Page 709 - Atlas of Histology with Functional Correlations

chloride but not to water. The hypertonicity (high osmotic pressure) of the

extracellular fluid in the medulla interstitium removes water from the
glomerular filtrate as it flows through the descending thin tubules, thereby
increasing its sodium and chloride concentration. In the ascending thin limb,
water remains behind, whereas sodium chloride leaves the fluid and is
concentrated in the interstitium. The countercurrent flow of ultrafiltrate in the

descending and ascending thin loops of Henle produces a gradient of
osmolarity in the interstitium of the medulla. The water that enters the
interstitium is then removed by the countercurrent blood flow in the capillary

loops of the vasa recta, thus maintaining the osmotic concentration gradient
in the medulla, resulting in water conservation and urine concentration.
These capillary loops are permeable to water and take up the water from the
medullary interstitium to return it to the systemic circulation.

Distal Convoluted Tubules

The distal convoluted tubules are shorter, less convoluted than the proximal
tubules, and less frequently observed in the cortex and near the renal
corpuscles. In comparison with the proximal convoluted tubules, the distal
convoluted tubules do not exhibit brush borders, the cells are smaller, and

more nuclei are seen per tubule. The basolateral membranes of distal
convoluted tubule cells also show increased cell membrane interdigitations
and elongated mitochondria within these infoldings. The distal convoluted
tubules actively reabsorb sodium ions from the tubular filtrate and excrete

hydrogen, potassium, and ammonium ions into the tubular fluid. The
excretion of hydrogen ions is connected with the absorption of bicarbonate
ions and increasing the acidification of urine.

Sodium reabsorption in the distal convoluted tubules is controlled by the

hormone aldosterone secreted by the adrenal cortex. Aldosterone hormone
induces cells of the distal convoluted tubules to actively absorb sodium and
chloride ions from the filtrate and transport them into the interstitium. From
the interstitium, sodium chloride ions are quickly absorbed by the
peritubular capillaries and returned back to the systemic circulation,

thereby decreasing sodium loss in urine. These functions of the distal
convoluted tubules are vital for maintaining the proper acid–base balance of
body fluids and blood.

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