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32 MAINTAINING THE INTERNAL ENVIRONMENT
CHAPTER OUTLINE
LEARNING OBJECTIVES
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List the various ways vertebrates employ to flush nitrogenous wastes from their bodies.
Describe how invertebrates excrete wastes.
Explain the changes that occurred in the evolution of the vertebrate kidney.
Describe the basic features of the kidney and its nephrons.
List the five steps involved in the formation of urine.
Homeostasis (p. 608)
32.1
How the Animal Body Maintains Homeostasis (p. 608; Figs. 32.1, 32.2)
A. Homeostasis is the dynamic constancy of the internal environment; body conditions fluctuate
constantly within narrow limits.
B. Negative Feedback Loops
1. A feedback loop has sensors, and integrating center, and effectors.
2. In negative feedback loops correct deviations in from a set point and promote stability.
C. Regulating Body Temperature
1. The hypothalamus coordinates temperature and homeostasis.
2. Homeothermic vertebrates have physiologic means to adjust body temperature.
3. Ectothermic vertebrates must move to a more desirable temperature within their habitats.
D. Regulating Blood Glucose
1. Excess glucose is stored in the liver as glycogen under the influence of the hormone
insulin from the pancreas.
2. When glucose levels are low in the blood, the pancreas releases the hormone glucagon,
which stimulates the liver to convert glycogen back to glucose.
Osmoregulation (p. 610)
32.2
Regulating the Body’s Water Content (p. 610; Figs. 32.3, 32.4, 32.5, 32.6)
A. Animals use various mechanisms for osmoregulation, the regulation of the body’s osmotic
composition, or how much water and salt it contains.
B. Flatworms use protonephridia that branch throughout the body into bulb-like flame cells.
C. Earthworms employ nephridia to obtain fluid from the body cavity through filtration into
nephrostomes.
1. As fluid passes through the tubules of the nephridia, salts are reabsorbed, and urine is
more dilute than body fluids.
D. The excretory organs of insects are called Malpighian tubules, extensions of the digestive
tract.
1. Insects create an excretory fluid by secreting potassium ions into tubules, which draws
water osmotically.
E. Kidneys are the excretory organs of vertebrates, and unlike the Malpighian tubules of insects,
kidneys create a tubular fluid by filtration of the blood under pressure.
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Osmoregulation in Vertebrates (p. 612)
32.3
32.4
Evolution of the Vertebrate Kidney (p. 612; Figs. 32.7, 32.8, 32.9, 32.10, 32.11, 32.12)
A. The kidney is a complex organ made up of thousands of nephrons.
B. Fluids and wastes from the blood are filtered through a glomerulus.
C. Useful nutrients and water are reabsorbed into the bloodstream.
D. Only birds and mammals can reabsorb water from their glomerular filtrate to produce a urine
that is hypertonic to blood.
E. Freshwater Fish
1. Kidneys are thought to have evolved first in the freshwater bony fish.
2. Body fluids of fish are hypertonic to surrounding fresh water, so water tends to enter the
body, and solutes tend to leave.
3. Fish counter this problem by not drinking water, by excreting very dilute urine, and by
reabsorbing ions from the filtrate back into the blood.
F. Marine Bony Fish
1. It is likely that marine bony fish evolved from their freshwater ancestors.
2. To compensate for water loss to the environment from their hypotonic bodies, marine
fish drink lots of seawater, actively excrete excess ions, and the urine they excrete is
hypotonic to their body fluids.
G. Cartilaginous Fish
1. Rather than drinking copious amounts of seawater like the marine bony fish,
cartilaginous fish concentrate blood urea, making their blood isotonic to surrounding
seawater.
H. Amphibians and Reptiles
1. Amphibians, when not in water, spend most of their time in wet places on land.
2. Amphibians produce a very dilute urine and compensate for lost sodium ions by actively
pumping in sodium through their skin from the surrounding water.
3. Reptiles live in a variety of habitats, wet or dry.
4. Those that live in aquatic habitats have excretory systems similar to fish and amphibians.
5. The kidneys of terrestrial reptiles reabsorb much of the salt and water in the nephron
tubules, helping to conserve water and thus blood volume.
I. Mammals and Birds
1. Modifications in the mammals and birds, especially in the loop of Henle, allow these
groups to reabsorb water and produce a hypertonic urine.
2. A long loop of Henle dips deep into the renal medulla and can reabsorb more water.
The Mammalian Kidney (p. 616; Fig. 32.13)
A. Within the human kidney, the ureter opens into an expanded area, called the renal pelvis.
B. Renal tissue is divided into a renal cortex and a renal medulla.
C. The basic functional unit of the kidney is the nephron, which has three major segments.
1. The first, Bowman's capsule, is a catcher's mitt-shaped portion that wraps around a wad
of blood capillaries called a glomerulus.
2. The fluid portion of the blood is forced out of the glomerulus into Bowman's capsule. The
second portion of the nephron is a hairpin-shaped tube called the loop of Henle.
3. This loop is where reabsorption of nutrients from the plasma occurs.
4. The loop of Henle is lined with active transport channels that capture and save
electrolytes, amino acids, and glucose; without selective reabsorption, these nutrients
would be lost into the urine.
5. The third segment of the nephron is called the collecting duct.
6. Reabsorption of water occurs in the collecting duct when the body needs to conserve
water.
7. Blood vessels surround the loop of Henle and the collecting duct, and these gather up
reabsorbed nutrients and water, carrying them back to the blood.
D. The Kidney at Work
1. Five steps are involved in the formation of urine.
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2.
32.5
Driven by blood pressure, glomerular filtrate is produced as fluids are forced from the
blood in the glomerulus and are collected by Bowman's capsule.
3. The filtrate travels through the loop of Henle where reabsorption of water occurs first,
followed by the selective reabsorption of nutrients.
4. At the end of the loop of Henle, tubular excretion of substances occurs in order to add
extra waste materials to the urine.
5. Finally, further reabsorption of water occurs in the collecting duct if the body is in need
of water.
6. The end product of these processes is urine, which is eventually expelled from the body.
Eliminating Nitrogenous Wastes (p. 619; Fig. 32.14)
A. Eliminating Nitrogenous Wastes
1. When animals consume food containing amino acids and nucleic acids, they produce
nitrogenous wastes that must be eliminated from the body.
2. Vertebrates first convert these wastes to ammonia, which is so toxic to cells it must be
excreted in a very dilute urine.
3. In many animals, the ammonia is first converted to its less toxic form, urea, and excreted.
4. Still other animals excrete uric acid, which is an adaptation for water conservation.
KEY TERMS
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homeostasis (p. 608)
glycogen (p. 609) Glycogen is the starch-like compound the liver uses to store energy.
insulin (p. 609) A hormone produced by the pancreas to lower blood glucose levels
osmoregulation (p. 610) The regulation of the body’s salt content.
kidney (p. 611) An excretory organ found in vertebrates.
nephron (p. 612) Repeating disposal units of the vertebrate kidney.
glomerulus (p. 616) A fine network of capillaries that act as the filtration device in the mammalian
kidney.
loop of Henle (p. 616) Bent portion of the renal tubule that concentrates urine and conserves water.
collecting duct (p. 616) Also acts as a water-conservation device when water is limited.
urea (p. 619) A less-toxic form of nitrogenous waste excreted by the body.
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
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Kidney Structure. Demonstrate kidney structure to your students. Obtain a beef or pork kidney from
your local meat locker. Pork kidneys are more similar to those of humans. Beef kidneys look like
human kidneys during fetal development before the lobules fuse to form a smooth kidney. Section the
kidney longitudinally, and show students the medulla and cortex regions. Discuss where the nephrons
are found and how urine formation proceeds. Show them the renal sinus and where the ureter leaves the
kidney.
Kidney Dialysis. Kidney dialysis, or more accurately, hemodialysis, involves removing waste materials
from the blood by artificial means when kidneys are no longer fully functional. Explain this procedure
to students. Dialysis refers to the separation of larger particles from smaller ones through a membrane
that is selectively permeable. The artificial kidney machine is usually the device that accomplishes
dialysis of human blood. A tube is used to connect the kidney machine to the patient's radial artery.
Blood leaves the artery and enters the machine where it flows to one side of a selectively permeable
membrane made out of cellulose acetate. The other side of the membrane is bathed with an artificial
fluid called dialysate. The dialysate has the same electrolyte concentration of normal plasma. Any
excess electrolytes in the patient's blood move through the selectively permeable membrane, much the
way normal diffusion occurs. Waste materials from the blood, such as urea, also diffuse into the
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dialysate. Blood proteins and blood cells are too large to filter across the membrane so they remain in
the blood. Since only about 500 ml of blood is cleansed at one time, a single dialysis session can last
from 4 to 6 hours. Dialysis is normally performed three times weekly. Disadvantages to this type of
dialysis are that it is time-consuming, and anticoagulants must be added to the patient's blood to keep it
flowing. Another form of hemodialysis has been recently developed. Continuous ambulatory peritoneal
dialysis employs the peritoneum as the selectively permeable membrane. A catheter connects the
individual's peritoneal cavity with a bag of dialysis solution. The solution enters the peritoneal cavity
by gravity, and exchange occurs between the blood and dialysate. When finished, the fluid is removed
from the peritoneal cavity and discarded. The advantage of this method is that it allows the person to
complete the dialysis process at home during sleeping hours. A drawback is the danger of infection.
CRITICAL THINKING QUESTION
1.
Freshwater and marine bony fishes have evolved means to compensate for hypertonic and hypotonic
body fluids, respectively. Explain why humans cannot drink seawater and survive for long.
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