Final answer:
The macula densa secretes renin, which initiates a cascade that ultimately increases blood pressure and promotes water retention through the actions of angiotensin II and aldosterone. ADH assists in water retention but is not secreted by the macula densa.
Step-by-step explanation:
The macula densa in mammalian nephrons secretes renin, not aldosterone, ADH, water, or sodium. Renin, which is secreted by part of the juxtaglomerular complex, acts on angiotensinogen from the liver to form angiotensin I, which is then converted to angiotensin II by the enzyme ACE (angiotensin-converting enzyme). Angiotensin II is a potent vasoconstrictor that increases blood pressure and stimulates the release of aldosterone from the adrenal cortex. Aldosterone causes the kidneys to reabsorb sodium, which leads to water retention and hence an increase in blood pressure. Although ADH, or antidiuretic hormone, also plays a role in water regulation by promoting water reabsorption in the kidneys, it is not secreted by the macula densa.
ADH works by increasing the permeability of the kidney's collecting ducts to water, thereby facilitating water reabsorption. This is achieved through the stimulation of aquaporin water channels, which are inserted into the cell membranes when ADH is present. The released aldosterone further assists in water retention by prompting the kidneys to reabsorb sodium. The mechanism involving renin, angiotensinogen, angiotensin II, aldosterone, and ADH is essential for maintaining both blood pressure and volume in the body.