Regulation of Sodium and Potassium

Four key hormones regulate blood sodium: aldosterone from the adrenal cortex stimulates sodium reabsorption in response to low blood volume or pressure; atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) promote sodium excretion during high blood volume; and ADH indirectly regulates sodium by increasing water reabsorption, which reduces plasma sodium concentration.

Aldosterone is the primary regulator of potassium in blood plasma. High potassium levels trigger aldosterone secretion, which stimulates the renal collecting duct to excrete excess potassium into urine. Conversely, low potassium levels reduce aldosterone secretion, decreasing potassium excretion and conserving potassium within the body.

When plasma sodium concentration drops below 135 mEq/L, a condition called hyponatremia, ADH release is inhibited. This reduces water reabsorption in the kidneys, increasing water excretion and helping restore normal plasma osmolarity and sodium concentration to healthy levels throughout the body.

In response to high blood volume and pressure, atrial cardiac cells release atrial natriuretic peptide (ANP) and ventricular cells release B-type natriuretic peptide (BNP). Both peptides aid sodium and water excretion by inhibiting their reabsorption in the collecting ducts and suppressing aldosterone, renin, and ADH release.

Aldosterone is released from the adrenal cortex in response to low blood volume, low blood pressure, low sodium levels, or high potassium levels. Once released, aldosterone stimulates active reabsorption of sodium ions in the kidneys, followed by water reabsorption, which increases extracellular fluid volume.

ADH indirectly regulates sodium concentration by increasing water reabsorption in the kidneys. When more water is reabsorbed, plasma volume increases and plasma osmolarity decreases, which dilutes sodium concentration. This mechanism helps maintain the balance between water and sodium in the blood.

Normal blood plasma sodium concentration ranges from 136 to 148 mEq/L, with sodium making up approximately 90% of extracellular cations. Potassium ion concentration in blood plasma is normally maintained between 3.5 and 5.0 mEq/L, with aldosterone as its primary regulator.