Functions of the Urinary System

Objectives

When you have completed this section, you should be able to

• name and locate the organs of the urinary system;
• list several functions of the kidneys in addition to urine formation;
• name the major nitrogenous wastes and identify their sources;
• define excretion and identify the systems that excrete wastes; and
• define osmolarity and state its relevance to urinary function.

Key Point Review

1. State four functions of the kidneys other than forming urine.

2. List four nitrogenous wastes and their metabolic sources.

3. Name some wastes eliminated by three systems other than the urinary system.

4. Calculate the osmolarity of a solution that contains 0.1 M urea, 0.2 M NaCl, and 0.5 M CaCl₂. Assume urea does not ionize and NaCl and CaCl₂ ionize completely.

Anatomy of the Kidney

Objectives

When you have completed this section, you should be able to

• identify the major external and internal features of the kidney;
• trace the flow of blood through the kidney; and
• trace the flow of fluid through the renal tubules.

Think About It

Can you identify a portal system in the renal circulation?

Key Point Review

5. Arrange the following in order from the most numerous to the least numerous structures in a kidney: glomeruli, major calices, minor calices, interlobular arteries, interlobar arteries.

6. Trace the path taken by one red blood cell from the renal artery to the renal vein.

7. Consider one molecule of urea in the urine. Trace the route that it took from the point where it left the bloodstream to the point where it left the body.

Urine Formation I: Glomerular Filtration

Objectives

When you have completed this section, you should be able to

• describe the glomerular filtration membrane and how it excludes blood cells and proteins from the filtrate;
• explain the forces that promote and oppose glomerular filtration, and calculate net filtration pressure if given the magnitude of these forces; and
• describe how glomerular filtration rate is modified by renal autoregulation, the sympathetic nervous system, and hormones.

Think About It

How low would capillary BP have to be for glomerular filtration and urine output to cease?

Think About It

Describe or diagram a similar negative feedback loop to show how the macula densa could compensate for a drop in systemic blood pressure.

Think About It

Hypertension is commonly treated with drugs called ACE inhibitors. Explain how such drugs could lower the blood pressure.

Key Point Review

8. Name the four major processes in urine production.

9. Trace the movement of a urea molecule from the blood to the capsular space, and name the barriers it passes through.

10. Calculate the net filtration pressure in a patient whose blood COP is only 10 mmHg due to hypoproteinemia. Assume other relevant variables to be normal.

11. Assume a person is moderately dehydrated and has low blood pressure. Describe the homeostatic mechanisms that would help the kidneys maintain a normal GFR.

Urine Formation II: Tubular Reabsorption and Secretion

Objectives

When you have completed this section, you should be able to

• describe how the renal tubules reabsorb useful solutes from the glomerular filtrate and return them to the blood;
• describe how the tubules secrete solutes from the blood into the tubular fluid; and
• describe how the nephron regulates water excretion.

Key Point Review

12. The reabsorption of water, Cl^-, and glucose by the PCT are all linked to the reabsorption of Na⁺, but in three very different ways. Contrast these three mechanisms.

13. Explain why a substance appears in the urine if its concentration in the glomerular filtrate exceeds the T_m of the renal tubule.

14. Contrast the effects of aldosterone and ANF on the renal tubule.

Urine Formation III: Concentrating the Urine

Objectives

When you have completed this section, you should be able to

• explain how the collecting duct and antidiuretic hormone regulate the volume and concentration of urine; and
• explain how the countercurrent multiplier and countercurrent exchange systems maintain the osmotic gradient of the renal medulla.

Key Point Review

15. Predict and explain the effect of ADH hypersecretion on the sodium concentration of the urine.

16. Concisely contrast the role of the countercurrent multiplier with that of the countercurrent exchanger.

17. How would the function of the collecting duct change if the nephron loop did not exist?

Urine and Renal Function Tests

Objectives

When you have completed this section, you should be able to

• describe the composition and properties of urine; and
• carry out some calculations to evaluate renal function.

Think About It

In table 23.1, what process accounts for most of the difference between values in the middle column and those on the right?

Think About It

What would you expect the value of renal clearance of glucose to be in a healthy individual? Why?

Key Point Review

18. Define oliguria and polyuria. Which of these is characteristic of diabetes?

19. Identify two causes of glycosuria other than diabetes mellitus.

20. How is the diuresis produced by furosemide like the diuresis produced by diabetes mellitus? How are they different?

21. Explain why GFR could not be determined from measurement of the amount of NaCl in the urine.

Urine Storage and Elimination

Objectives

When you have completed this section, you should be able to

• describe the functional anatomy of the ureters, urinary bladder, and male and female urethra; and
• explain how the nervous system and dual urethral sphincters control the voiding of urine.

Key Point Review

22. Describe the location and function of the detrusor muscle.

23. Compare and contrast the functions of the internal and external urethral sphincters.

24. How would micturition be affected by a spinal cord lesion that prevented voluntary nerve impulses from reaching the sacral part of the cord?

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