The kidneys regulate the blood concentration of Na+,K+, HCO3-, and H+. Aldosterone stimulates the reabsorption of Na+ in exchange for K+ in the tubule. Aldosterone thus promotes the renal retention of Na+ and the excretion of K+. Secretion of aldosterone from the adrenal cortex is stimulated directly by a high blood K+ concentration and indirectly by a low Na+ concentration via the renin-angiotensin system.
The kidneys help to regulate the concentrations of plasma electrolytes—sodium, potassium, chloride, bicarbonate, and phosphate—by matching the urinary excretion of these compounds to the amounts ingested. The control of plasma Na+ is important in the regulation of blood volume and pressure; the control of plasma K+ is required to maintain proper function of cardiac and skeletal muscles.
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