Review Question - QID 107148

The proximal convoluted tubule (PCT) reabsorbs the majority of filtered water. In the PCT, water is absorbed passively following the gradient created by the absorption of sodium ions.

The PCT is the site of reabsorption of all of the filtered glucose and amino acids, and most of the bicarbonate, sodium, chloride, and water. Under normal circumstances, about two-thirds of the filtered water is reabsorbed here. The filtrate in the PCT is isosmotic to plasma. Reabsorption of water in the PCT is accomplished via passive diffusion of water molecules along the gradient created by active reabsorption of electrolytes and glucose.

Costanzo discusses glomerulotubular balance in the PCT. Glomerulotubular balance is responsible for maintaining a constant fraction of water reabsorption. Glomerulotubular balance is accomplished because of the proximity and association of the peritubular capillaries to the PCT lumen. If an increased fraction of sodium is filtered, Starling forces will drive an increased fraction of sodium to be reabsorbed into the peritubular capillaries. This will also passively increase the fractional absorption of water thereby maintaining the ratio of filtered to reabsorbed at two-thirds.

Eaton discusses water absorption throughout the remainder of the nephron. The descending limb of the loop of Henle is highly permeable to water, while the thick ascending limb is essentially impermeable to water. The net affect is to produce dilute tubular fluid via absorption of much of the remaining ions. The late distal convoluted tubule and collecting duct exhibit variable permeability to water reabsorption as directed by hormonal control (vasopressin/ADH). This final step is important in the maintenance of serum osmolarity and volume status.

Illustration A is a drawing illustrating water reabsorption along the length of the nephron.

Incorrect Answers:
Answer 1: The early distal convoluted tubule is impermeable to water, while the late distal convoluted tubule can absorb a small amount of water.
Answer 2: The collecting duct can absorb a relatively small fraction of the filtered water via aquaporin channels.
Answer 3: The thick ascending loop of Henle is largely impermeable to water.
Answer 5: The late distal convoluted tubule absorbs a small amount of the total filtered fraction of water. This is done in part under the action of ADH via aquaporins.