Final answer:
Low cardiac output due to heart failure causes the kidneys to release renin, which leads to a series of reactions resulting in increased blood pressure and volume to compensate for the initial perceived decrease.
Step-by-step explanation:
The increased secretion of renin in heart failure is primarily caused by low cardiac output. In such a condition, the kidneys receive less blood due to the weakened pumping action of the heart. This reduced renal blood flow is misinterpreted as a decrease in blood volume or blood pressure, triggering the release of renin. Renin is not released from the liver but rather from the kidneys, specifically from cells known as the juxtaglomerular cells. Furthermore, renin does not act directly on blood pressure or volume; it initiates a biochemical cascade that involves the conversion of angiotensinogen to angiotensin I, which is then converted to angiotensin II by ACE (Angiotensin-Converting Enzyme). Angiotensin II increases blood pressure and volume by prompting vasoconstriction and stimulating the release of aldosterone and antidiuretic hormone (ADH), which prompts the kidneys to reabsorb sodium and reduce urine output, thereby conserving water.