Corticopapillary osmotic gradienthigh ADH

- is the gradient of osmolarity from the cortex (300 mOsm/L) to the papilla (1200 mOsm/L), and is composed primarily of NaCl and urea.

- is established by countercurrent multiplication and urea recycling.

- is maintained by countercurrent exchange in the vasa recta, a. Countercurrent multiplication in the loop of Henle

- depends on NaCl reabsorption in the thick ascending limb and countercurrent flow in the descending and ascending limbs of the loop of Henle.

- is augmented by ADH, which stimulates NaCl reabsorption in the thick ascending limb. Therefore, the presence of ADH increases the size of the corticopapillary osmotic gradient.

Deprive of water

i

Increases plasma osmolality

i

Stimulates osmoreceptors in anterior hypothalamus

Stimulates osmoreceptors in anterior hypothalamus

Increases secretion of ADH from posterior pituitary

Increases water permeability of late distal tubule and collecting duct

Increases water reabsorption

Increases urine osmolarity and decreases urine volume

i

r

Decreases plasma osmolarity toward normal

Figure 5-13. Responses to water deprivation. ADH = antidiuretic hormone.

Figure 5-13. Responses to water deprivation. ADH = antidiuretic hormone.

b. Urea recycling from the inner medullary collecting ducts into the medullary interstitial fluid also is augmented by ADH.

c. Vasa recta are the capillaries that supply the loop of Henle. They maintain the corticopapillary gradient by serving as osmotic exchangers. Vasa recta blood equilibrates osmotically with the interstitial fluid of the medulla and papilla.

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