Juxtaglomerular cell

The juxtaglomerular cell is a kidney cell with a very precise address: it lives in the wall of the afferent arteriole, the small vessel that carries blood into the glomerulus, the kidney's filter. That is where the name comes from: juxta means next to, and glomerular, of the glomerulus.

What it does is store and release. It contains prorenin, the inactive form of an enzyme. When the cell is activated, it cleaves that prorenin; the now mature renin is stored in granules, which are packets inside the cell, and from there it is released into the blood. Renin matters because it is the rate-limiting step of the renin angiotensin aldosterone system, the hormone chain that adjusts blood volume and blood pressure: nothing in that chain moves faster than renin allows. It stores the messenger already made, and all it needs is the cut that turns it on.

Two of the signals that put it to work are these. One is mechanical: the arteriole has mechanoreceptors, stretch sensors, that register how much blood is reaching the kidney. The other is chemical and comes from the macula densa, a group of cells that keeps measuring how much sodium and how much chloride reach the distal tubule.