Final answer:
Juxtaglomerular cells secrete renin in response to decreased blood pressure, elevating blood pressure and aiding in homeostasis.
Step-by-step explanation:
Granular cells (juxtaglomerular cells) secrete renin when there is a fall in blood pressure. These cells are modified smooth muscle cells in the walls of the afferent arteriole at the juxtaglomerular apparatus (JGA) of the kidney. Renin is essential in regulating blood pressure and electrolyte balance. It specifically acts by converting angiotensinogen to angiotensin I, which is further converted to the active vasoconstrictor angiotensin II, leading to increased blood pressure.
The macula densa, part of the JGA, also plays a role in regulating renin release. It monitors Na+ concentration in the kidney and can signal juxtaglomerular cells to adjust the glomerular filtration rate (GFR). This tubuloglomerular feedback helps maintain a stable environment in the body regardless of changes in osmolarity or blood pressure.
Thus, a decrease in blood pressure or a signal from the macula densa due to variations in Na+ concentration induces the juxtaglomerular cells to release renin, which ultimately influences systemic blood pressure and volume through the renin-angiotensin-aldosterone system.