Regulation of Sodium and Potassium

The regulation of sodium and potassium ion concentrations in the human body is a complex process governed primarily by hormones such as aldosterone, antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP).

Sodium Regulation
Sodium ions make up approximately 90% of extracellular cations, with a normal blood plasma concentration of 136–148 mEq/L. A decrease in blood volume and pressure triggers the release of renin from granular cells in the juxtaglomerular complex (JGC), primarily in response to reduced renal perfusion, sympathetic nervous activation, or low sodium delivery to the macula densa. Renin initiates the formation of angiotensin II, which stimulates the adrenal cortex to secrete aldosterone.

Aldosterone increases sodium reabsorption in the kidneys, particularly in the distal convoluted tubules and collecting ducts. When aldosterone levels are high, the majority of sodium reabsorption in the distal convoluted tubules and collecting ducts is enhanced, leading to an increase in extracellular fluid volume as water follows sodium. Conversely, low aldosterone levels result in minimal sodium reabsorption beyond the distal convoluted tubule.

In conditions of high blood volume and pressure, the hormone atrial natriuretic peptide (ANP) is released. ANP promotes the excretion of sodium and water by inhibiting their reabsorption in the collecting ducts and suppressing the release of ADH, renin, and aldosterone. If plasma sodium concentration drops below 135 mEq/L, a condition called hyponatremia, the associated plasma hypo-osmolarity typically suppresses ADH release. This reduces water reabsorption, increasing water excretion and helping to restore normal sodium levels.

Potassium Regulation
Potassium ion concentration in blood plasma is normally maintained between 3.5 and 5.0 mEq/L. Potassium regulation is primarily controlled by aldosterone. Elevated potassium levels in the blood stimulate aldosterone secretion, prompting the renal collecting ducts to excrete excess potassium into the urine. Conversely, low potassium levels reduce aldosterone secretion, leading to decreased potassium excretion and conservation of potassium within the body.