Final answer:
Low blood pressure stimulates granular cells to release renin, which activates a system that increases blood pressure and volume through vasoconstriction and retention of sodium and water.
Step-by-step explanation:
True: Low blood pressure directly stimulates granular cells to release renin. When there is a decrease in blood pressure, it is sensed by granular cells located in the afferent arteriole of the juxtaglomerular apparatus (JGA). These cells respond by secreting the enzyme renin. Renin is crucial for the activation of the renin-angiotensin-aldosterone system, which ultimately increases blood pressure. Renin converts angiotensinogen, produced by the liver, into angiotensin I, which is then converted to angiotensin II by angiotensin-converting enzyme (ACE).
Angiotensin II is a potent vasoconstrictor that raises blood pressure and stimulates the release of aldosterone from the adrenal cortex. Aldosterone further increases blood pressure and volume by promoting sodium and water retention in the kidneys. This mechanism is vital for maintaining blood pressure homeostasis and is a key point of control for blood pressure medications, such as ACE inhibitors.