Physiology of electrolytes in the body

2021-06-17 04:16 PM

ADH increases water reabsorption in the distal tubule and collecting duct. When blood Na+ is below 135 mEq/l, the posterior pituitary gland stops secreting ADH, causing a large amount of dilute urine to be excreted.

Sodium

Normal plasma Na+ concentration is 136-142 mEq/l. Na+ plays a major role in water and electrolyte balance and is an essential ion to conduct impulses in nerve and muscle organizations. Na+ levels are controlled by aldosterone, ADH and ANP.

Aldosterone acts on the distal tubule and collecting duct of the renal unit to increase Na+ reabsorption. When Na+ moves from the filtrate back into the blood, it creates an osmotic gradient that causes water to follow. Aldosterone is secreted when blood volume or cardiac output decreases, extracellular Na+ decreases, and extracellular K+ increases.

ADH increases water reabsorption in the distal tubule and collecting duct. When blood Na+ is below 135 mEq/l, the posterior pituitary gland stops secreting ADH, causing a large amount of dilute urine to be excreted.

ANP increases the glomerular filtration rate and decreases Na+ reabsorption in the collecting tubules.

Chloride

Normal plasma Cl- concentration is 95-103 mEq/l. Cl- can help balance anion concentrations between different body fluid compartments.

Aldosterone indirectly regulates Cl- balance in body fluids, as it regulates Na+ reabsorption in the distal tubule. In many cases, Cl- passively follows Na+ due to charge attraction.

Potassium

The K+ ion is the most abundant cation in the intracellular fluid. K+ plays a key role in establishing the resting membrane potential and in the repolarization phase of the action potential in nerve and muscle tissue. K+ also plays a role in maintaining intracellular fluid volume. When K+ is swapped with H+, it helps to regulate pH.

Normal plasma K+ concentration is 3.8-5.0 mEq/l. This concentration is controlled mainly by aldosterone. When plasma K+ is elevated, more aldosterone is secreted into the blood. Aldosterone will stimulate the secretion of K+ into the urine to increase the amount of K+ out of the body. When the plasma K+ concentration is low, the opposite occurs.

Bicarbonate

The HCO3- ion is the second most abundant anion in the extracellular fluid. Normal plasma HCO3- concentrations are 22-26 mEq/l in the arteries and 19-24 mEq/l in the veins. The Cl- for HCO3- swap helps to maintain the correct extracellular and intracellular anion balance.

The kidney is the major regulator of HCO3- concentration in the blood. The kidneys can form HCO3- and release it into the bloodstream when HCO3- is low or excrete a lot of HCO3- into the urine when it's too high.

Calcium

About 98% of adult calcium is in bones (and teeth), where it works with phosphate to form a crystal lattice of mineral salts. The normal total calcium concentration in plasma is about 5mEq/l. Of this, about 50% (2.4-2.5 mEq/l) exists in the ionized form, an amount about 40% is in the conjugated form with plasma proteins, and about 10% is in the conjugated form of phosphate or citrate. . In addition to regulating the strength of bones and teeth, calcium plays an important role in blood clotting, neurotransmitter release, maintenance of muscle tone, and nerve and muscle excitability.

Plasma calcium concentrations are mainly regulated by the following two hormones:

Parathyroid hormone (PTH): released when plasma Ca2+ concentration is low. PTH stimulates osteoclasts in the bone to release calcium (and phosphate) from the mineral salts of the bone matrix. PTH also increases Ca2+ absorption from the gastrointestinal tract and promotes Ca2+ reabsorption from the glomerular filtrate.

Calcitonin: is released by the thyroid gland when the plasma Ca2+ concentration is high. It reduces Ca2+ by stimulating osteoclast activity and inhibiting osteoclast activity.

Phosphate

About 85% of adult phosphate is present in calcium phosphate salts. The remaining 15% is ionized (H2PO4-, HPO42-, and PO43-). Most phosphate ions are in the conjugated form. At normal pH, HPO42- is the most common form. H2PO4- and HPO42- both play important roles in the buffer reaction.

Normal plasma concentrations of ionized phosphate are only 1.7-2.6 mEq/l. The main mechanism for the regulation of phosphate concentration is the mechanism of phosphate transport in the rental unit. PTH also plays a role in the regulation of phosphate levels.    

Magnesium

In adults, about 54% of body magnesium is deposited in the bone matrix as magnesium salts. The remaining 46% is in the magnesium ion form of intracellular fluid (45%) and extracellular fluid (1%). Mg2+ is the second most abundant intracellular cation after K+. Functionally, Mg2+ is a cofactor of enzymes involved in carbohydrate and protein metabolism and Na+/K+ ATPase (Na+ pumping enzyme). Mg2+ is also important in neuromuscular activity, impulse conduction, and myocardial function.

The normal concentration of Mg2+ in plasma is only 1.3-2.1 mEq/l. Many factors regulate blood levels of Mg2+ by varying the rate at which it is excreted in the urine. The kidneys increase Mg2+ excretion in the presence of elevated blood Ca2+, increased blood Mg2+, increased extracellular fluid volume, decreased PTH, and acidosis. The opposite conditions will reduce Mg2+ excretion.