Chapter 25 The Urinary System

Question 1

describe the anatomy of a nephron

Answer 1

the nephron is divided into 6 distinct parts;

1. proximal (convoluted) tubule
2. Descending loop of Henle
3. Ascending loop of Henle
4. Distal (convoluted) tubule
5. Cortical collecting duct
6. Distal Collecting duct

Question 2

Describe the forces (pressures) that promote or counteract glomerular filtration.

Answer 2

Glomerular filtration is a passive, nonselective process in which hydrostatic pressure forces fluid through the glomerular membrane

Question 3

describe the mechanisms underlying water and solute reabsoption from the renal tubules into the peritubular capillaries

Answer 3

Tubular reabsorption begins as soon as the filtrate enters the proximal convoluted tubule, and involves near total reabsorption of organic nutrients, and the hormonally regulated reabsorption of water and ions.

• The most abundant cation of the filtrate is Na+, and reabsorption is always active.
• Passive tubular reabsorption is the passive reabsorption of negatively charged ions that travel along an electrical gradient created by the active reabsorption of Na+.
• Obligatory water reabsorption occurs in water-permeable regions of the tubules in response to the osmotic gradients created by active transport of Na+.
• Secondary active transport is responsible for absorption of glucose, amino acids, vitamins, and most cations, and occurs when solutes are cotransported with Na+ when it moves along its concentration gradient.
• Substances that are not reabsorbed or incompletely reabsorbed remain in the filtrate due to a lack of carrier molecules, lipid insolubility, or large size (urea, creatinine, and uric acid).

Question 4

describe how sodium and water re-absorption is regulated in the distal tubule and collecting duct.

Answer 4

Different areas of the tubules have different absorptive capabilities.
•The proximal convoluted tubule is most active in reabsorption, with most selective reabsorption occurring there.

• The descending limb of the loop of Henle is permeable to water, while the ascending limb is impermeable to water but permeable to electrolytes.
• The distal convoluted tubule and collecting duct have Na+ and water permeability regulated by the hormones aldosterone, antidiuretic hormone, and atrial natriuretic peptide.

Question 5

Describe the importance of tubular secretion and list several substances that are secreted

Answer 5

• Tubular secretion disposes of unwanted solutes, eliminates solutes that were reabsorbed, rids the body of excess K+, and controls blood pH.
• Tubular secretion is most active in the proximal convoluted tubule, but occurs in the collecting ducts and distal convoluted tubules as well.

Question 6

Describe the mechanisms responsible for the medullary osmotic gradient.

Answer 6

Regulation of Urine Concentration and Volume

•One of the critical functions of the kidney is to keep the solute load of body fluids constant by regulating urine concentration and volume.

•The countercurrent mechanism involves interaction between filtrate flow through the loops of Henle (the countercurrent multiplier) of juxtamedullary nephrons and the flow of blood through the vasa recta (the countercurrent exchanger).
o Because water is freely absorbed from the descending limb of the loop of Henle, filtrate concentration increases and water is reabsorbed.
o The ascending limb is permeable to solutes, but not to water.
o In the collecting duct, urea diffuses into the deep medullary tissue, contributing to the increasing osmotic gradient encountered by filtrate as it moves through the loop.
o The vasa recta aids in maintaining the steep concentration gradient of the medulla by cycling salt into the blood as it descends into the medulla, and then out again as it ascends toward the cortex.

Question 7

Explain the formation of dilute versus concentrated urine

Answer 7

• Because tubular filtrate is diluted as it travels through the ascending limb of the loop of Henle, production of a dilute urine is accomplished by simply allowing filtrate to pass on to the renal pelvis.
• Formation of a concentrated urine occurs in response to the release of antidiuretic hormone, which makes the collecting ducts permeable to water and increases water uptake from the urine.
• Diuretics act to increase urine output by either acting as an osmotic diuretic or by inhibiting Na+ and resulting obligatory water reabsorption.