts or through endothelial cells. c. Lipid-soluble substances can diffuse easily through the plasma membrane of endothelial cells. d. Plasma protein molecules are too large and the filtrate is nearly protein-free. e. Fluid exchange across a capillary wall is a balance of hydrostatic pressure and osmotic pressure. f. If fluids leave the capillaries and do not reenter circulation, the tissues accumulate fluid (edema). g. In a capillary, blood pressure is higher at the arteriole end and declines toward the venule side. h. However, osmotic pressure is created by proteins that cannot pass across the capillary wall. i. As a result, water and solutes are filtered out at the arteriole end and drawn in at the venule end. j. However, outflow exceeds inflow and the excess fluid is lymph that remains in interstitial spaces. k. This excess is removed by lymph capillaries and eventually returned to the circulatory system. F. Veins 1. Venules and veins are thinner walled, less elastic, and larger than arteries and arterioles. 2. Blood pressure is low (10 mm Hg) where capillaries drain into venules and nearly zero at the heart. 3. Venous blood is assisted back to the heart by valves in veins, body muscles surrounding veins, suction created during diastole of the heart, and movement of the lungs. 4. Blood would pool in the long extremities without the veins to segment the blood column. 5. Valves are formed as infoldings of the endothelial cell layer and underlying connective tissue. 6. Skeletal muscle action squeezes the veins, and valves keep the flow going toward the heart. 7. Negative pressure in the thorax, created by breathing, speeds venous return by sucking blood up the large vena cava. G. Lymphatic System (Figure 31.18) 1. Thin-walled vessels extend into most body tissues to collect lymph. 2. Lymphatic vessels merge into larger vessels that drain into veins in the lower neck. 3. Lymph has a lower concentration of protein but carries some fat molecules absorbed from the gut. 4. Lymph nodes are located along the lymph vessels and trap and remove foreign particles. 5. Lymph nodes are also a center, along with bone marrow and thymus gland, for lymphocytes. 31.4. Respiration A. Processes 1. Cellular respiration is defined as the oxidative processes that occur inside a cell. 2. External respiration is an exchange of oxygen and carbon dioxide between organism and environment. B. Problems of Aquatic and Aerial Breathing 1. Water and land are vastly different in their physical characteristics. 2. Air contains about 20 times more oxygen than does water; fully saturated water contains 9 ml of oxygen per liter compared to 209 ml of oxygen per liter in air. 3. Water is 800 times more dense and 50 times more viscous than air. 4. Gas molecules diffuse about 10,000 times more rapidly in air than in water. 5. Advanced fishes still must use up to 20% of their energy to extract oxygen from water. 6. Mammals use only 1–2% of their resting metabolic energy to breathe. 7. Respiratory surfaces must be thin and moist; this is not a problem for aquatic animals. 8. Air breathers have respiratory surfaces invaginated, and pumping actions move air in and out. 9. Evaginations of the body surface, such as gills, are used for aquatic respiration. 10. Invaginations such as tracheae and lungs are used for air breathing. C. Respiratory Organs 1. Gas Exchange by Direct Diffusion a. Protozoans, sponges, cnidarians and many worms use direct diffusion to exchange gases. b. Cutaneous respiration is not sufficient if the body exceeds 1 mm in diameter. c. However, flatworms extend a thin body to achieve adequate gas exchange. d. Larger animals can use cutaneous respiration as a supplement to gills or lungs. e. Eels secure 60% of their oxygen and carbon dioxide exchange through highly vascular skin. f. During winter hibernation, frogs and turtles can meet their lowered respiratory requirements. g. Lungless salamanders usually lack lungs as adults; they are limited in body size. 2. Gas Exchange Through Tubes: Tracheal Systems (Figure 31.19) a. Insects and some other arthropods have a direct and efficient system of tracheae. b. Air enters through valve-like spiracles. c. Tracheal channels narrow to fluid-filled tracheoles 1 micrometer in diameter embedded in tissues. d. Air enters and leaves the tracheal system through valvelike openings (spiracles). e. The spiracle opening is regulated to reduce water loss. f. Oxygen diffuses in along a gradient as oxygen is absorbed by tissues. g. Carbon dioxide diffuses out along a gradient as carbon dioxide builds up in tissues. h. Some insects ventilate the tracheal system with body movements. i. The tracheal system is independent of the hemolymph that has no direct role in respiration. 3. Efficient Exchange in Water: Gills (Figure 31.20) a. Gills, or branchia, may be simple external extensions of the body surface (e.g., dermal papulae of sea stars or branchial tufts of marine worms). b. The dorsal lobe of parapodia may also serve as an external respiratory surface for some polychaete worms. c. Internal gills of fishes and arthropods are thin filamentous structures supplied with vessels. d. In gills, blood flow is opposite the flow of water to provide the maximum extraction of oxygen; this is countercurrent flow. e. Water is washed over the gills in a steady stream, pulled and pushed by an efficient, two-valved, branchial pump. f. The fish’s forward movement through water assists some gill ventilation. 4. Lungs (Figures 31.21–31.22) a. Despite the high oxygen levels in air, gills do not function in air because they dry out. b. Some invertebrates including snails, scorpions, some spiders, etc. have inefficient “lungs.” c. Terrestrial vertebrates generally have lungs that can be ventilated by muscle movements. d. The most rudimentary lungs exist in lungfishes. 1) The lungfish lung has a rich supply of capillaries on unfurrowed walls. 2) A tube connects it to the pharynx. 3) It uses a primitive ventilating system to move air in and out of the lung. e. Amphibian lungs vary from smooth-walled, bag-like salamander lungs to divided lungs of frogs. f. Reptiles’ lungs have greater surface area because they are subdivided further into air sacs. g. The mammalian lung has millions of small sacs, called alveoli. h. Human lungs have 1000 kilometers of capillaries and 50–90 square meters of surface area. i. However, in contrast to flow over a gill, the air does not continuously enter a lung. j. About one-sixth the air in human lungs is replenished each inspiration. k. Bird Lungs 1) Bird lungs have an extensive system of air sacs as reservoirs during ventilation. 2) On inspiration, 75% of air bypasses the lungs to enter the air sacs. 3) At expiration, the fresh air flows through lung passages providing continuous gas exchange. l. Amphibians and lungfishes force air into their lungs by positive pressure breathing; this requires external nares and the ability to seal nostrils and mouth. m. Most nonavian reptiles, birds and mammals ventilate lungs by negative pressure, sucking air in by expanding the thoracic cavity. D. Structure and Function of the Mammalian Respiratory System (Figure 31.23) 1. Structure a. Air enters the mammalian respiratory system through nostrils. b. It passes through nasal chambers lined with mucus-secreting epithelium. c. The internal nares are openings leading to the pharynx where the pathway crosses with digestion. d. Inhaled air passes out a narrow opening, the glottis, while food crosses to enter the esophagus. e. The glottis opens into the larynx or voice box and then into the trachea or windpipe. f. The trachea branches into two bronchi, one to each lung. g. The bronchus divides and subdivides into small bronchioles that lead to alveoli. h. Alveolar walls are made of single-layered endothelium. i. Air passageways are lined with mucus-secreting and ciliated epithelial cells. j. Partial cartilage rings in the tracheae, bronchi, and bronchioles prevent collapsing. k. During this passage, inhaled air is filtered free from most dust, warmed, and moistened. l. The lungs are mostly elastic tissue and a little muscle. m. A thin layer of visceral pleura encloses the lung; parietal pleura lines the inner wall of the chest. n. The two layers are lubricated and slide past each other during ventilation. o. The spine, ribs and breastbone surround the thoracic cavity. p. The diaphragm forms the floor, and a muscular diaphragm is only found in mammals. 2. Ventilating the Lungs (Figure 31.24) a. The chest cavity is an airtight chamber. b. In inspiration, the ribs are pulled upward and the diaphragm flattens; this enlarges the chest. c. The increase in volume causes intrapleural pressure to fall to a more negative value and intrapulmonary pressure to fall below atmospheric pressure. d. Air rushes in through the air passageways to equalize the pressure. e. Tidal volume is the amount of air that is moved during this process. f. Normal expiration involves relaxation of ribs and diaphragm that return to the normal position. g. Chest cavity size decreases and air exits. h. During forced expiration, the ribs are pulled down and inward by the internal intercostal muscles, abdominal muscles force the diaphragm upward to a greater degree; these mechanisms expel more air and enhance inspiratory volume. 3. Coordination of Breathing a. Breathing is normally involuntary and automatic but can come under voluntary control. b. Neurons in the medulla of the brain regulate normal, quiet breathing. c. They produce regular spontaneous bursts that stimulate the external intercostal muscles. d. Respiration must increase dramatically when there is a high requirement for oxygen. e. However, the body cues on the increasing carbon dioxide level rather than the decrease in oxygen. f. As carbon dioxide increases, an increase in hydrogen ions makes the cerebrospinal fluid acidic. g. Carbon dioxide combines with water to form carbonic acid that releases hydrogen ions. 4. Gaseous Exchange in Lungs and Body Tissues: Diffusion and Partial Pressure (Figure 31.25) a. Air is a mixture of 71% nitrogen, 20.9% oxygen, 0.03% carbon dioxide and a few other gases. b. Gravity attracts the mass of atmosphere to the earth; total air pressure is 760mm Hg. c. Each component gas contributes to this total; each component gas therefore has a partial pressure. (Table 31.1) d. Partial pressure of oxygen is 0.209 x 760 or 159mm Hg. e. Partial pressure of carbon dioxide is 0.0003 x 760 or 0.23mm Hg in dry air. f. Water vapor likewise exerts a partial pressure. g. Air entering the respiratory tract changes in composition; it becomes wet and mixes with residual air. h. The partial pressure of oxygen in the lung alveoli is greater (100mm Hg) than in the venous blood of lung capillaries (40mm Hg), oxygen diffuses into the lung capillaries. i. The carbon dioxide in the blood of lung capillaries has a higher concentration (46mm Hg) than in the lung alveoli (40mm Hg) and carbon dioxide diffuses from blood to alveoli. j. In tissues, respiratory gases continue to move along concentration gradients. 5. How Respiratory Gases Are Transported (Figure 31.26) a. In some invertebrates, respiratory gases are merely dissolved in body fluids. b. Only animals with low metabolism can survive on such low levels of oxygen. c. Only one percent of the human oxygen requirement could be provided by dissolved oxygen. d. In many invertebrates and all vertebrates, respiratory pigments are used to transport oxygen. e. In most animals and in all vertebrates, the pigments are packaged in blood cells. f. Hemoglobin i. Hemoglobin is the most widespread respiratory pigment among animals. ii. Each molecule is made of 5% heme, an iron-compound and 95% globin, a colorless protein. iii. The heme portion has a great affinity for oxygen; each gram can carry 1.3 ml of oxygen. iv. Heme also holds oxygen in a loose enough chemical state that tissues can take it away. g. Hemoglobin has a 200 times greater affinity for carbon monoxide than for oxygen and can displace oxygen, resulting in death. h. Hemoglobin Saturation Curves i. Also called oxygen dissociation curves, they show the relationship to surrounding oxygen levels. ii. The lower the surrounding oxygen is tension, the more oxygen released. iii. This allows more oxygen to be released to tissues that need it most. iv. Carbon dioxide shifts the hemoglobin saturation curve to the right; this is the Bohr effect. v. Therefore, as carbon dioxide enters the blood from respiring tissues, it causes hemoglobin to unload more oxygen. vi. The opposite occurs in the lungs and more oxygen is loaded onto hemoglobin. i. Other Pigments i. Hemocyanin is a blue, copper-containing protein present in crustaceans and most molluscs. ii. Hemerythrin is a red pigment found in some polychaete worms; it does not have a heme group and it has lower oxygen-holding properties. j. Carbon Dioxide Transport (Figure 31.27) i. About 7% of carbon dioxide is carried dissolved in the blood. ii. The remainder diffuses into red blood cells where 70% of it becomes carbonic acid through the action of the enzyme carbonic anhydrase. iii. Carbonic acid immediately dissociates into hydrogen ions and bicarbonate ions. iv. Several systems buffer the hydrogen ions to prevent blood acidity. v. About 23% of the carbon dioxide combines reversibly with hemoglobin, not with the heme, but with the amino acids to form carbaminohemoglobin. vi. The reaction is reversible and the carbon dioxide diffuses into alveoli in the lungs. vii. Increased carbon dioxide in the blood lowers blood pH, as does addition of acid to the blood. Lecture Enrichment 1. Note that the short four-month lifetime of the red blood cell is directly related to its inability to repair itself since it has lost nuclear DNA, ribosomes, mitochondria, etc. 2. Red blood cells become damaged and fragment from wear-and-tear because, unlike other cells, they cannot repair themselves. Therefore, they are pulled from circulation by macrophages as they pass through the liver and some components are recycled, a process that can be compared with worn out money being pulled from circulation as it passes through banks. 3. In discussion of the heart structures, the terms “right” and “left” are particularly important. This is a point where you can remind students that directions and positions are named from the perspective of the organism that possesses the structure, not right and left as an observer would designate viewing the organism or person head-on. 4. It can be noted that pulmonary circulation is not used for oxygen before birth in placental mammals because oxygenated blood is received from the umbilical connection to the placenta. Therefore, it is not critical that the heart have a completely separated septum before birth, and this is indeed the case where the foramen ovale, or opening across the heart wall, does not close until late in pregnancy. If it fails to close, the baby is a “blue baby” which reflects the lower level of oxygenated blood; and the inability of this baby to live very long without an operation to close the opening is obvious evidence of our need to completely separate pulmonary and systemic circulation. 5. The historical notes of Stephan Hales (blood pressure of a mare) and Marcello Malpighi (capillaries in living frog lung) are just a few of many critical breakthrough experiments that relied on experiments with living animals. Such examples may elicit revulsion from students who have been sheltered from meat processing farm experiences and surgical and emergency room procedures. While discussion of these historical discoveries may provide an intellectual perspective on reality-based laboratory work, only actual labwork by students will give them a fuller understanding of the concepts and the need for such research practices. 6. An instructor can illustrate a simple diagnostic test of edema. Pressing on the surface of soft tissue in the hand, arm, ankle, etc. with a thumb will leave a white thumbprint for about 3–5 seconds as blood has been pressed out of the capillaries in the surface tissue and takes this much time to return. However, if there is fluid in these tissues, the thumbprint depression will remain long after the white blanched area has returned to pink. Note that you are using a teaching technique and not practicing medicine. 7. Some zoology teachers are accomplished at simulating positive pressure breathing of a frog by taking a mouthful of air, sealing the nose and lips, and pressing the bloated cheeks so the air appears to be forced into the lungs. 8. Gases dissolved in fluids are not beyond student experience; the difference between a fizzy soda and a flat soda is dissolved carbon dioxide, and the bubbles of gas can be seen sparkling off the top of a newly poured drink. Commentary/Lesson Plan Background: To the extent students have participated in blood drives and given blood, they will have experiences with some blood properties, the speed of replenishment, the viscosity, and the equipment involved, including the long tube that is crimped to provid samples for cross-matching. Students who have run on a cold, dry winter day have experienced a mild pleurisy where the pleural membranes stick together from dryness. Misconceptions: Sadly, blood has moved from a public image of “river of life” to potential “river of death.” With the advent of AIDS and greater awareness of hepatitis (a far greater infection risk than AIDS), the fear is overblown and a rational discussion of its biology may help restore some objectivity. A few people still believe that some aspects of heredity including temperament “run in the blood line”; this is ironic since red blood cells are the only common body cells that lack hereditary material and this old concept could not be farther from the truth. Some students have the wrong perception that humans have the “best” or “most advanced” of all systems and yet the bird has a far more efficient lung. Such a high efficiency lung is not needed by a lower metabolism human, just as an alveolar lung is not useful to an ectothermic frog. Schedule: HOUR 1 31.1. Internal Fluid Environment A. Fluids B. Composition of the Body Fluids 31.2. Composition of the Blood A. Elements B. Hemostasis; Prevention of Blood Loss 31.3. Circulation A. General Design B. Open and Closed Circulations C. Plan of Vertebrate Circulatory Systems HOUR 2 D. Arteries E. Capillaries F. Veins G. Lymphatic System 31.4. Respiration A. Processes B. Problems of Aquatic and Aerial Breathing C. Respiratory Organs D. Structure and Function of the Mammalian Respiratory System E. Coordination of Breathing F. Gaseous Exchange in Lungs and Body Tissues: Diffusion and Partial Pressure G. Respiratory Gas Transport ADVANCED CLASS QUESTIONS: 1. What factors limit the size and life span of a red blood cell? How could an experiment be constructed that demonstrated the inability of a red blood cell to repair itself? Answer: Several factors limit the size and lifespan of a red blood cell (RBC), including: 1. Lack of Nucleus: Mature red blood cells in mammals lack a nucleus and other organelles, such as mitochondria and endoplasmic reticulum. This absence of a nucleus allows RBCs to have a flexible, biconcave shape, which facilitates their passage through narrow capillaries and maximizes their surface area for gas exchange. However, it also means that RBCs are unable to repair DNA damage or synthesize new proteins necessary for cellular repair and maintenance. 2. Limited Lifespan: The lack of a nucleus and other organelles also limits the lifespan of red blood cells. Without the ability to repair cellular damage or replace worn-out components, RBCs gradually accumulate damage over time and become less functional. The average lifespan of a red blood cell in humans is approximately 120 days, after which they are removed from circulation by the spleen and liver. 3. Oxygen Transport: Red blood cells are specialized for the transport of oxygen from the lungs to tissues throughout the body. Their small size and flexible shape allow them to squeeze through narrow capillaries and deliver oxygen efficiently. However, these features also make them more susceptible to mechanical stress and damage during circulation. To demonstrate the inability of a red blood cell to repair itself, an experiment could be constructed as follows: 1. Isolation of Red Blood Cells: Red blood cells could be isolated from a blood sample obtained from a donor using standard laboratory techniques, such as centrifugation and density gradient separation. 2. Induction of Damage: The isolated red blood cells could then be subjected to various forms of damage, such as exposure to oxidative stress (e.g., hydrogen peroxide), mechanical stress (e.g., shear forces), or exposure to ultraviolet (UV) radiation. These treatments would induce different types of damage, including DNA damage, membrane disruption, and protein denaturation. 3. Assessment of Repair Mechanisms: Following the induction of damage, the ability of red blood cells to repair themselves could be assessed by monitoring changes in cellular morphology, membrane integrity, and function over time. For example, changes in cell shape and membrane integrity could be visualized using microscopy techniques, while changes in cellular function could be assessed by measuring parameters such as oxygen transport capacity or membrane fluidity. 4. Comparison with Control Cells: To confirm that any observed changes are due to the inability of red blood cells to repair themselves, the damaged cells could be compared with control cells that have not been subjected to damage. Control cells could be maintained under identical conditions but without exposure to damaging agents. By systematically inducing damage to red blood cells and monitoring their ability to repair themselves, researchers can demonstrate the limitations of RBC repair mechanisms and further our understanding of the factors that govern the size and lifespan of these vital blood cells. 2. If there is no DNA in red blood cells, then how do forensic scientists identify the blood from a crime scene and establish the DNA match with the accused? [This involves students comprehending that RBC proteins are placed on the membrane when the cells are formed and that there are nuclei present in the WBCs of a blood sample.] Answer: Forensic scientists use a different component of blood, namely white blood cells (WBCs) or leukocytes, to obtain DNA profiles for identification purposes. While red blood cells (RBCs) lack a nucleus and do not contain DNA, white blood cells retain their nuclei and contain genetic material that can be used for DNA analysis. Here's how the process typically works: 1. Blood Sample Collection: Forensic investigators collect blood samples from a crime scene using appropriate protocols to preserve the integrity of the evidence. 2. Separation of Blood Components: In the laboratory, the collected blood sample is processed to separate its components, including red blood cells and white blood cells. This can be achieved through centrifugation or other separation techniques. 3. Isolation of White Blood Cells: White blood cells, which contain nuclei and DNA, are isolated from the blood sample. This step involves separating the leukocytes from the other components of the blood, such as plasma and platelets. 4. DNA Extraction: The isolated white blood cells are then subjected to DNA extraction procedures to isolate the DNA from the cells. This typically involves breaking open the cells to release the DNA and removing other cellular components. 5. DNA Profiling: The extracted DNA is then analyzed using techniques such as polymerase chain reaction (PCR) and gel electrophoresis to generate a DNA profile or fingerprint. This profile consists of specific genetic markers or regions of the DNA that are unique to each individual. 6. Comparison with Suspect's DNA: The DNA profile obtained from the blood sample collected at the crime scene is compared with the DNA profiles of known suspects or individuals associated with the case. If there is a match between the DNA profiles, it provides strong evidence linking the individual to the crime scene. It's important to note that while red blood cells themselves do not contain DNA, other components of blood, such as white blood cells, contain genetic material that can be used for DNA analysis. By isolating and analyzing the DNA from white blood cells, forensic scientists can establish DNA matches and identify individuals involved in criminal investigations. 3. Why is an opening across the heart septum, called the foramen ovale and a normal fetal condition, not a problem before birth? Answer:The foramen ovale is a normal opening in the septum, or wall, between the right and left atria of the fetal heart. This opening serves a crucial function during fetal development but typically closes shortly after birth. Before birth, the foramen ovale is not a problem for several reasons: 1. Fetal Circulation: During fetal development, the circulatory system operates differently than after birth. Oxygenation of blood occurs primarily through the placenta rather than the lungs. The foramen ovale allows oxygenated blood returning from the placenta to bypass the fetal lungs and flow directly from the right atrium to the left atrium, where it is then pumped out to the body by the left ventricle. This helps ensure that oxygenated blood is distributed to the developing fetal tissues. 2. Pressure Differences: In the fetal circulatory system, there are specific pressure gradients and physiological mechanisms that promote blood flow through the foramen ovale. The pressure in the right atrium is slightly higher than that in the left atrium, which facilitates the flow of blood from right to left across the foramen ovale. This pressure difference is maintained by factors such as the relatively low resistance of the fetal lungs and the presence of specialized structures in the fetal heart, such as the fetal shunts (ductus arteriosus and ductus venosus). 3. Functional Closure: Although the foramen ovale remains open during fetal life, it typically begins to close shortly after birth due to changes in circulatory dynamics and pressure gradients. As the newborn takes its first breaths and the lungs become functional, pulmonary vascular resistance decreases, and the pressure in the left atrium increases. This reversal of pressure gradients across the atrial septum helps push the flaps of the foramen ovale together, sealing the opening. Over time, the foramen ovale undergoes anatomical closure and is eventually obliterated, resulting in a fully developed interatrial septum. In most cases, the closure of the foramen ovale occurs within the first few months to years of life, ensuring the normal separation of the right and left sides of the heart. However, in some individuals, the foramen ovale may fail to close completely, leading to a persistent opening known as a patent foramen ovale (PFO). While a PFO may not cause symptoms or problems for many people, it can be associated with certain medical conditions, such as paradoxical embolism (a type of stroke), and may require medical evaluation and treatment in some cases. 4. Arteries and veins are named by their relationship in blood flow from or to the heart. Generally, arteries carry oxygenated blood and veins carry deoxygenated blood, but the pulmonary arteries and veins are the reverse of this. What other human circulatory circuit has such a reversal? [Answer: the umbilical cord leading to the placenta, before birth.] Answer: Yes, that's correct! In addition to the pulmonary circulation, where the pulmonary arteries carry deoxygenated blood from the heart to the lungs and the pulmonary veins return oxygenated blood from the lungs to the heart, another human circulatory circuit with a reversal of oxygenation status is the fetal circulation through the umbilical cord leading to the placenta before birth. In the fetal circulation, the umbilical arteries carry deoxygenated blood from the fetus to the placenta, where it is oxygenated through the exchange of gases with the maternal blood. The oxygenated blood is then returned to the fetus through the umbilical vein, providing oxygen and nutrients to support fetal growth and development. This reversal of oxygenation status in the umbilical arteries and veins is a unique adaptation of fetal circulation to meet the metabolic needs of the developing fetus before birth. After birth, when the umbilical cord is clamped and severed, the fetal circulation transitions to the postnatal circulation, and the umbilical arteries and vein close and become fibrous remnants known as the medial umbilical ligaments. 5. Why would a heavier person be more likely to have higher blood pressure? Answer: Several factors can contribute to higher blood pressure in heavier individuals, including: 1. Increased Cardiac Output: Heavier individuals often have a larger body mass, which requires the heart to pump more blood to supply oxygen and nutrients to tissues. This increased cardiac output can lead to higher blood pressure. 2. Increased Peripheral Resistance: Adipose (fat) tissue releases substances known as adipokines, which can contribute to inflammation and insulin resistance. These conditions can lead to increased peripheral resistance in blood vessels, making it harder for blood to flow through them and resulting in higher blood pressure. 3. Increased Blood Volume: Adipose tissue is associated with higher levels of certain hormones, such as leptin, which can lead to increased blood volume by promoting sodium retention and stimulating the release of aldosterone. Higher blood volume can elevate blood pressure. 4. Obstructive Sleep Apnea (OSA): Heavier individuals are more prone to obstructive sleep apnea, a condition characterized by pauses in breathing during sleep due to airway obstruction. OSA is associated with hypertension, as episodes of low oxygen levels during sleep can lead to increased sympathetic nervous system activity and elevated blood pressure. 5. Insulin Resistance and Metabolic Syndrome: Heavier individuals are at a higher risk of developing insulin resistance and metabolic syndrome, which are associated with hypertension. Insulin resistance can lead to increased sympathetic nervous system activity and sodium retention, both of which contribute to elevated blood pressure. 6. Physical Inactivity: Heavier individuals may be less physically active, which can contribute to weight gain and exacerbate other risk factors for hypertension, such as insulin resistance and high cholesterol levels. 7. Genetic Predisposition: There may be a genetic predisposition to both obesity and hypertension, with certain genetic factors influencing both conditions independently. In summary, the relationship between weight and blood pressure is multifactorial, involving complex interactions between physiological, metabolic, and lifestyle factors. While weight alone may not always directly cause hypertension, it can contribute to other risk factors that increase the likelihood of developing high blood pressure. 6. Why are the substantial plasma proteins found in blood not used by cells as a source of metabolic energy? Answer: Several factors contribute to the likelihood of higher blood pressure in heavier individuals: 1. Increased Blood Volume: Heavier individuals typically have a larger body mass, which often correlates with higher blood volume. More blood circulating in the body means that there is greater pressure exerted on the walls of the blood vessels, leading to elevated blood pressure. 2. Increased Cardiac Output: To meet the metabolic demands of a larger body, the heart may need to pump more blood per minute, resulting in increased cardiac output. Higher cardiac output can elevate blood pressure, especially if the blood vessels have reduced elasticity or if there is increased resistance to blood flow. 3. Greater Peripheral Resistance: Adipose tissue, or fat, secretes various hormones and inflammatory substances that can affect blood vessel function and increase peripheral resistance, the resistance to blood flow in the smaller arteries and arterioles. Increased peripheral resistance can lead to higher blood pressure. 4. Insulin Resistance and Metabolic Syndrome: Excess body weight, particularly abdominal obesity, is associated with insulin resistance, a condition in which cells become less responsive to the effects of insulin. Insulin resistance can lead to metabolic syndrome, a cluster of conditions including high blood pressure, high blood sugar, abnormal cholesterol levels, and abdominal obesity, all of which increase the risk of cardiovascular disease. 5. Obstructive Sleep Apnea: Heavier individuals are more prone to obstructive sleep apnea, a condition characterized by pauses in breathing during sleep due to airway obstruction. Sleep apnea is associated with hypertension (high blood pressure), likely due to repeated episodes of oxygen deprivation and arousal from sleep, which can lead to increased sympathetic nervous system activity and elevated blood pressure. 6. Inactivity and Poor Diet: Sedentary lifestyle and poor dietary habits, which are more common in heavier individuals, can contribute to higher blood pressure. Lack of physical activity and consumption of high-calorie, high-sodium diets can lead to weight gain, sodium retention, and other metabolic abnormalities that elevate blood pressure. Overall, the relationship between body weight and blood pressure is complex and multifactorial, involving a combination of physiological, metabolic, and lifestyle factors. Maintaining a healthy weight through regular physical activity, balanced diet, and lifestyle modifications is important for preventing and managing high blood pressure. 7. Some terrestrial mammals ignore declining oxygen levels, and cue on to the reciprocal increase in carbon dioxide to regulate breathing. However, some diving marine mammals pace breathing on oxygen sensors. Why would this have evolved? Answer: The evolutionary adaptations of terrestrial and diving marine mammals in regulating breathing are influenced by their specific physiological and environmental challenges: 1. Terrestrial Mammals: Terrestrial mammals typically rely on detecting changes in carbon dioxide (CO2) levels rather than oxygen (O2) levels to regulate breathing. The primary stimulus for breathing in terrestrial mammals is the buildup of CO2 in the bloodstream, which leads to an increase in carbonic acid and a decrease in blood pH (acidosis). This increase in acidity triggers the respiratory center in the brainstem to stimulate breathing, facilitating the removal of CO2 from the body and restoring normal blood pH levels. The reliance on CO2 rather than O2 as the primary respiratory stimulus in terrestrial mammals is likely due to the relatively stable O2 levels in terrestrial environments compared to aquatic environments. Terrestrial mammals have constant access to atmospheric O2, which is replenished through breathing, while CO2 levels can fluctuate more readily due to factors such as metabolic activity and environmental conditions. 2. Diving Marine Mammals: Diving marine mammals, such as whales, dolphins, and seals, face unique challenges related to breath-holding and oxygen management during prolonged dives underwater. Unlike terrestrial mammals, diving marine mammals cannot breathe atmospheric air while submerged and must rely on stored O2 reserves and efficient oxygen utilization strategies to prolong their dives. Diving marine mammals have evolved specialized physiological adaptations to optimize oxygen uptake, storage, and utilization during diving. These adaptations include: - Increased O2 storage capacity: Diving marine mammals have larger blood volume, higher hemoglobin concentrations, and greater muscle myoglobin content compared to terrestrial mammals, allowing them to store more O2 in their blood and tissues. - Enhanced diving reflex: Diving marine mammals exhibit a strong diving reflex, characterized by bradycardia (slowing of the heart rate), peripheral vasoconstriction (reduction of blood flow to non-essential tissues), and preferential shunting of blood flow to vital organs such as the heart, brain, and lungs. This diving reflex helps conserve O2 and prolongs the duration of dives. - Sensitivity to O2 levels: Some diving marine mammals have specialized oxygen sensors, called arterial O2 chemoreceptors, located in their carotid arteries and aortic bodies. These sensors detect changes in arterial O2 levels and help regulate breathing and cardiovascular responses during dives. By monitoring O2 levels, diving marine mammals can adjust their diving behavior and respiratory patterns to optimize O2 uptake and utilization. The evolution of these specialized adaptations in diving marine mammals reflects the importance of efficient O2 management in their aquatic lifestyle and the challenges they face in maintaining O2 supply during prolonged dives. By pacing breathing on O2 sensors, diving marine mammals can optimize their diving behavior and maximize their diving efficiency, allowing them to thrive in the marine environment. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214