Blood Supply
The loop of Henle is supplied by blood in a series of straight capillaries descending from the cortical efferent arterioles. These capillaries (called the vasa recta; recta is from the Latin for "straight") also have a countercurrent multiplier mechanism that prevents washout of solutes from the medulla, thereby maintaining the medullary concentration. As water is osmotically driven from the descending limb into the interstitium, it readily enters the vasa recta. 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.
Also, the vasa recta still has the large proteins and ions which were not filtered through the glomerulus, which provides an oncotic pressure for ions to enter the vasa recta from the interstitium.
The main function of the Loop of Henle is to set up a concentration gradient.
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