Mechanisms for achieving Hyperosmolality of the Medullary Interstitium  

To be able to excrete a concnetrated urine, the first step is the creation of a hyperosmolal medullary interstitial fluid. The normal osmolality of the fluids is about 300 mOsm/L. However in the medullary interstitium, osmolarilities of around 1200 mOsm/L can be reached. Four different solute concentrating mechanisms are responsible to achieve this.

1. Active Transport of Sodium Ions out of the Thick Portion of the Ascending Loop of Henle.

Active transport of sodium ions together with cotransport of potasssium and chloride ions from the thick ascending limb of the loop of Henle produces an increase in the concnetrationof these solutes in outer medulla. This part of the loop of Henle is impermeable to water so the osmolality within the nephron is decreased.