The loop of Henle of the nephron can be separated into an ascending limb and the descending limb. The descending limb is highly impermeable to solutes such as sodium, but permeable to water. Conversely, the ascending limb is highly impermeable to water, but permeable to solutes. Chloride, potassium, and sodium are co-transported across the apical membrane (closest to the lumen) via transporters from the filtrate. The transporter requires all three ions present to be effective and to maintain electroneutrality. In addition, the three ions are transported across the basolateral membrane (closest to the renal interstitium) via other means such as the sodium potassium ATPase transports and the chloride channels in the membrane. As these solutes are being actively transported out of the ascending limb and into the renal interstitium/capillary network without water following (due to the lack of water permeability), the filtrate becomes more diluted. Furthermore, these ions simultaneously causes an increase in osmotic pressure that contributes to water reabsorption in the descending limb. This effect can be magnified with the help of vasopressin, which is a hormone that is typically involved with water reabsorption. However, when it acts on the ascending limb, it aids in increasing sodium reabsorption which will increase water reabsorption in the latter parts of the nephron (the distal tubule and collecting duct).

Kidney Function - Ascending Limb of The Loop of Henle References