In the blood supply of the kidney, the vasa recta renis (or straight arteries of kidney, or straight arterioles of kidney) form a series of straight capillaries in the medulla. They lie parallel to the loop of Henle. These vessels branch off the efferent arterioles of juxtamedullary nephrons (those nephrons closest to the medulla), enter the medulla, and surround the loop of Henle. [ http://en.wikipedia.org/wiki/Vasa_recta ]