The vasa recta, or arteriolae rectae renis, are peritubular capillaries in the kidneys that are situated parallel to and surrounding the loop of Henle. This term is a combination of the Latin words recta, which means straight, and vasa, which pertains to blood vessels. These capillaries are important parts of the kidney anatomy that form long, descending and ascending capillary loops. They participate in a mechanism called countercurrent exchange, which is needed for the production of concentrated urine.
These structures are small branches of the efferent arterioles of the juxtamedullary nephrons, which is why they are also called vasa recta renis, meaning “straight arteries or arterioles of kidney." Juxtamedullary nephrons are excretory units nearest the kidney medulla. The “renis” part is often not mentioned, but it is important when making the distinction between structures in the kidneys and those in both the intestines and seminiferous tubules.
The straight arteries maintain the hypertonicity, or saltiness, of the kidney’s medulla through countercurrent exchange. In the hypertonic medullary environment, salt or sodium chloride and solutes, such as urea, are present in high concentrations. As blood passes through the vasa recta, salt and other solutes diffuse from the medullary tissue fluid into the blood, passing through a descending capillary loop. This process increases the solute concentrations in the descending loop, but as blood goes through the ascending loop, the solutes passively diffuse out of the ascending loop and go back into the descending loop due to the presence of a high-low concentration gradient.
As a result, salt and other solutes become trapped within the medulla and are recirculated instead of being transported by blood into the renal cortex. The walls of the capillaries are freely permeable to the solutes, so the concentrations of these solutes within them and in the medullary interstitial fluid eventually become equal. When blood from the cortex goes through the descending capillary loop, it loses water and gains salt and solutes. As blood ascends, the reverse occurs, and it gains water and gradually loses salt and solutes.
The walls of the vasa recta renis do not permit the passage of small proteins, so there is greater pressure within the capillary loops, or greater colloid pressure, compared to the interstitial pressure. The result is the osmotic movement of water into the blood vessels and its transport from the kidney medulla to the outer kidney cortex.