Excretory System
Workbook
MCHS – Biology 20 – Mr. Korotash
Name: __________ Date: _______
1
Study the diagram above. Name the structures and indicate their functions by completing the
following table:
Structure
1.
Function
2.
3.
4.
5.
6.
7.
8.
2
Study the diagrams above and answer the questions below.
9. The volume of blood entering the kidney through the renal artery in one day is more than the
volume leaving through the renal vein. What does this tell you about where urine comes from?
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If you study the overall structure of the kidney (Diagrams A and B), you will learn very little
about how it cleanses the blood. This is because the cleansing occurs in millions of microscopic
nephrons. The renal cortex of the kidney contains the curly parts of the nephrons. The renal
medulla contains the long loops and collecting ducts. These are shown in Diagram C.
How the nephron cleanses the blood
Cleansing the blood of wastes, excess salts, and water can be compared to house cleaning. Cleaning
out a desk drawer involves separating useful items from garbage, then keeping the useful items and
discarding the garbage. You can choose one of two ways to do this. In one method, you would dump
everything out of the drawer and then reclaim the useful items, leaving a pile of garbage. An
alternative would be to sort through the materials in the drawer, removing the garbage and leaving
the useful items in place.
The cleansing of blood in nephrons involves mostly the first method. Useful molecules as well as
wastes and excess substances are filtered out of the blood. Then useful molecules are reclaimed back
into the blood, leaving wastes (urine) to be excreted. A review of blood composition will help you to
understand how it is cleansed.
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10. List all the components of whole blood that you can think of (i.e. what is in blood?). Note that
some components are waste molecules and some are useful. Some useful components occur in
excess amounts.
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Study the diagram of a nephron with its surrounding blood vessels that follows. Remember that
blood passes through two capillary beds as it flows around the nephron, and that exchange of
components between blood and surrounding tissues can only occur through thin capillary walls.
11.
Identify where useful molecules and wastes are filtered out of the blood and into the nephron.
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12. Where are useful molecules reabsorbed from the nephron back into the blood?
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13. Which structure transports the waste molecules (urine) left behind after reabsorption?
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14. Which parts of the nephron are in the cortex of the kidney? Which parts penetrate deep into the
medulla?
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15. A common misconception is that “kidneys filter wastes out of the blood.” Use the words
“filtration” and “reabsorption” to explain why this statement is false.
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Urine formation occurs as blood pressure forces filtrate from the glomerulus into the capsule. This
bulk flow of fluids into the capsules of the nephrons in both kidneys creates about 180 L of filtrate
per day. All but about 1 L will be actively reabsorbed back into the blood, with great expenditure of
ATP.
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16. In what ways is nephric filtration similar to formation of interstitial fluid (lymph) in other tissues
in the body?
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17. One of the effects of a drug overdose is a serious decrease in blood pressure. How might this
affect kidney function?
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18. Why are useful molecules like glucose and other nutrients found in the filtrate along with urea
and other wastes?
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19. Sometimes bacterial infection causes nephritis—an inflammation of membranes in the
glomerulus and capsule. Large pores are created through which blood cells enter the nephron.
What symptom would indicate this problem?
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20. What other useful substances, in addition to Na+, Cl–, and water, are reabsorbed in the proximal
tubule?
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Use the following diagram, which shows a nephron lying within the cortex and the medulla, and
your textbook or other resource to answer the questions below.
21. Explain the process of reabsorption from the descending loop.
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22. Where is the highest concentration of Na+ found?
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23. Explain the process of reabsorption from the ascending loop.
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24. Explain how reabsorption of ions and water occurs from the distal tubule.
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The following table compares the composition of blood plasma, nephric filtrate, and urine. Study it
carefully and answer the questions that follow.
Comparison of concentrations of substances in Plasma, Filtrate and Urine (mg/100mL)
Substance
Plasma
Filtrate
Urine
Concentration
Change
Inorganic ions
0.9
0.9
<0.9 – 3.6
<1 – 4×
(all)
K+
0.02
0.02
0.15
7.5×
Amino acids
0.05
0.05
none
–
Proteins
8.0
none
none
–
Glucose
0.01
0.01
none
–
Urea
0.03
0.03
1.8
60×
Note: The pH of blood plasma and filtrate is 7.4. The pH of urine is 4.8–7.5.
25. Study the concentrations of the ions, the amino acids, glucose and urea. Why are their
concentrations in the filtrate identical to those in the plasma?
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26. Neither glucose nor proteins are present in urine, but for different reasons. Explain.
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27. Give two reasons to explain why K+ is more concentrated in the urine than in the filtrate.
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28. Which ion accounts for the low pH of the urine? How and where is this ion transported into the
urine? Why is the elimination of this ion important to survival?
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When kidney failure occurs, blood plasma is said to become uremic, because urea molecules
accumulate to dangerous levels. Imbalances in other substances also occur.
Dialysis is a medical procedure in which the composition of the plasma can be corrected through
simple diffusion. Dialysing fluid (dialysate) is separated from the patient’s blood (uremic plasma) by
thin semipermeable membranes. Molecules and ions diffuse into or out of the patient’s plasma,
depending upon the composition of the dialysate. Thus, careful formulation of dialysate is the key to
correcting the composition of uremic plasma.
In hemodialysis, diffusion occurs across artificial membranes. In peritoneal dialysis, it occurs across
the intestinal lining (peritoneum). These procedures are illustrated on page 322 of your textbook.
Use the table below, which shows the composition of the dialysate compared to normal plasma and
uremic plasma, to answer the questions that follow.
Composition of Plasma and Dialysing Fluid (various units)
Component
Normal Plasma
Uremic Plasma
+
Potassium (K )
5
8
Bicarbonate (HCO3‾ )
27
14
Glucose
100
100
Urea
26
200
Dialysate
5
27
125
0
29. Explain why the dialysate is produced with a concentration of 5 units of potassium ions.
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30. What is the function of bicarbonate ions in the blood? Suggest an explanation for the decreased
concentration of bicarbonate ions in uremic plasma.
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31. What evidence is given in the table that kidney failure has little effect upon glucose metabolism?
Given the glucose concentration of normal plasma, why might dialysate be made to contain 125
units of glucose?
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32. What is the concentration of urea in normal plasma? Suggest an explanation for the lack of urea
in the dialysate.
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