macula densa

The macula densa is a tiny patch of sensor cells in the kidney that acts like a salt-taste gauge on a pipe. As fluid flows past it inside the kidney’s tubule, it samples how much salt is going by and uses that reading to help set blood pressure and kidney function.

It is a cluster of specialized cells in the wall of the distal tubule, positioned where the tubule loops back to touch its own glomerulus (the filter). When the salt (specifically sodium-chloride) reaching it is low — a sign of low blood volume or pressure — it signals the neighboring renin-producing cells to release more renin, kicking off the renin-angiotensin-aldosterone cascade that retains salt and raises pressure. When salt delivery is high, it suppresses renin.

The macula densa also adjusts the filtering rate of its own glomerulus through a local reflex called tubuloglomerular feedback, narrowing the incoming vessel when too much salt arrives so the kidney does not lose fluid too fast. In this way it links what the kidney is filtering to the whole body’s blood-pressure and sodium control, serving as the sensor that starts the RAAS.