Mastering Concepts
33.1
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. 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?
Mammals are endotherms that maintain a constant internal body temperature. A small
mammal has more surface area over which to lose heat; its higher metabolic rate
generates the heat needed to maintain its body temperature.
3. What are some physiological and behavioral adaptations to extreme cold and extreme
heat?
Some physiological adaptations to extreme cold include countercurrent exchange
systems, shunting blood away 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 seeking shade, digging burrows,
swimming, shedding clothes, and consuming cold drinks.
33.2
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 eliminate 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 eliminate 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 absorb ions 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.
33.3
1. Why do land animals convert ammonia to urea or uric acid?
Ammonia is very toxic. Animals that cannot spare enough water to dilute ammonia to
nontoxic concentrations must convert it to less toxic forms. Mammals convert ammonia
to urea, whereas birds and insects convert ammonia to uric acid.
2. Describe the function of Malpighian tubules in insects.
Malpighian tubules carry uric acid, water, and excess ions into the intestines to be
eliminated.
33.4
1. List the organs that make up the human urinary system. What is the function of each?
The kidneys produce urine, regulate blood volume, conserve ions and nutrients, and
regulate blood pH. The ureters carry urine from the kidneys to the bladder. The urinary
bladder stores urine. The urethra transports urine from the bladder to the outside of the
body
2. What is the role of sphincters in the urinary system?
Two sphincters (one involuntary and one voluntary) control the release of urine from the
bladder.
33.5
1. What three processes occur in urine formation?
The three processes are filtration, reabsorption, and secretion.
2. Select a point along the descending limb of the nephron loop in figure 33.13. Compare
the solute concentration at that point and in the adjacent interstitial fluid; explain why
water moves in the direction it does.
At each point along the descending limb, the solute concentration in the nephron loop is
lower than in the surrounding interstitial fluid. Osmosis is the movement of water from
an area with low solute concentration to an area of high solute concentration. As a result
of the solute concentration gradient, water moves by osmosis from the nephron loop to
the surrounding tissue.
3. Describe the roles of antidiuretic hormone and aldosterone.
Antidiuretic hormone reduces urine volume by causing more water to be reabsorbed into
blood at the collecting duct. 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.
33.6
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 the presence of turbinates corresponds to a
larger cross-sectional area of the nose. Bony turbinates in the fossils of ancient reptiles
suggested that the reptilian ancestors of mammals were endotherms (even though they
lacked fur). Measuring nasal cavities revealed that feathered bird ancestors were
ectotherms, not endotherms.
2. Suppose researchers discover a fossil of a 100-kilogram dinosaur with a nasal cavity
greater than 10 square centimeters. According to figure 33.16, 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. 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 at any
temperature, but they require much more food and oxygen than do ectotherms.
2. Birds and insects frequently collect nectar from plants. Birds are endothermic, and
insects are ectothermic. Do you think that a given amount of nectar can support a greater
mass of insects or of birds? Explain.
A given amount of nectar supports a greater mass of insects than birds because
ectotherms use much less energy and therefore require less food than do endotherms.
3. Explorers of Antarctica must eat thousands more Calories per day than people
exploring the tropics. Explain this observation.
Humans are endotherms. We use the energy that we obtain from food to maintain a
constant body temperature. As the temperature outside of the body decreases, the body
must work harder (burn more Calories) to maintain a constant temperature.
4. 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.
5. 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.
6. Compare how an iguana and human excrete metabolic wastes produced in protein
metabolism.
Protein metabolism generates ammonia waste. Both iguanas and humans convert
ammonia to less toxic forms. Iguanas convert ammonia to uric acid, which is eliminated
in highly concentrated urine. Humans convert ammonia to urea, which is also eliminated
in urine.
7. Explain why exercise increases the urea concentration in urine.
Exercise increases protein metabolism in muscles. The urea produced in protein
metabolism enters the blood, moves into the nephrons, and is excreted in urine. If a
person sweats during exercise, then the urine will contain less water after exercise, which
would also increase the concentration of urea in urine.
8. 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.
9. 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 loop, 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.
10. Redraw the nephron in figure 33.13, but reverse the direction of blood flow around
the nephron loop. How would this change affect how water and salts are reabsorbed into
the blood?
Normally, blood flows through the capillaries in the opposite direction as the fluid in the
nephron. However, if blood flows through the capillaries in the same direction as the
filtrate flows through the nephron, then less water and fewer salts will be reabsorbed. The
osmotic pressure between the nephron and the capillary will balance near the top of the
descending limb. Once the osmotic pressure is equal, water will no longer diffuse
between the nephron and the capillary. Many salts that return to the blood are actively
transported from the nephron, so salt reabsorption will be less affected than water
reabsorption.
11. 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.
12. How could chronic high blood pressure damage the kidneys? How could very low
blood pressure impair kidney function?
Blood pressure drives substances from the bloodstream into the nephron. High blood
pressure damages blood vessels throughout the body (including the kidneys), depriving
the kidney of its blood supply and reducing its ability to regulate blood volume. On the
other hand, if blood pressure is very low, fluid will not enter the nephrons, and wastes
could build up in the bloodstream to toxic levels.
13. 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.
14. Diuretic drugs prevent ions from being reabsorbed from the nephron. Explain why a
diuretic drug might lower blood pressure. Why do some patients have low levels of salts
in their blood while taking a diuretic?
Since the drug prevents ions from moving out of the nephron, the salt concentration of
the nephron increases, so more water is retained in the nephron. More water is therefore
excreted in urine, and less water is returned to the blood. The resulting decrease in blood
volume helps to lower blood pressure. Meanwhile, since fewer ions are reabsorbed into
the bloodstream from the nephron, the salt concentration in the blood is likely to decline.
15. 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.
“Glomerular capsule” connects with “receives fluid from the” to “glomerulus.”
“Proximal convoluted tubule” connects with “passes fluid to the” to “nephron loop,”
which connects with “passes fluid to the” to “distal convoluted tubule.” All three of these
terms could also connect with the phrase “are parts of the” to “renal tubule.”
2. Add water, salts, toxins, and urine to the concept map.
“Water” and “salts” connect with “is filtered from blood at the” to “glomerular capsule.”
“Urine” leads with “leaves the body through the” to “urethra.” “Urine” leads with “is
stored in the” to “urinary bladder.” “Urine” leads with “is formed in the” to “renal
tubule.” “Water,” “salts,” and “toxins” lead with “are excreted in” to “urine.” “Water”
and “salts” lead with “if needed, return to the” to “blood.” “Toxins” leads with “are
secreted into the” to “renal tubule.”
3. How do aldosterone and antidiuretic hormone influence kidney function?
Both hormones act on the kidney to return water to the blood. 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 reabsorb more
water at the collecting duct, so more water enters the bloodstream.