The distal tubules include the distal convoluted tubules and collecting ducts. Unlike earlier portions of the nephron tubules (the proximal convoluted tubule and loop of Henle), this part of the tubule system is under hormonal control. Two hormones largely determine the amount of water reabsorbed in the distal nephrons: aldosterone and ADH (vasopression). Aldosterone is released from the adrenal cortex under the influence of angiotensin II produced by the renin-angiotensin system. ADH is secreted into the posterior pituitary by the hypothalamus and enters the blood stream. Both cause the urine to be more concentrated (preserving body water) by increasing water reabsorption in the distal tubules.
NaCl reabsorption in the distal convoluted tubule is increased by aldosterone. In the presence of ADH, which inserts water pores (aquaporins) into the otherwise water-impermeable tubule, water is reabsorbed by osmosis, following the Na gradient. Salt uptake into the cells lining the tubule lumen is carried out by the NCC transporter, a thiazide-sensitive NaCl transporter in the apical membrane that is inhibited by thiazide diuretics. A basolateral Na-K ATPase pumps Na into the peritubular fluids and a Cl channel mediates Cl ion reabsorption. Aldosterone increases Na-K ATPase activity and number of Na-Cl cotransporters in the apical membrane.
K secretion in the distal nephron is required to maintain high Na reabsorption. In addition to the NCC transporter, an amiloride-sensitive ENaC channel in the apical membrane reabsorbs Na in the distal tubules. This Na uptake does not carry Cl ions with it, so a voltage imbalance builds up. The voltage imbalance drives K secretion into the tubule lumen. K passes through the apical membrane through a ROMK channel. Thus, Na reabsorption causes loss of K ions from the plasma. Aldosterone increases the activities of both ROMK and ENaC channels.
The collecting ducts pass through the deepest parts of the renal medulla that have high osmolarity. Dilute urine is produced in absence of ADH because the collecting ducts are impermeable to water. ADH inserts aquaporin channels that cause water reabsorption as it passes through the renal medulla on the way to the papillary ducts that eventually empty into the renal pelvis. The collecting tubules near the papillary ducts are permeable to urea. Urea concentration in the tubules increases as it passes deeper into the medulla as water leaves. Thus, the osmolarity of the medulla is further increased by high concentrations of urea.
Making a Difference in the World 