Birds and insects excrete uric acid, whereas mammals and most amphibians excrete mainly urea. What is the chief advantage of uric acid over urea as a waste product?
a. Uric acid is a much simpler molecule.
b. It takes less energy to make uric acid.
c. Less water is required to excrete uric acid.
d. More solutes are removed excreting uric acid.

In which of the following species should natural selection favor the highest proportion of juxtamedullary nephrons?

a. A river otter

b. A mouse species living in a temperate broadleaf forest

c. A mouse species living in a desert

d. A beaver

As filtrate passes through the loop of Henle, salt is reabsorbed and concentrated in the interstitial fluid of the medulla. This high solute concentration in the medulla enables nephrons to

a. Excrete the maximum amount of salt.

b. Neutralize toxins that might be found in the kidney.

c. Excrete a large amount of water.

d. Reabsorb water from the filtrate very efficiently.

Compare and contrast the types of nitrogenous wastes excreted by animals. Identify which type can be excreted with the least water, which is most toxic, and which waste is excreted by bony fishes, by mammals, and by insects. Which type would you expect to be produced by embryos inside eggs laid on land?

African lungfish, which are often found in small stagnant pools of fresh water, produce urea as a nitrogenous waste. What is the advantage of this adaptation?

a. Urea takes less energy to synthesize than ammonia.

b. Small stagnant pools do not provide enough water to dilute ammonia, which is toxic.

c. Urea forms an insoluble precipitate.

d. Urea makes lungfish tissue hypoosmotic to the pool.

The chloride cells of fish gills have a high density of mitochondria. How does this characteristic relate to the functional role of chloride cells?

Would you expect other epithelial cells involved in ion transport to contain large numbers of mitochondria? Explain.

A freshwater fish would be expected to

a. Pump salt out through its gills.

b. Produce copious quantities of dilute urine.

c. Have scales and a covering of mucus that reduce water loss to the environment.

d. Do all of the above.

Explain why mammals would not be able to produce concentrated urine if their nephrons lacked loops of Henle.

Match each of the following components of blood (on the left) with what happens to it as the blood is processed by the kidney (on the right). Note that each lettered choice may be used more than once.

8. Water

9. Glucose

10. Plasma protein

11. Toxins or drugs

12. Red blood cell

13. Urea

a. passes into filtrate; almost all excreted in urine

b. remains in blood

c. passes into filtrate; mostly reabsorbed

d. secreted and excreted