294 Notes to Instructors
Notes to Instructors
Chapter 44 Osmoregulation and Excretion
What is the focus of this activity?
The digestive system and the respiratory system function in the bulk flow of nutrients and
environment to gain control over these functions.
What is this particular activity designed to do?
Activity 44.1 What is nitrogenous waste, and how is it removed from the body?
This activity is designed to help students understand the structure and function of the
What misconceptions or difficulties can this activity reveal?
Activity 44.1
Most students can work through how the filtrate is formed and how various substances
are later removed from the filtrate. Many don’t understand what happens to these
Answers
Activity 44.1 What is nitrogenous waste, and how
is it removed from the body?
In the space below, draw a longitudinal section of a mammalian kidney. Next to this, draw
a blowup of a nephon (including Bowman’s capsule and the loop of Henle) and its
associated collecting duct. Be sure to include the afferent arteriole, glomerulus, and
efferent arteriole that are associated with this nephron.
You may do your drawing in chalk on a tabletop or blackboard if they are available.
Refer to Figure 44.14 on pages 962–963 of Campbell Biology, 9th edition.
Use your drawing and your understanding of the operation of the kidney to answer
the questions.
1. Define excretion, and indicate how it differs from elimination.
Excretion is the removal of nitrogenous waste from the body. Elimination is the
2. The removal of nitrogenous wastes (excess nitrogen) is a special problem in most
animals.
a. Where does the nitrogenous waste come from?
The primary source of nitrogenous waste is deamination of amino acids. This
b. What is it about the chemistry of nitrogen that makes it difficult for most animals
to deal with?
Amine groups are easily converted to ammonia (NH3), which is very soluble in
Activity 44.1 295
3. Work through parts a and b, and then use the information you gather there to answer
the question in part c.
a. Describe the composition of the newly filtered solution that enters Bowman’s
capsule. Then compare it to the composition of the blood entering and leaving
the glomerulus.
The solution that enters Bowman’s capsule contains water and anything soluble in
b. Starting with the solution that escapes into Bowman’s capsule from the
glomerulus, describe the changes that occur in its composition as it moves
through each of these regions:
i. Proximal convoluted tubule
In this region, NaCl and nutrients (for example, sugars) are actively
ii. Loop of Henle
In the thick segment of the ascending portion of the loop, NaCl is actively
iii. Distal convoluted tubule
In this region, NaCl and bicarbonate ions are actively transported out of the
iv. Collecting duct
The cortical and outer medullary regions of the collecting duct are permeable
296 Activity 44.1
v. Urinary bladder
The collecting ducts empty into the renal pelvis, which leads to the ureter.
c. Now explain how the general function of the kidney enables it to “remove”
(a better expression would be “let out”) from the body a wide variety of
unfamiliar substances (drugs, inorganic molecules, or ions of many kinds) that
the body has never encountered before. After answering this, explain why “let
out” from the body is a better expression than “remove.”
The membranes of the capillaries of the glomerulus contain numerous aquaporins,
4. It is useful to consider the excretory system (along with the digestive and gas-
exchange systems) as primarily involved in bulk exchange with the external
environment. The excretory system could also be interpreted as a specialized part of
the external surface of the organism, which in its own way encloses and modifies
part of the environment. Describe how this is true for the human kidney. For
example:
a. Where in the kidney does the organism end and the environment begin?
The organism ends at the glomerulus, where the fluid crosses the membrane of
b. Are changes in the glomerular filtrate changes in the organism, changes in the
environment, or both?
The filtrate in the tubule is technically in the external environment. Changes in
Activity 44.1 297
c. What do your answers in parts a and b indicate about the possible evolutionary
origins of the kidney?
As noted on pages 961–963 of Campbell Biology, 9th edition, excretory systems
5. a. What is the difference between hydrostatic pressure and osmotic pressure?
Hydrostatic pressure is the pressure exerted by a fluid (for example, water) on the
walls of the containing vessel. Water tends to move from a region of high hydrostatic
b. Where in the human excretory system is hydrostatic pressure responsible for
moving water across a membrane or layer of cells?
Hydrostatic pressure is responsible for moving fluid from the glomerulus into
c. Where in the excretory system is osmotic pressure responsible for such
movement?
Osmotic pressure is responsible for moving water from regions of lower solute
298 Activity 44.1
44.1 Test Your Understanding
1. a. You examine the kidney structure and function of a two species of mice, one from
the desert and another from a meadow or grassland. What differences would you
expect to find? Use drawings of the two systems and their functions to explain
your reasoning.
To survive in the desert, organisms need to conserve much more water than do
b. Would you expect the excretory systems of organisms that live in the sea to
resemble more closely those of animals that live in deserts or animals that live in
fresh water? Explain.
The fluids of fish living in the sea contain a lower concentration of salts than is
2. Many medicines are taken orally—that is, swallowed and absorbed from the
digestive system. Aspirin and many antibiotics are examples. Even though these
drugs may be very different chemically, the instructions for taking them often say
“Repeat dose every 4 to 6 hours.” What is the reasoning behind this dose rate? In
other words, why do you need to take the medicine every 4 to 6 hours?
Medicines and anything else that is soluble in the blood are filtered out into
Indicate whether the following are true or false. Explain your reasoning.
Questions 3 to 5. In a normal, healthy human, urine:
Activity 44.1 299
Questions 6 to 8. Injection of an individual with antidiuretic hormone (ADH)
would cause:
T/F6. increased urine volume.
False—ADH would make the distal tubules and collecting ducts more
permeable to water. This allows water in the tubules/ducts to move back into
the interstitial fluid. It would result in a more concentrated, lower volume of
urine.
300 Activity 44.1