22nd June 2023

Proximal Convoluted Tubule

The main role of the proximal convoluted tubule (PCT) is reabsorption. It is lined by cuboidal epithelial cells covered in microvilli forming a ‘brush border’, increasing surface area for reabsorption. The epithelial cells are packed with mitochondria, providing an energy supply for the active transport of solutes.

Approximately 65% of sodium and water in the glomerular filtrate is reabsorbed in the PCT.

Na⁺-K⁺-ATPase transporters on the basolateral membrane form a primary active transport mechanism in which 3 Na⁺ ions are pumped out of the cell in exchange for 2 K⁺ ions pumped into the cell. This forms an electrochemical gradient which allows diffusion of sodium from the lumen of the renal tubule into the cell.

Sodium can be co-transported with other solutes via symporters, whereby sodium moves down its concentration gradient whilst actively co-transporting other solutes, such as glucose and amino acids, against their concentration gradients. In normal physiological states, all organic solutes, such as glucose and amino acids, are reabsorbed in the PCT via these symporters.

Sodium can also be transported out of the lumen via an antiporter, which enables reabsorption of sodium with the concurrent secretion of other ions, in particular hydrogen ions, which have a role in maintaining acid/base homeostasis. H⁺ ions secreted via the Na⁺/H⁺ antiporter combine with bicarbonate ions, forming carbonic anhydrase (H₂CO₃), which subsequently splits to form CO₂ and H₂O. These are absorbed by the PCT epithelial cells, where the previous reaction reverses, releasing bicarbonate (which is absorbed into the blood) and hydrogen ions (which are recycled for further reactions). Approximately 90% of bicarbonate is reabsorbed in the PCT.

Ion absorption and secretion in the PCT.

Image Source: Wikimedia Commons (2007). Renal Diuretics, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Renal_Diuretics.gif (Accessed on: 13 February 2023).

As the concentration of solutes within PCT decreases due to reabsorption, the solute concentration in the renal interstitium increases, and water can move out of the lumen into the interstitium by osmosis.

The PCT is also a site of secretion of organic acids and bases (e.g. bile salts, oxalate and catecholamines), and certain drug metabolites, from the peritubular capillaries into the renal tubule for rapid removal from the body.