Collecting Duct Effect of Antidiuretic Hormone ADH

As a result of the recycling of salt between the ascending and descending limbs and the recycling of urea between the collecting duct and the loop of Henle, the interstitial fluid is made very hypertonic. The collecting ducts must channel their fluid through this hypertonic environment in order to empty their contents of urine into the calyces. Whereas the fluid surrounding the collecting ducts in the medulla is hypertonic, the fluid that passes into the collecting ducts in the cortex is hypotonic as a result of the active extrusion of salt by the ascending limbs of the loops.

The medullary region of the collecting duct is impermeable to the high concentration of NaCl that surrounds it. The wall of the collecting duct, however, is permeable to water. Since the surrounding interstitial fluid in the renal medulla is very hypertonic because of the countercurrent multiplier system, water is drawn out of the collecting ducts by osmosis. This water does not dilute the surrounding interstitial fluid because it is transported by capillaries to the general circulation. In this way, most of the water remaining in the filtrate is returned to the vascular system (fig. 17.19).

Note that it is the osmotic gradient created by the counter-current multiplier system that provides the force for water reabsorption through the collecting ducts. The rate at which this osmotic movement occurs, however, is determined by the permeability of the collecting duct to water. This depends on the number of aquaporins (water channels) in the plasma membranes of the collecting duct epithelial cells.

Aquaporins are produced as proteins within the membranes of vesicles that bud from the Golgi apparatus (chapter 3). In the absence of stimulation, these vesicles are present in the cytoplasm of the collecting duct cells. When antidiuretic hormone (ADH) binds to its membrane receptors on the collecting duct, it acts (via cAMP as a second messenger) to stimulate the fusion of these vesicles with the cell membrane (chapter 6; see fig. 6.15). This is identical to exocytosis, except that here there is no secretion of product. The importance of this process in the collecting duct is that the water channels are incorporated into the plasma membrane when the vesicles and membrane fuse. In response to ADH, therefore, the collecting duct becomes more permeable to water. When ADH is no longer available to bind to its membrane receptors, the water channels are removed from the plasma membrane by a process of endocytosis. Endocytosis is the opposite of exocytosis;

Physiology of the Kidneys

Cortex tttt

Distal tubule

Outer medulla

Inner medulla

- Collecting duct

Inner medulla

Loop of Henle

■ Figure 17.18 The role of urea in urine concentration. Urea diffuses out of the inner collecting duct and contributes significantly to the concentration of the interstitial fluid in the renal medulla. The active transport of Na+ out of the thick segments of the ascending limbs also contributes to the hypertonicity of the medulla, so that water is reabsorbed by osmosis from the collecting ducts.

Table 17.2 Transport Properties of Different Segments of the Renal Tubules and the Collecting Ducts

Passive Transport

Nephron Segment Active Transport Salt Water

Urea

Proximal tubule

Na+

Cl-

Yes

Yes

Descending limb of Henle's loop

None

Maybe

Yes

No

Thin segment of ascending limb

None

NaCl

No

Yes

Thick segment of ascending limb

Na+

Cl-

No

No

Distal tubule

Na+

Cl-

No**

No

Collecting duct*

Slight Na+

No

Yes (ADH) or slight (no ADH)

Yes

*The permeability of the collecting duct to water depends on the presence of ADH. **The last part of the distal tubule, however, is permeable to water.

*The permeability of the collecting duct to water depends on the presence of ADH. **The last part of the distal tubule, however, is permeable to water.

the plasma membrane invaginates to reform vesicles that again contain the water channels. Alternating exocytosis and endocytosis in response to the presence and absence of ADH, respectively, is believed to result in the recycling of water channels within the cell.

When the concentration of ADH is increased, the collecting ducts become more permeable to water and more water is reabsorbed. A decrease in ADH, conversely, results in less reabsorption of water and thus in the excretion of a larger volume of more dilute urine (fig. 17.20). ADH is produced by neurons in the hypothalamus and is released from the posterior pituitary (chapter 11; see fig. 11.13). The secretion of ADH is stimulated when osmoreceptors in the hypothalamus respond to an increase in blood osmolality. During dehydration, therefore, when the plasma becomes more concentrated, increased secretion of ADH promotes increased permeability of the collecting ducts to water. In severe dehydration only the minimal amount of water needed to eliminate the body's wastes is excreted. This minimum, an obligatory water loss of about 400 ml per day, is limited by the

© The McGraw-Hill Companies, 2003

Chapter Seventeen

Proximal tubule

Vasa recta

Proximal tubule

Vasa recta

Ascending Vasa Recta

1200

1400

Descending limb of loop

Ascending limb of loop

Descending limb of loop

Ascending limb of loop

Collecting duct

600 600

Outer medulla

1200

1400

— Inner medulla

■ Figure 17.19 The osmolality of different regions of the kidney. The countercurrent multiplier system in the loop of Henle and countercurrent exchange in the vasa recta help to create a hypertonic renal medulla. Under the influence of antidiuretic hormone (ADH), the collecting duct becomes more permeable to water, and thus more water is drawn out by osmosis into the hypertonic renal medulla and peritubular capillaries.

fact that urine cannot become more concentrated than the medullary tissue fluid surrounding the collecting ducts. Under these conditions about 99.8% of the initial glomerular ultrafil-trate is reabsorbed.

A person in a state of normal hydration excretes about 1.5 L of urine per day, indicating that 99.2% of the glomerular ultrafil-trate volume is reabsorbed. Notice that small changes in percent reabsorption translate into large changes in urine volume. Drinking more water—and thus decreasing ADH secretion (fig. 17.20 and table 17.3)—results in correspondingly larger volumes of urine excretion. It should be noted, however, that even in the complete absence of ADH some water is still reabsorbed through the collecting ducts.

B it Diabetes insipidus is a disease associated with the inadequate secretion or action of ADH. When the se-^ cretion of ADH is adequate, but a genetic defect in the ADH receptors or the aquaporin channels renders the kidneys incapable of responding to ADH, the condition is called nephrogenetic diabetesinsipidus. Without proper ADH secretion or action, the collecting ducts are not very permeable to water, and so a large volume (5 to 10 L per day) of dilute urine is produced. The dehydration that results causes intense thirst, but a person with this condition has difficulty drinking enough to compensate for the large volumes of water lost in the urine.

© The McGraw-Hill Companies, 2003

Physiology of the Kidneys 539

Table Í7.3

Antidiuretic Hormone Secretion and Action

Stimulus

Receptors

Secretion of ADH

Effects on Urine Volume

Effects on Blood

TOsmolality (dehydration)

Osmoreceptors in hypothalamus

Increased

Decreased

Increased water retention; decreased blood osmolality

^Osmolality

Osmoreceptors in hypothalamus

Decreased

Increased

Water loss increases blood osmolality

ÎBlood volume

Stretch receptors in left atrium

Decreased

Increased

Decreased blood volume

iBlood volume

Stretch receptors in left atrium

Increased

Decreased

Increased blood volume

Low water intake (dehydration)

High water intake (over-hydration)

Low water intake (dehydration)

f Plasma osmolality-

High water intake (over-hydration)

J Plasma osmolality

Osmoreceptors in hypothalamus

Posterior pituitary fADH

Kidneys

Negative feedback correction f Water reabsorption

Less water reted in urin

Negative feedback correction j Water reabsorption o

More water excreted in urine

Negative feedback correction

■ Figure 17.20 Homeostasis of plasma concentration is maintained by ADH. In dehydration (left side of figure), a rise in ADH secretion results in a reduction in the excretion of water in the urine. In overhydration (right side of figure), the excess water is eliminated through a decrease in ADH secretion. These changes provide negative feedback correction, maintaining homeostasis of plasma osmolality and, indirectly, blood volume.

Test Yourself Before You Continue

1. Describe the mechanisms for salt and water reabsorption in the proximal tubule.

2. Compare the transport of Na+, Cl-, and water across the walls of the proximal tubule, ascending and descending limbs of the loop of Henle, and collecting duct.

3. Describe the interaction between the ascending and descending limbs of the loop and explain how this interaction results in a hypertonic renal medulla.

4. Explain how ADH helps the body to conserve water. How do variations in ADH secretion affect the volume and concentration of urine?

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Responses

  • Karen
    Is water reabsorption within the collecting duct influenced by a persons hydration state?
    7 years ago
  • Lucia
    How does adh affect the activity of the collecting duct?
    6 years ago
  • aulis salama
    How does AHD affect the activity of the collectiing duct?
    5 years ago
  • PEARL
    Does adh cause urea to move from the collecting duct to the interstitial fluid?
    3 years ago
  • eros
    How does adh affect the wall of the collecting duct?
    2 years ago
  • robinia
    How do variations in ADH secretion affect the volume and concentration of urine?
    2 years ago
  • gimja
    How ADH concentrates urine within collecting ducts?
    1 year ago
  • Miska
    What is the effect of antidiuretic hormone on the cells of the collecting ducts?
    1 year ago
  • sinit
    How are collecting ducts affected by adh?
    1 year ago
  • Brigitte
    How does adh work in the duct wall?
    10 months ago
  • carson
    What is the effect of antidiuretic hormone on the cells of the collecting duct?
    10 months ago

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