Mastering Concepts
32.1
1. Differentiate between ectotherms and endotherms.
Ectotherms use the environment, not their own internal mechanisms, to regulate their
body temperature. Endotherms regulate their body temperature through internal
mechanisms.
2. Which animals are ectotherms, and which are endotherms?
Ectotherms include invertebrates, fishes, amphibians, and nonavian reptiles. Endotherms
include birds and mammals.
3. What are the advantages and disadvantages of ectothermy and endothermy?
Ectothermy requires less food and oxygen than does endothermy. One disadvantage of
ectothermy, however, is that an animal must be able to successfully complete the
behaviors needed to raise or lower its body temperature. Ectotherms also become
sluggish at low temperatures. Endotherms are typically more active animals, but they
require much more food and oxygen than do ectotherms.
4. What are some physiological and behavioral adaptations to extreme cold and extreme
heat?
Some physiological adaptations to extreme cold include countercurrent exchange
systems, shunting of blood from extremities, and shivering. Behavioral adaptations to
extreme cold include fluffing fur or feathers, huddling, and hibernating. Some
physiological adaptations to extreme heat include evaporative cooling, dilating surface
blood vessels near the skin, and rerouting skin-cooled blood to towards the brain.
Behavioral adaptations to extreme heat include hiding in shade, digging burrows,
swimming, shedding clothes, and consuming cold drinks.
32.2
1. What is osmoregulation, and why is it important?
Osmoregulation is the maintenance of constant concentrations of ions in body fluids,
even in changing environmental conditions. This is important since a cell will die if
fluids are too concentrated or too dilute.
2. Describe osmoregulation in marine and freshwater fishes.
Marine fish must conserve water and get rid of excess salts. They drink seawater, have
low urine volume, and use active transport to remove salt at the gills. Freshwater fish
must conserve salts and get rid of excess water. They take in a lot of water through their
skin and gills by osmosis, and excrete the excess in their urine. They also use active
transport to take ions in through the gills.
3. How do land animals gain and lose water?
Land animals take in water through food, drink and metabolism. They lose it through
their lungs, skin, feces, and urine.
32.3
1. What are three nitrogenous wastes that animals produce?
Three examples of nitrogenous wastes are ammonia, uric acid, and urea.
2. Describe the function of Malpighian tubules in insects.
Malpighian tubules carry uric acid, water, and excess ions into the intestines to be
eliminated.
32.4
1. List the organs that make up the human urinary system.
The organs of the human urinary system are the kidneys, ureters, urinary bladder, and
urethra.
2. What are the functions of the kidneys?
The kidneys remove urea from the bloodstream and maintain the proper concentrations of
water, salt, and ions in the bloodstream. The kidneys also regulate blood pH.
32.5
1. Trace the pathway of blood as it moves through a kidney.
As blood moves through a kidney, it follows this path: renal artery -- incoming arteriole –
glomerulus –outgoing arteriole – peritubular capillary –venule – renal vein – back to the
heart.
2. What are the parts of a nephron?
The parts of the nephron are the glomerular capsule, the proximal convoluted tubule, the
nephron loop, and the distal convoluted tubule
3. What three processes occur in urine formation?
The three processes are filtration, reabsorption, and secretion.
4. Describe the events that occur at the glomerular capsule, proximal convoluted tubule,
nephron loop, and distal convoluted tubule.
In the glomerular capsule water and dissolved materials enter the nephron under pressure,
forming the filtrate In the proximal convoluted tubule, nutrients and other useful
materials are reabsorbed into the capillaries by osmosis and active transport. Some
secretion also occurs in this region. In the descending portion of the nephron loop, water
is absorbed back into the blood; in the ascending portion, salt is reabsorbed into the
blood. In the distal convoluted tubule, additional water and salt are reabsorbed, and the
final concentrations of hydrogen and potassium ions are adjusted..
5. What is the function of the collecting duct?
The collecting duct receives the fluid from many nephrons and continues to concentrate
the urine by reabsorbing more water into the blood.
6. Describe the roles of antidiuretic hormone and aldosterone in regulating kidney
function.
Antidiuretic hormone stimulates the formation of water channels in the walls of the distal
convoluted tubule and collecting duct, reducing urine volume by causing more water to
be reabsorbed into blood. Aldosterone also conserves water and boosts blood pressure by
stimulating the formation of sodium channels in the distal convoluted tubule, causing
sodium to reabsorbed into the blood. Water follows by osmosis.
32.6
1. How did Ruben and his colleagues use turbinates and nasal cavities to draw their
conclusions?
Turbinates are ridges of bone or cartilage inside the nose. They are characteristic of
endotherms such as birds and mammals, and their presence of turbinates corresponds to a
larger cross-sectional area of the nose. Measuring the nasal cavity area of fossil bird
ancestors revealed that, though they possessed feathers, they were ectothermic.
2. Suppose researchers discover a fossil of a 100-kilogram dinosaur with a nasal cavity
greater than 10 square centimeters. According to figure 32.16, how would that finding
affect Ruben’s conclusions?
Such a fossil would suggest the presence of endothermy in some dinosaurs and would
lead to a different conclusion, perhaps that the evolution of feathers and endothermy went
hand-in-hand.
Write It Out
1. How do humans and iguanas differ in body temperature regulation?
Iguanas are ectotherms; they lack an internal temperature-regulating mechanism so they
must move to areas where they can gain or lose heat. Humans are endotherms; we
regulate body temperature internally to maintain a relatively constant body temperature.
2. Birds and insects frequently collect nectar from plants. Birds are endothermic, and
insects are ectothermic. For the same investment of nectar, do you think a plant can
support a greater mass of insects or of birds?
For the same investment of nectar, a plant could support a greater mass of insects than
birds because ectotherms use much less energy and therefore require less food than do
endotherms.
3. On a per-kilogram basis, why does a small mammal such as a shrew require so much
more energy than does an elephant or other large mammal?
A small mammal has more surface area over which to lose heat and so needs a higher
metabolism to generate the heat needed to maintain body temperature.
4. Cite three adaptations that help keep an animal’s feet warm.
(1) One adaptation is the hypothalamus, which regulates many of the physiological or
behavioral responses that maintain homeostasis. (2) In countercurrent exchange, two
adjacent currents flow in opposite directions and exchange heat with each other, which
conserves heat near the body’s core rather than allowing it to escape through the
extremities. (3) Shivering helps keep extremities warm as contraction of skeletal muscle
generates heat.
5. Whales and seals are mammals that swim in ice-cold water. Explain how they might
use countercurrent exchange to keep the blood in their flippers from freezing.
Arteries carrying warm blood from the body’s core run alongside veins with colder blood
from the extremities. Heat from the arteries transfers to the blood in the veins. In
extremely cold water, the animal may also constrict its blood vessels to divert blood away
from the extremities.
6. Woolly mammoths are extinct relatives of modern-day elephants. The mammoths were
heavier and shaggier than today’s elephants, their ears were smaller, and they had a thick
fat layer under their skin. Explain each of these differences in light of the fact that today’s
elephants originate in Asia and Africa, whereas mammoths lived on the tundra.
On the tundra the temperatures are much colder than they are in Asia and Africa. The
mammoths’ extra body mass reduced the surface area relative to the animal’s volume,
reducing heat loss to the environment. Fat and the shaggy coat added insulation. The
smaller ears presented less surface area for heat loss.
7. Imagine you are adrift at sea. If you drink seawater, you will dehydrate much faster
than if you have access to fresh water. Explain.
Drinking salty water will cause your cells to lose water by osmosis, and you will rapidly
become dehydrated. Drinking fresh water will have the opposite effect; your cells will
absorb water by osmosis.
8. What are the three types of nitrogenous wastes, and under which conditions is each
most likely to be used?
Ammonia is used in watery environments where the ammonia can be heavily diluted.
Urea is used by animals that live in water or on land and do not need to heavily conserve
water. Uric acid, the most concentrated nitrogenous waste, is used by animals that need
to heavily conserve water.
9. How are Malpighian tubules similar to and different from the human kidney?
Like the kidney, the Malpighian tubules remove nitrogenous waste, excess ions, and
water from the blood. Unlike the kidneys, the tubules empty this material into the
intestine for removal rather than connecting to its own removal system (the urethra). Also
unlike the kidneys, the tubules remove uric acid rather than urea.
10. List the organs that make up the human urinary system. What is the function of each?
Kidney—form urine, regulate blood volume, conserve ions and nutrients, regulate blood
pH
Ureter—carries urine from the kidneys to the bladder
Urinary bladder—store urine
Urethra—transport urine from the bladder to the outside of the body
11. Kidney stones are calcium-rich crystals that form inside the kidney. What symptoms
would you expect if the stones lodge in a ureter?
If the stones lodge in a ureter, symptoms might include blocked urine flow, pain, and
bleeding from the walls of the urinary tract.
12. Urinary tract infections frequently accompany sexually transmitted diseases,
especially in women. Why?
The entrance to the urinary tract is very close to the entrance to the vagina. Microbes that
enter the vagina easily gain entrance to the urinary tract as well.
13. Draw a nephron and label the parts.
[See diagram in text].
14. Shortly after you drink a large glass of water, you will feel the urge to urinate.
Explain this observation. Begin by tracing the path of the water, starting at the
glomerulus and ending with the arrival of urine in the bladder.
You will feel the urge to urinate shortly after you drink a large glass of water because the
water causes the blood to be too dilute; the kidneys subsequently remove the excess water
from the blood. The excess water first moves into the glomerular capsule and passes
through the nephron, where some is reabsorbed into the blood. From the nephron the
fluid moves to the collecting duct, a ureter, and then into the urinary bladder. Throughout
this process, ADH secretion has stopped because the blood is fully hydrated, which
causes the walls of the distal convoluted tubule and collecting duct to be less permeable
to water. This allows for more water to be retained in the urine.
15. How does the kidney reduce the volume of urine to a small fraction of the volume of
filtrate that enters the glomerular capsule?
Capillaries surrounding the nephron reabsorb most of the water that enters the glomerular
capsule. Ion gradients cause water to move from the nephron into the blood by osmosis.
Because of this process, once the filtrate has passed through the distal convoluted tubule,
97% of the water in the original glomerular filtrate has been reabsorbed.
16. Why is protein in the urine a sign of kidney damage? What structures in the kidney
are probably affected?
Ordinarily, plasma proteins are too large to fit through the pores of the glomerular
capsule. Their presence in urine suggests therefore suggests damage to the glomerular
capsule.
17. How could very low blood pressure impair kidney function?
Blood pressure drives substances from the bloodstream into the nephron. If blood
pressure is very low, fluid will not enter the nephrons to be cleansed, and wastes could
build up in the bloodstream to toxic levels.
18. The fluids deep in the medulla of the kidney have a much higher solute concentration
than those in the cortex. Where do the solutes in the medulla come from? What is the role
of that high solute concentration in the functioning of the nephron loop?
The solutes in the medulla come from the ascending portion of the nephron loop where
they are actively transported out of the loop. This high solute concentration draws out
water by osmosis in the descending loop and therefore helps conserve water.
19. An amphibian’s kidneys lack nephron loops. Why can’t a human also live without
nephron loops?
Amphibians live in a watery environment; water conservation is not a selective force on
these animals. In humans and many other animals, nephron loops allow for water
reabsorption and conservation. Without nephron loops, most water would not be
reabsorbed into the bloodstream, and the person would dehydrate.
20. Many pharmaceutical drugs leave the body in urine. As we age, the number of
nephrons in the kidneys declines. Do you predict that an older person would need a
higher or lower dose of a drug to compensate for the amount lost in urine? Explain your
answer.
An person with fewer nephrons would be less efficient at cleansing the blood of the drug.
He or she would therefore need a lower dose of the drugs.
21. How is the descending limb of the nephron loop different from the ascending limb?
Considering their roles in transport of sodium ions, which cells do you think have the
most mitochondria: the cells lining the descending limb or the cells lining the ascending
limb? Explain.
The cells of the descending limb are permeable to water by osmosis. Since osmosis is a
passive transport process, few mitochondria would be needed. The cells of the ascending
limb transport salts out of the nephron by active transport. This requires the use of ATP,
and so many more mitochondria would be needed.
22. Which of the substances in the following table are excreted, and which are reabsorbed
into the bloodstream?
Urea, uric acid, and creatinine are all excreted. Glucose is reabsorbed into the
bloodstream.
23. Review the action of steroid and peptide hormones in Chapter 27. Which hormone
should act faster, ADH or aldosterone? Why?
ADH is a peptide hormone, so it is water soluble. Aldosterone is a steroid hormone. In
general, peptide hormones act more rapidly than do steroid hormones, so ADH should
have faster effects.
24. In a disease called diabetes insipidus, ADH activity is insufficient. Would a person
with this disease produce more or less urine than normal? Explain.
A person with insufficient ADH would produce more urine than normal, because ADH
secretion conserves water.
25. Use the Internet to find a list of diseases of the kidneys or other organs of the urinary
system. Select one to research further. What causes the illness you chose, and what are
the symptoms? How does the disease interfere with the function of the urinary system? Is
there a treatment or cure? Who is most at risk of the disease?
[Answers will vary]
Pull it Together
1. Add the terms glomerulus, proximal convoluted tubule, nephron loop, and distal
convoluted tubule to this concept map.
All of these terms represent parts of the nephron. Fluid moves from the glomerulus to the
proximal convoluted tubule to the nephron loop to the distal convoluted tubule, and from
there to the collecting duct.
2. Where do filtration, secretion, and reabsorption occur in the nephron?
Filtration occurs at the glomerulus. Secretion occurs at the proximal and distal
convoluted tubules. Reabsorption occurs at the nephron loop and at the proximal and
distal convoluted tubules.
3. What is the relationship between the kidneys, ureters, bladder, and urethra?
Kidneys release urine into the ureters, which squirt the urine into the muscular bladder.
Urine then leaves the body via the urethra.
4. How do aldosterone and antidiuretic hormone influence kidney function?
Both hormones act on the kidney to conserve water. Aldosterone causes the distal
convoluted tubule to produce additional sodium channels; water follows the sodium by
osmosis and enters the bloodstream. ADH stimulates the kidneys to produce more water
channels at the distal convoluted tubule and collecting duct, so more water enters the
bloodstream.