The cells of the macula densa are sensitive to the ionic content and water volume of the fluid in the DCT, producing molecular signals that promote renin secretion by other cells of the juxtaglomerular apparatus. The release of renin is an essential component of the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure and volume.
The cells of the macula densa cells are taller and have more prominent nuclei than surrounding cells of the distal convoluted tubule.
The close proximity and prominence of the nuclei cause this segment of the DCT's wall to appear darker in microscopic preparations, hence the name macula densa.
A decrease in blood pressure results in a decreased concentration of sodium ions in the distal convoluted tubule. (This is due to reduced filtration by the glomerulus: less filtrate is expelled into Bowman's space and the proximal convoluted tubule; the resulting fluid reaching the distal convoluted tubule will have a lower sodium concentration after the water is removed.)
In response, the macula densa cells release prostaglandins, which triggers granular juxtaglomerular cells lining the afferent arterioles to release renin into the bloodstream. (The juxtoglomerular cells can also release renin independently of the macula densa, as they are also triggered by baroreceptors lining the arterioles, and release renin if a fall in blood pressure in the arterioles is detected.)
- Organology at UC Davis Urinary/mammal/cortex1/cortex5 - "Mammal, kidney cortex (LM, Medium)"
- Essentials of Human Physiology by Thomas M. Nosek. Section 7/7ch03/7ch03p17. - "The Nephron: Juxtaglomerular Apparatus"