ALDOSTERONE
1. What is the function of aldosterone? How does aldosterone secretion assist in restoring blood volume?
2. Does aldosterone regulate ECF Na+ concentration?
It is noted in Figure 11-21 that angiotensin II production not only increases tubular Na+ reabsorption but also increases aldosterone secretion, which in turn also increases tubular Na+ reabsorption.
Aldosterone is a hormone of the adrenal cortex (see Chapter 6) and is more particularly involved with the regulation of K+ concentration in the ECF and promotes the secretion of K+. The mechanism of K+ secretion, however, involves the reabsorption of Na+. Therefore, the secretion of aldosterone in response to angiotensin II has as its function the reabsorption of Na+.Aldosterone regulates the K+ concentration of the ECF by its activity in the cortical collecting tubules and in medullary collecting ducts. In this regard, aldosterone is secreted in response to elevated K+ concentrations in the ECF. Although Na+ reabsorption is coupled with K+ secretion (not a 1:1 exchange), aldosterone is not involved with the regulation of Na+ concentration. The ADH thirst mechanism (osmoregulation) regulates the concentration of Na+ concentration in the ECF.
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