Chapter 41 Summary
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Core Concepts Summary
41.1 All animals regulate the water and electrolyte levels within their cells.
Cell membranes act as selectively permeable membranes, allowing the passage of water but restricting or controlling the movement of many solutes. page 876
Water moves across selectively permeable membranes by osmosis from a region of lower solute concentration to a region of higher solute concentration. page 876
Osmotic pressure is a measure of the tendency for water to move by osmosis across a selectively permeable membrane into a solution with higher solute concentration. page 876
Osmoregulation is the regulation of osmotic pressure inside cells or organisms. page 877
Osmoconformers maintain an internal solute concentration similar to that of the environment, whereas osmoregulators have an internal solute concentration different from that of the environment. page 878
Both osmoconformers and osmoregulators regulate the levels of particular solutes, especially sodium, potassium, and chloride. page 878
Osmoregulators that live in high-
Osmoregulators that live in low-
Osmoregulators that live on land minimize water loss. page 879
41.2 Excretory organs eliminate nitrogenous wastes and regulate water and electrolyte levels.
Osmoregulation and excretion are closely coordinated processes controlled by the excretory organs of animals. page 881
Ammonia, urea, and uric acid are three major forms of nitrogenous waste that animals excrete, depending on their evolutionary history and habitat. page 881
Ammonia is toxic but readily diffuses into water; urea is less toxic and can be stored before it is eliminated; uric acid is a semisolid and can be eliminated with minimal water loss as an adaptation for living on land. page 881
All animal excretory systems first isolate or filter fluid into an extracellular space. Key electrolytes, solutes, and water are then reabsorbed from the filtrate. Additional wastes are then secreted into the remaining filtrate before it is eliminated from the body. page 882
Examples of excretory organs are the protonephridia of flatworms, the metanephridia of segmented annelid worms, the Malpighian tubules of insects, and the kidneys of vertebrates. page 884
The urine produced by the kidneys and other excretory organs is stored in a bladder until it is eliminated from the body. page 885
The functional unit of the vertebrate kidney is the nephron, which consists of a glomerulus, capsule, renal tubules, and collecting duct. page 885
41.3 The mammalian kidney can produce urine that is more concentrated than blood as an adaptation for living on land.
The kidney has an outer cortex and an inner medulla. page 886
The renal tubules have specialized regions, including the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. page 888
The loop of Henle of some nephrons extends all the way into the medulla before looping back to the cortex. These nephrons create a concentration gradient from the outer cortex to the deeper medulla of the kidney. page 888
The ability to create a concentration gradient from the cortex to the medulla results from a countercurrent multiplier mechanism of the loops of Henle. page 890
The final concentration of urine is regulated by adjusting water permeability within the walls of the collecting ducts, which pass from the cortex to the medulla. page 891
Release of antidiuretic hormone by the posterior pituitary gland increases the water permeability of the collecting ducts, concentrating the urine that empties from vertebrate kidneys. page 891
The kidneys help to regulate blood volume and pressure by secreting the hormone renin, leading to the production of angiotensin II, which constricts blood vessels, and aldosterone, which increases reabsorption of salt and water by the kidneys. page 892
Self-Assessment
Given a selectively permeable membrane that is permeable to water but not to a particular solute and given that there are different solute concentrations on the two sides of the membrane, show the direction of water movement and label the side with the higher osmotic pressure.
Self-Assessment 1 Answer
Describe how animals gain and lose water and electrolytes.
Self-Assessment 2 Answer
Animals gain water and electrolytes through drinking and the foods they eat. Freshwater animals also gain water through their gills. Additionally, water is generated by cellular respiration. Water and electrolytes are lost through urine, feces, evaporation from the lungs, and sweating. Most marine animals lose water through their gills as well. Specialized glands in some animals can also aid in removal of excess electrolytes.
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Name two animals that are osmoconformers and two that are osmoregulators. Explain the difference between the two types of animal.
Self-Assessment 3 Answer
Osmoconformers are animals that regulate their internal osmotic pressure by maintaining their internal solute concentration at a level similar to that of their environment. Osmoconformers include animals such as sharks and marine invertebrates like sea stars. Some marine vertebrates are also osmoconformers, including hagfish, lampreys, rays, and coelacanths. All freshwater animals, including fishes and amphibians, and all terrestrial animals, including humans, are osmoregulators because they actively regulate their internal osmotic pressure, expending considerable energy to maintain an internal solute concentration that is different from that of their environment. The largest group of marine vertebrates, the teleosts, are also osmoregulators.
List three forms of nitrogenous waste and describe how each is an adaptation for the environment in which the animal lives.
Self-Assessment 4 Answer
The three forms of nitrogenous waste are ammonia, urea, and uric acid. Ammonia is the nitrogenous waste produced by most aquatic animals, including many fishes. Although ammonia is highly toxic, fish can excrete waste in this form because it can be easily diluted in their aquatic environment; they don’t have to expend additional energy converting this waste into a less toxic form. Since terrestrial animals do not live in water, they must convert ammonia to less toxic forms of nitrogenous waste. Mammals, many amphibians, sharks, and some bony fish excrete nitrogen in the form of urea, which can be stored in a concentrated form and excreted with water in the urine. Urea is less toxic than ammonia, but requires energy to produce it and water to eliminate it. For those animals that must conserve water, like those that live in hot, dry climates, the ammonia that is generated by the breakdown of proteins is converted into uric acid, which is much less toxic and does not dissolve in water, so waste can be excreted with limited water loss. Birds, insects, reptiles, and land snails excrete waste in the form of uric acid.
Describe the three steps in which organisms excrete wastes.
Self-Assessment 5 Answer
The three steps of waste excretion are filtration, reabsorption, and secretion. Filtration involves the filtering of solutes, waste, and water from the blood into an extracellular space, creating a filtrate. Reabsorption returns water and necessary solutes from the filtrate to the blood. Secretion involves active transport of excess solutes and toxins from the blood into the filtrate.
Draw a mammalian nephron, label and describe the primary function of each part, and show the direction of water and electrolytes in each part.
Self-Assessment 6 Answer
See also Fig. 41.18.
Explain how the loop of Henle creates a concentration gradient from the cortex to the medulla.
Self-Assessment 7 Answer
The loop of Henle is long enough to descend into the medulla before looping back into the cortex. It helps to establish a concentration gradient based on the permeability of different parts of the loop. The descending loop of Henle is only permeable to water, so water moves out of the filtrate by osmosis into the interstitial space, which has a higher concentration of solute than the filtrate as it moves from the cortex to the medulla. The movement of water out of the loop of Henle in the descending limb results in concentration of the filtrate to match the concentration of solute in the interstitial space. As the filtrate then passes through the ascending loop of Henle, which is impermeable to water, electrolytes are pumped into the interstitial space.
Describe the role of ADH in the regulation of urine concentration.
Self-Assessment 8 Answer
ADH affects urine concentration by altering the permeability of the collecting duct to water. In the presence of ADH, aquaporins are inserted into the membrane of the cells surrounding the collecting duct, increasing reabsorption of water and concentrating the urine. In the absence of ADH, the collecting duct is impermeable to water, so water remains in the filtrate and the urine is dilute.
Explain how the kidneys help to regulate blood volume and blood pressure.
Self-Assessment 9 Answer
If blood pressure drops, cells within the juxtaglomerular apparatus release renin into the blood. The release of renin, in turn, leads to activation of angiotensin II, which causes blood vessels to constrict, increasing blood pressure. In addition, angiotensin II causes release of aldosterone, which triggers increased reabsorption of water and electrolytes in the distal convoluted tubule and collecting duct, leading to an increase in blood volume and blood pressure.