As stated above, the mineralocorticoids control the balance of potassium,
sodium, and chloride in the body. The cortex secretes several different types of
mineralocorticoids. The principal mineralocorticoid is aldosterone, since aldosterone is
responsible for over 90 percent of the total mineralocorticoid activity.
a. Physiological Actions of Aldosterone. Aldosterone acts to increase the
amount of sodium reabsorbed by the renal tubular epithelium. That is, the more
aldosterone secreted by the cortex, the more sodium that is reabsorbed into the blood.
Conversely, when extremely small amounts of aldosterone are secreted, very small
amounts of sodium are reabsorbed into the blood and passed out of the body in the
urine. Such control of sodium is crucial to the physiological balance in the body.
Remember that sodium is the primary electrolyte in extracellular fluid. If there is too
little sodium reabsorbed into the blood, the volume of extracellular fluid (and circulating
blood volume) in the body could decrease to levels that could injure the body.
Therefore, aldosterone helps to control the level of sodium in the body. In addition,
aldosterone helps to decrease the amount of potassium reabsorbed (and thus increases
the amount of potassium removed from the body in the urine) and increase the amount
of chloride reabsorbed into the blood. To summarize, aldosterone helps to increase the
amount of sodium and chloride present in the extracellular fluid and to decrease the
amount of potassium present in the extracellular fluid.
b. Hyposecretion of Mineralocorticoids. As stated, hyposecretion of
aldosterone can result in a lack of water in extracellular fluid (due to decreased amounts
of sodium in the extracellular fluid). This can lead to decreased blood volume that can
result in decreased cardiac output and hypotension.
c. Hypersecretion of Mineralocorticoids. Hypersecretion of
mineralocorticoids (aldosterone) can lead to increased sodium reabsorption. This can
also lead to decreased reabsorption of potassium into the blood. Ultimately,
hypersecretion of aldosterone can result in an increased volume of extracellular fluid,
which leads to increased volume of blood. This can increase cardiac output, ultimately
resulting in hypertension.
THE GLUCOCORTICOIDS (HYDROCORTISONE (CORTISOL) AND OTHERS)
As stated previously, the glucocorticoids affect the metabolism of fat, glucose,
and protein in the body. The principal glucocorticoid is hydrocortisone (cortisol).