Sodium Homeostasis

Sodium (Electrolyte) Balance – Physiologists often refer to sodium as the backbone of the extracellular fluid and emphasize that water follows sodium. Although separate hormonal axes exist for water and sodium reuptake, sodium plays a crucial role in maintaining intravascular fluid volume in addition to contributing to serum osmolality. As will be explained, the kidneys conserve much of the water (a.k.a. plasma volume) in the glomerular filtrate by retaining sodium.

Serum sodium concentrations must be maintained between 130-145 mEq/mL. At levels below 130 mEq (hyponatremia), the CNS becomes swollen with water; cerebral edema leads to seizure, coma and death if not corrected. At levels above 145 mEq (hypernatremia), nervous system function is disrupted along with cardiac and renal functions. To avoid these extremes, the body has developed an entire hormonal axis devoted exclusively to sodium homeostasis. The kidney is the primary organ dedicated to sodium homeostasis; in most scenarios this means sodium retention. Between the glomeruli and distal convoluted tubule (DCT) in each nephron, a sensor called the juxtaglomerular apparatus (JGA) measures the flow rate

As Angiotensin I flows through the renal and pulmonary circulations, a second enzyme called Angiotensin Converting Enzyme (ACE) cleaves Angiotensin I into Angiotensin II. Angiotensin II acts in three ways to conserve ECF volume. First, AT-II is a powerful vasoconstrictor. AT-II constricts the renal arteries and arterioles in order to increase perfusion pressure in the renal cortex where most glomeruli are located. Second, AT-II crosses into 2 areas of brain lacking the blood-brain barrier (the SFO- Subfornical Organ and OVLT – Vascular Organ of the Lamina Terminalis) to trigger the sensation of thirst. Third, AT-II travels to the adrenal cortex, where it stimulates zona glomerulosa cells to release