Roles of Electrolytes: Calcium and Phosphate

Calcium and phosphate are essential electrolytes in the human body, with calcium being the most abundant mineral. Around 99% of the body's calcium is stored in the skeleton and teeth, forming a crystal lattice of mineral salts in combination with phosphates. Calcium plays crucial roles in various bodily functions such as blood clotting, neurotransmitter release, muscle tone maintenance, and nervous and muscle tissue excitability.

The calcium concentration in blood plasma is primarily regulated by Parathyroid Hormone (PTH). PTH gets released when the plasma levels of calcium decline. This hormone targets bones, kidneys, and the small intestine to increase calcium levels. In bones, PTH activates osteoclasts that break down the bone matrix, releasing calcium and phosphate ions into the blood. In kidneys, it boosts calcium reabsorption and decreases phosphate ion reabsorption. In the small intestine, PTH promotes calcium absorption indirectly by stimulating the kidneys to activate vitamin D. Activated vitamin D increases the absorption of calcium from the diet by the small intestine.

About 85% of the adult body's phosphate exists as calcium phosphate salts in bones and teeth. Of the remaining 15% , the majority is found in soft tissues as part of organic molecules and intracellular stores, while only about 1% is present in extracellular fluids, including ionized forms in blood plasma. Phosphates function as important intracellular anions and play a significant role as buffers in body fluids and urine. They are also covalently bound to organic molecules like lipids, proteins, carbohydrates, nucleic acids, and ATP.

PTH, calcitriol, and Fibroblast Growth Factor 23 (FGF 23) regulate the phosphate level in blood plasma. While PTH stimulates the resorption of bone extracellular matrix, releasing phosphate and calcium ions into the bloodstream, both PTH and FGF23 inhibit the reabsorption of phosphate ions in the kidneys. Calcitriol enhances the absorption of both phosphates and calcium from the gastrointestinal tract. FGF23 primarily decreases phosphate levels in the blood by increasing its excretion by the kidneys. Additionally, FGF23 reduces intestinal phosphate absorption indirectly by suppressing calcitriol production, which in turn lowers gastrointestinal phosphate transport.