Hypoaldosteronism

- The lack of aldosterone has three direct effects on the kidney; decreased Na+ reabsorption, decreased K+ secretion, and decreased H+ secretion.  As a result, there is ECF volume contraction (caused by decreased Na+ reabsorption), hyperkalemia (caused by decreased K+ secretion), and metabolic acidosis (caused by decreased H+ secretion).
- The ECF volume contraction is responsible for orthostatic hypotension.  The decreased arterial pressure produces an increased pulse rate via the baroreceptor mechanism.
- ECF volume contraction also stimulates ADH secretion from the posterior pituitary via volume receptors.  ADH causes increased water reabsorption from the collecting ducts, which results in decreased serum [Na+] (hyponatremia) and decreased serum osmolarity.  Thus, ADH released by a volume mechanism is "inappropriate" for the serum osmolarity in this case.
- Hyperpigmentation is caused by adrenal insufficiency.  Decreased levels of cortisol produce increased secretion of adrenocorticotropic hormone (ACTH) by negative feedback.  ACTH has pigmenting effects similar to those of melanocyte-stimulating hormone.