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Chapter 20: Urinary System

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Chapter Summary

Introduction (page 820)

The urinary system consists of the kidneys, ureters, urinary bladder, and urethra.

Kidneys (page 820)

1. Location of the kidneys a. The kidneys are bean-shaped organs on either side of the vertebral column, high on the posterior wall of the abdominal cavity.

b. They are positioned posterior to the parietal peritoneum and held in place by adipose and connective tissue.

2. Kidney structure a. A kidney contains a hollow renal sinus.

b. The ureter expands into the renal pelvis, which, in turn, is divided into major and minor calyces.

c. Renal papillae project into the renal sinus.

d. Kidney tissue is divided into a medulla and a cortex.

3. Functions of the kidneys a. The kidneys remove metabolic wastes from the blood and excrete them to the outside.

b. They also help regulate red blood cell production, blood pressure, calcium ion absorption, and the volume, composition, and pH of the blood.

4. Renal blood vessels a. Arterial blood flows through the renal artery, interlobar arteries, arcuate arteries, interlobular arteries, afferent arterioles, glomerular capillaries, efferent arterioles, and peritubular capillaries.

b. Venous blood returns through a series of vessels that correspond to those of the arterial pathways.

5. Nephrons a. Structure of a nephron

(1) A nephron is the functional unit of the kidney.

(2) It consists of a renal corpuscle and a renal tubule.

(a) The corpuscle consists of a glomerulus and a glomerular capsule.

(b) Portions of the renal tubule include the proximal convoluted tubule, the nephron loop (ascending and descending limbs), and the distal convoluted tubule.

(3) The nephron joins a collecting duct, which empties into the minor calyx of the renal pelvis.

b. Juxtaglomerular apparatus

(1) The juxtaglomerular apparatus is located at the point of contact between the distal convoluted tubule and the afferent and efferent arterioles.

(2) It consists of the macula densa and the juxtaglomerular cells.

c. Cortical and juxtamedullary nephrons

(1) Cortical nephrons are the most numerous and have corpuscles near the surface of the kidney.

(2) Juxtamedullary nephrons have corpuscles near the medulla.

d. Blood supply of a nephron

(1) The glomerular capillary receives blood from the afferent arteriole and passes it to the efferent arteriole.

(2) The efferent arteriole gives rise to the peritubular capillary system, which surrounds the renal tubule.

(3) Capillary loops, called vasa recta, dip down into the medulla.

Urine Formation (page 830)

Nephrons remove wastes from the blood and regulate water and electrolyte concentrations. Urine is the product of these functions.

1. Glomerular filtration a. Urine formation begins when water and dissolved materials are filtered out of the glomerular capillary.

b. The glomerular capillaries are much more permeable than the capillaries in other tissues.

2. Filtration pressure a. Filtration is mainly due to hydrostatic pressure inside the glomerular capillaries.

b. The osmotic pressure of the blood plasma and hydrostatic pressure in the glomerular capsule also affect filtration.

c. Filtration pressure is the net force acting to move material out of the glomerulus and into the glomerular capsule.

d. The composition of the filtrate is similar to that of tissue fluid.

3. Filtration rate a. The rate of filtration varies with the filtration pressure.

b. Filtration pressure changes with the diameters of the afferent and efferent arterioles.

c. As the osmotic pressure in the glomerulus increases, filtration decreases.

d. As the hydrostatic pressure in a glomerular capsule increases, the filtration rate decreases.

e. The kidneys produce about 125 milliliters of glomerular fluid per minute, most of which is reabsorbed.

f. The volume of filtrate varies with the surface area of the glomerular capillary.

4. Control of filtration rate a. Glomerular filtration rate (GFR) remains relatively constant but may be increased or decreased when the need arises. Increased sympathetic nerve activity can decrease GFR.

b. When tubular fluid NaCl concentration decreases, the macula densa causes the juxtaglomerular cells to release renin. This triggers a series of changes leading to vasoconstriction, which may affect GFR, and secretion of aldosterone, which stimulates tubular sodium reabsorption.

c. Autoregulation is the ability of an organ or tissue to maintain a constant blood flow under certain conditions when the arterial blood pressure is changing.

5. Tubular reabsorption a. Substances are selectively reabsorbed from the glomerular filtrate.

b. The peritubular capillary is adapted for reabsorption.

(1) It carries low-pressure blood.

(2) It is very permeable.

c. Most reabsorption occurs in the proximal tubule, where the epithelial cells possess microvilli.

d. Different modes of transport reabsorb various substances in particular segments of the renal tubule.

(1) Glucose and amino acids are reabsorbed by active transport.

(2) Water is reabsorbed by osmosis.

(3) Proteins are reabsorbed by pinocytosis.

e. Active transport mechanisms have limited transport capacities.

f. If the concentration of a substance in the filtrate exceeds its renal plasma threshold, the excess is excreted in the urine.

g. Substances that remain in the filtrate are concentrated as water is reabsorbed.

h. Sodium ions are reabsorbed by active transport.

(1) As positively charged sodium ions are transported out of the filtrate, negatively charged ions accompany them.

(2) Water is passively reabsorbed by osmosis as sodium ions are actively reabsorbed.

6. Tubular secretion a. Tubular secretion transports certain substances from the plasma to the tubular fluid.

b. Some substances are actively secreted.

(1) These include various organic compounds and hydrogen ions.

(2) The proximal and distal convoluted tubules secrete hydrogen ions.

c. Potassium ions are secreted both actively and passively in the distal convoluted tubule and collecting duct.

7. Regulation of urine concentration and volume a. Most of the sodium ions are reabsorbed before the urine is excreted.

b. Sodium ions are concentrated in the renal medulla by the countercurrent mechanism.

(1) Chloride ions are actively reabsorbed in the ascending limb, and sodium ions follow them passively.

(2) Tubular fluid in the ascending limb becomes hypotonic as it loses solutes.

(3) Water leaves the descending limb by osmosis, and NaCl enters this limb by diffusion.

(4) Tubular fluid in the descending limb becomes hypertonic as it loses water and gains NaCl.

(5) As NaCl repeats this circuit, its concentration in the medulla increases.

c. The vasa recta countercurrent mechanism helps maintain the NaCl concentration in the medulla.

d. The distal convoluted tubule and collecting duct are impermeable to water, which therefore is excreted in urine.

e. ADH from the posterior pituitary gland increases the permeability of the distal convoluted tubule and collecting duct, promoting water reabsorption.

8. Urea and uric acid excretion a. Urea is a by-product of amino acid metabolism.

(1) It is passively reabsorbed by diffusion.

(2) About 50% of the urea is excreted in urine.

(3) A countercurrent mechanism involving urea helps in the reabsorption of water.

b. Uric acid results from the metabolism of nucleic acids. (1) Most is reabsorbed by active transport.

(2) Some is secreted into the renal tubule. 9. Urine composition a. Urine is about 95% water, and it usually contains urea, uric acid, and creatinine.

b. It may contain a trace of amino acids and varying amounts of electrolytes, depending upon dietary intake.

c. The volume of urine varies with the fluid intake and with certain environmental factors.

Elimination of Urine (page 843)

1. Ureters a. The ureter is a tubular organ that extends from each kidney to the urinary bladder.

b. Its wall has mucous, muscular, and fibrous layers.

c. Peristaltic waves in the ureter force urine to the bladder.

d. Obstruction in the ureter stimulates strong peristaltic waves and a reflex that decreases urine production.

2. Urinary bladder a. The urinary bladder is a distensible organ that stores urine and forces it into the urethra.

b. The openings for the ureters and urethra are located at the three angles of the trigone in the floor of the urinary bladder.

c. Muscle fibers in the wall form the detrusor muscle.

d. A portion of the detrusor muscle forms an internal urethral sphincter.

3. Urethra a. The urethra conveys urine from the bladder to the outside.

b. In females, it empties between the labia minora.

c. In males, it conveys products of reproductive organs as well as urine.

(1) Three portions of the male urethra are prostatic, membranous, and penile.

(2) The urethra empties at the tip of the penis.

4. Micturition a. Micturition is the process of expelling urine.

b. It involves contraction of the detrusor muscle and relaxation of the external urethral sphincter.

c. Micturition reflex

(1) Distension stimulates stretch receptors in the bladder wall.

(2) The micturition reflex center in the sacral portion of the spinal cord sends parasympathetic motor impulses to the detrusor muscle.

(3) As the bladder fills, its internal pressure increases, forcing the internal urethral sphincter open.

(4) A second reflex relaxes the external urethral sphincter, unless its contraction is voluntarily controlled.

(5) Nerve centers in the brain stem and cerebral cortex aid control of urination.

Life-Span Changes (page 848)

Distinctive changes occur in the kidneys, ureters, and urethra with age, but nephrons are so numerous that a healthy person is usually unaware of kidney shrinkage and slowed cleansing of the blood.

1. With age, the kidneys appear grainy and scarred.

2. GFR drops significantly with age as glomeruli atrophy, fill with connective tissue, or unwind.

3. Renal tubules accumulate fat on their outsides and become asymmetric. Reabsorption and secretion may slow or become impaired. Drugs remain longer in the circulation as a person ages.

4. Changes in the circulatory system slow the rate of processing through the urinary system. The kidneys slow

Critical Thinking Questions in their response to changes, and are less efficient at activating vitamin D. 5. The bladder, ureters, and urethra lose elasticity, with effects on the urge and timing of urination.

1. If an infant is born with narrowed renal arteries, what effect would this condition have on the volume of urine produced? Explain your answer.

2. Why are people with nephrotic syndrome, in which plasma proteins are lost into the urine, more susceptible to infections?

3. If a patient who has had major abdominal surgery receives intravenous fluids equal to the volume of blood lost during surgery, would you expect the volume of urine produced to be greater or less than normal? Why?

4. If a physician prescribed oral penicillin therapy for a patient with an infection of the urinary bladder, how would you describe for the patient the route by which the drug would reach the bladder?

5. If the blood pressure of a patient who is in shock as a result of a severe injury decreases greatly, how would you expect the volume of urine to change? Why?

6. Inflammation of the urinary bladder is more common in women than in men. How might this observation be related to the anatomy of the female and male urethras?

Review Exercises

1. Name the organs of the urinary system, and list their general functions.

2. Describe the external and internal structure of a kidney.

3. List the functions of the kidneys.

4. Name the vessels the blood passes through as it travels from the renal artery to the renal vein.

5. Distinguish between a renal corpuscle and a renal tubule.

6. Name the structures fluid passes through as it travels from the glomerulus to the collecting duct.

7. Describe the location and structure of the juxtaglomerular apparatus.

8. Distinguish between cortical and juxtamedullary nephrons.

9. Distinguish among filtration, reabsorption, and secretion as they relate to urine formation.

10. Define filtration pressure.

11. Compare the composition of the glomerular filtrate with that of the blood plasma.

12. Explain how the diameters of the afferent and efferent arterioles affect the rate of glomerular filtration.

13. Explain how changes in the osmotic pressure of the blood plasma may affect the rate of glomerular filtration.

14. Explain how the hydrostatic pressure of a glomerular capsule affects the rate of glomerular filtration.

15. Describe two mechanisms by which the body regulates the filtration rate.

16. Define autoregulation.

17. Discuss how tubular reabsorption is a selective process.

18. Explain how the peritubular capillary is adapted for reabsorption.

19. Explain how the epithelial cells of the proximal convoluted tubule are adapted for reabsorption.

20. Explain why active transport mechanisms have limited transport capacities.

21. Define renal plasma threshold, and explain its significance in tubular reabsorption.

22. Explain how amino acids and proteins are reabsorbed.

23. Describe the effect of sodium reabsorption on the reabsorption of negatively charged ions.

24. Explain how sodium ion reabsorption affects water reabsorption.

25. Explain how hypotonic tubular fluid is produced in the ascending limb of the nephron loop.

26. Explain why fluid in the descending limb of the nephron loop is hypertonic.

27. Describe the function of ADH.

28. Explain how the renal tubule is adapted to secrete hydrogen ions.

29. Explain how potassium ions may be secreted passively.

30. Explain how urine may become concentrated as it moves through the collecting duct.

31. Compare the processes by which urea and uric acid are reabsorbed.

32. List the more common substances found in urine and their sources.

33. List some of the factors that affect the volume of urine produced each day.

34. Describe the structure and function of a ureter.

35. Explain how the muscular wall of the ureter aids in moving urine.

36. Discuss what happens if a ureter becomes obstructed.

37. Describe the structure and location of the urinary bladder.

38. Define detrusor muscle.

39. Distinguish between the internal and external urethral sphincters.

40. Compare the urethra of a female with that of a male.

41. Describe the micturition reflex.

42. Explain how the micturition reflex can be voluntarily controlled.

43. Describe the changes that occur in the urinary system with age?

V. Absorption and Excretion

21. Water, Electrolyte, and Acid-Base Balance

© The McGraw-H Companies, 2001

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