Expansion
Macula densa, juxtaglomerular cells and extraglomerular mesangial cells
Expansion
Three components, where the distal tubule returns to touch its own glomerulus:
- Macula densa: specialised cells of the thick ascending limb that sense luminal sodium chloride concentration
- Juxtaglomerular (granular) cells: modified smooth muscle in the wall of the afferent arteriole that store and secrete renin
- Extraglomerular mesangial (lacis) cells: connecting the two
Function:
- Low sodium chloride at the macula densa signals low GFR, so renin is released, generating angiotensin II which constricts the efferent arteriole and restores filtration
- High sodium chloride signals excessive GFR, so adenosine is released, constricting the afferent arteriole: tubuloglomerular feedback
Renin release is also stimulated directly by reduced afferent arteriolar stretch and by sympathetic beta-1 stimulation.
This anatomy explains why beta blockers reduce renin, and why loop diuretics, by blocking macula densa sodium uptake, both stimulate renin and abolish tubuloglomerular feedback.