Porth's Essentials of Pathophysiology, 4e

608

Kidney and Urinary Tract Function

U N I T 7

How the Kidney

U N D E R S T A N D I N G

The osmolarity of body fluids relies heavily on the ability of the kidney to produce dilute or concentrated urine. Urine concentration depends on three factors: (1) the osmolarity of interstitial fluids in the urine-concentrating part of the kidney, (2) the antidiuretic hormone (ADH), and (3) the action of ADH on the cells in the collecting tubules of the kidney.

Osmolality (mOsm)

H 2 O

1

NaCl

H 2 O

Osmolarity. In approximately one fifth of the juxtamedullary neph- rons, the loops of Henle and special hairpin-shaped capillaries called the vasa recta descend into the medullary portion of the kidney to form a coun- tercurrent system—a set of parallel passages in which the contents flow in opposite directions. The counter- current design serves to increase the osmolarity in this part of the kidney by promoting the exchange of solutes between the adjacent descending and ascending loops of Henle and between the descending and ascending sec- tions of the vasa recta. Because of these exchange processes, a high con- centration of osmotically active par- ticles (approximately 1200 mOsm/ kg of H 2 O) collects in the interstitium surrounding the collecting tubules where the ADH-mediated reabsorp- tion of water takes place. 2 Antidiuretic Hormone. Antid­ iuretic hormone, which regulates the ability of the kidneys to concentrate urine, is synthesized by neurons in the hypothalamus and transported down their axons to the posterior pituitary gland and then released into the circulation. One of the main stimuli for synthesis and release of ADH is an increase in serum osmo- larity. Antidiuretic hormone release is also controlled by cardiovascular reflexes that respond to changes in blood pressure or blood volume.

K + Na +

300

NaCl NaCl H 2 O

Cortex Medulla

NaCl Urea

H 2 O

NaCl Urea

H 2 O

H 2 O

600

NaCl Urea

NaCl Urea

NaCl

NaCl H 2 O Urea

H 2 O

800

H 2 O

Vasa recta Loop of Henle

Collecting tubule

1200

Hypothalamus

Osmoreceptors

Posterior pituitary gland

ADH

Urine: decreased flow and concentrated