Vasa recta


 * For the intestinal structure, see Vasa recta (intestines)

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 (recta is from the Latin for "straight") 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.

Histology
On a slide, vasa recta can be distinguished from the tubules of the loop of Henle by the presence of blood.

Function
Each of the vasa recta has a hairpin turn in the medulla and carries blood at a very slow rate, two factors crucial in the maintenance of countercurrent exchange that prevent washout of the concentration gradients established in the renal medulla.

The maintenance of this concentration gradient is one of the components responsible for the kidney's ability to produce concentrated urine.

On the descending portion of the vasa recta, NaCl and urea are reabsorbed into the blood, while water is secreted. On the ascending portion of the vasa recta, NaCl and urea are secreted into the interstitium, while water is reabsorbed.

Nomenclature
According to Terminologia Anatomica, the term "vasa recta renis" is an alternate name for "arteriolae rectae renis", and a separate term, venulae rectae renis, is used to identify the venous portion.

However, other sources consider "vasa recta renis" to refer to both the arterial and venous portions.

The "renis" is often omitted, but there do exist two other structures with the same name:
 * vasa recta (intestines) (in the ileum and jejunum)
 * the straight portion of the seminiferous tubule, though this is usually called the tubuli recti.

Pathology
The slow blood flow in vasa recta makes them a likely location of thrombosis in hypercoagulable states, or tissue loss due to erythrocyte sickling in sickle cell disease. Ischemia that results may lead to renal papillary necrosis.