Juxtaglomerular apparatus

The juxtaglomerular apparatus is a small group of specialized cells at the spot where a nephron's tubule loops back to touch its own glomerulus, between the arteriole that brings blood in (afferent) and the one that carries it out (efferent). It has three parts. The macula densa is a patch of crowded tubule cells, where the loop of Henle gives way to the distal convoluted tubule, that senses how much salt flows past. Juxtaglomerular cells, also called granular cells, are modified smooth muscle cells in the afferent arteriole wall that store and release the enzyme renin. Extraglomerular mesangial cells sit between the two and are thought to help relay signals.

It has two linked jobs. In tubuloglomerular feedback, a rise in filtration delivers more salt to the macula densa, which releases ATP and adenosine; these narrow the afferent arteriole and bring the glomerular filtration rate back down. Low salt at the macula densa, low pressure in the afferent arteriole and sympathetic nerve signals each increase renin release. Renin cuts angiotensinogen into angiotensin I, which is then converted to angiotensin II, a hormone that tightens blood vessels and triggers aldosterone release, so the kidneys keep more sodium and water and blood pressure rises.