Final answer:
True, low blood pressure stimulates granular cells to release renin, initiating the renin-angiotensin-aldosterone system which increases blood pressure by causing vasoconstriction and promoting sodium and water retention.
Step-by-step explanation:
The statement that low blood pressure directly stimulates granular cells to release renin is true. When blood pressure falls, the granular cells in the afferent arteriole of the juxtaglomerular apparatus (JGA) sense this decrease and respond by releasing renin. This enzyme is a key regulator in the renin-angiotensin-aldosterone system, which leads to an increase in blood pressure. Renin converts angiotensinogen, produced by the liver, into angiotensin I, which is subsequently converted into angiotensin II by the angiotensin-converting enzyme (ACE). Angiotensin II causes vasoconstriction, which raises blood pressure, and triggers the release of aldosterone from the adrenal cortex. Aldosterone promotes sodium reabsorption in the kidneys, which increases water retention and thus blood volume, further supporting a rise in blood pressure.