Unit Six. Animal Life


26. Maintaining the Internal Environment


26.5. Eliminated Nitrogenous Wastes


Amino acids and nucleic acids are nitrogen-containing molecules. When animals catabolize these molecules for energy or convert them into carbohydrates or lipids, they produce nitrogen-containing by-products called nitrogenous wastes that must be eliminated from the body.

The first step in the metabolism of amino acids and nucleic acids is the removal of the amino (—NH2) group and its combination with H+ to form ammonia (NH3) in the liver, 1 in figure 26.11. Ammonia is quite toxic to cells and therefore is safe only in very dilute concentrations. The excretion of ammonia is not a problem for the bony fish and tadpoles, which eliminate most of it by diffusion through the gills and the rest by excretion in very dilute urine 2. In sharks, adult amphibians, and mammals, the nitrogenous wastes are eliminated in the far less toxic form of urea 3. Urea is water-soluble and so can be excreted in large amounts in the urine. It is carried in the bloodstream from its place of synthesis in the liver to the kidneys, where it is excreted in the urine.



Figure 26.11. Nitrogenous wastes.

When amino acids and nucleic acids are metabolized, the immediate nitrogen by-product is ammonia 1, which is quite toxic but can be eliminated through the gills of bony fish 2. Mammals convert ammonia into urea 3, which is less toxic. Birds and terrestrial reptiles convert it instead into uric acid 4, which is insoluble in water.


Reptiles, birds, and insects excrete nitrogenous wastes in the form of uric acid 4, which is only slightly soluble in water. As a result of its low solubility, uric acid precipitates and thus can be excreted using very little water. Uric acid forms the pasty white material in bird droppings. The ability to synthesize uric acid in these groups of animals is also important because their eggs are encased within shells, and nitrogenous wastes build up as the embryo grows within the egg. The formation of uric acid, while a lengthy process that requires considerable energy, produces a compound that crystallizes and precipitates. As a precipitate, it is unable to affect the embryo’s development even though it is still inside the egg.

Mammals also produce some uric acid, but it is a waste product of the degradation of purine nucleotides (see chapter 3), not of amino acids. Most mammals have an enzyme called uricase, which converts uric acid into a more soluble derivative, allantoin. Only humans, apes, and the dalmatian dog lack this enzyme and so must excrete the uric acid. In humans, excessive accumulation of uric acid in the joints produces a condition known as gout.


Key Learning Outcome 26.5. The metabolic breakdown of amino acids and nucleic acids produces ammonia as a by-product. Ammonia is excreted by bony fish, but other vertebrates convert nitrogenous wastes into urea and uric acid, which are less toxic nitrogenous wastes.


Unquiry & Analysis

How Do Sleeping Birds Stay Warm?

Mammals and birds are endothermic—they maintain body temperatures regardless of the temperature of their surroundings. This lets them reliably run their metabolism even when external temperatures fall—the rates of most enzyme-catalyzed reactions slow two- to threefold for every 10°C temperature drop. Your body keeps its temperature within narrow bounds at 37°C (98.6°F), and birds maintain even higher temperatures.

To stay warm like this, mammals and birds continuously carry out oxidative metabolism, which generates heat. This requires a several-fold increase in metabolic rate, which is expensive, particularly when the animal is not active. The logical solution is to give up the struggle to keep warm and let the body temperature drop during sleep, a condition known as torpor. While humans don't adopt this approach, many other mammals and birds do. This raises an interesting question: What prevents a sleeping bird in torpor from freezing? Does its body simply adopt the temperature of its surroundings, or is there a body temperature below which metabolic heating kicks in to avoid freezing?

The graph to the right displays an experiment examining this issue in the tropical hummingbird Eulampis. The study examines the effect on metabolic rate (measured as oxygen consumption) of decreasing air temperature. Oxygen consumption was assessed over a range of air temperatures from 3° to 37°C, for two contrasting physiological states: The blue data were collected from birds that were awake, the red data from sleeping, torpid birds. The blue and red lines, called regression lines, were plotted using curve-fitting statistics that provide the best fit to the data.



1. Applying Concepts

a. Variable. In the graph, what is the dependent variable?

b. Comparing Two Data Sets. Do awake hummingbirds maintain the same metabolic rate at all air temperatures? sleeping, torpid ones? At a given temperature, which has the higher metabolic rate, an awake bird or a sleeping one?

2. Interpreting Data

a. How does the oxygen consumption of awake hummingbirds change as air temperature falls? Why do you think this is so? Is the change consistent over the entire range of air temperatures examined?

b. How does the oxygen consumption of sleeping torpid birds change as air temperature falls? Is this change consistent over the entire range of air temperatures examined? Explain any difference you detect.

c. Are there any significant differences in the slope of the two regression lines below 15°C? What does this suggest to you?

3. Making Inferences

a. For each 5-degree air temperature interval, estimate the average oxygen consumption for awake and for sleeping birds, and plot the difference as a function of air temperature.

b. Based on this curve, what would you expect to happen to a sleeping bird's body temperature as air temperatures fall from 30° to 20°C? from 15° to 5°C?

4. Drawing Conclusions. How do Eulampis hummingbirds avoid becoming chilled while sleeping on cold nights?

5. Further Analysis. Flying hummingbirds would be expected to use more metabolic energy than perched, awake ones. How would you expect the level of activity to influence the birds' regulation of body temperature? Explain.


Test Your Understanding

1. The monitoring and adjusting of the body’s condition, such as temperature and pH, is known as

a. exothermy.

b. homeostasis.

c. osmoregulation.

d. ectothermy.

2. Which of the following describes the method of regulation whereby the response of an effector returns conditions to a set point?

a. inhibitory regulation

b. homeostasis

c. positive feedback loop

d. negative feedback loop

3. If your blood sugar is too low, the hormone glucagon is released by the pancreas. This hormone will cause

a. the release of insulin.

b. glycogen to break down.

c. glycogen to be formed.

d. fat to be formed.

4. Which of the following is not involved in osmoregulation?

a. pancreas

b. nephridia

c. Malpighian tubules

d. flame cells

5. Which of the following animals use Malpighian tubules for excretion?

a. birds

b. ants

c. kangaroo rats

d. earthworms

6. To keep the proper concentrations of water and solutes in their blood, freshwater bony fish must drink

a. lots of water and excrete large volumes of urine that are hypotonic to body fluids.

b. no water and excrete large volumes of urine that are hypotonic to body fluids.

c. lots of water and excrete large volumes of urine that are isotonic to body fluids.

d. no water and excrete large volumes of urine that are isotonic to body fluids.

7. Which of the following animals has the least concentrated urine relative to its blood plasma?

a. bird

b. freshwater fish

c. camel

d. shark

8. Selective reabsorption of components of the filtrate occurs where?

a. Bowman’s capsule

b. glomerulus

c. loop of Henle

d. ureter

9. Water is removed from kidney filtrate by the process of

a. diffusion.

b. active transport.

c. facilitated diffusion.

d. osmosis.

10. Humans excrete their excess nitrogenous wastes as

a. uric acid crystals.

b. compounds containing protein.

c. ammonia.

d. urea.