[citation needed] The loop of Henle is supplied by blood in a series of straight capillaries descending from the cortical efferent arterioles.

The low bloodflow through the vasa recta allows time for osmotic equilibration, and can be altered by changing the resistance of the vessels' efferent arterioles.

[citation needed] As well, blood in the vasa recta still has large proteins and ions which were not filtered through the glomerulus.

[citation needed] The main function of the loop of Henle is to set up a concentration gradient.

[citation needed] The descending loop of Henle receives isotonic (300 mOsm/L) fluid from the proximal convoluted tubule (PCT).

Substances reabsorbed in the PCT include urea, water, potassium, sodium, chloride, glucose, amino acids, lactate, phosphate, and bicarbonate.

This area represents the highest concentration in the nephron, but the collecting duct can reach this same tonicity with maximum ADH[clarification needed] effect.

Increases in vasa recta flow wash away metabolites and cause the medulla to lose osmolarity as well.

[5] Aquaporin-2 (AQ2) sits in collecting duct and is selectively inserted into cell membranes, according to the body's needs, to reabsorb water to create that balance.

Therefore, some of them have a longer loop of Henle which creates a saltier medulla, leading them to reabsorb more water from the pre-urine.

Another example is the Australian mouse whose loop of Henle, 5.2 mm, can make the medulla as salty as 9000 mOsm.