Electrolytes

Electrolytes

An electrolyte in any chemical that dissociates to ions when dissolved in a solution. Ions can be positively charged (cations) or negatively charged (anions). The major electrolytes found in the body are sodium, potassium, calcium, magnesium, chloride, phosphate, sulphate and bicarbonate. The primary electrolytes of the ECF are sodium, chloride and bicarbonate and in the ICF are potassium and phosphates.

1. Sodium

Sodium is the major cation of ECF. About 45% of sodium stored is found in ECF, 45% in the bone and the remaining intracelullarly. It plays an important role in the excitability of muscles and neurons and in regulating the fluid balance in the body.

A constant sodium equilibrium in maintained in the ECF by two mechanisms-long term and short term control. The short term control is achieved by the ADH- thirst control system whereas long term control is by both ingestion and urinary excretion of sodium.

Short term Control (ADH-Thirst Control System)

If ECF sodium rises, it stimulates the release of ECF and thirst. Water gained by this mechanism, dilutes the ECF thereby restoring normal sodium level. This increases the blood volume and a slight increase in BP, which in turn increases the GFR and excretion of excess sodium and water there by restoring the ECF volume to its normal level.

Long term Control (Salt hunger/ Sodium ingestion)

A deficiency of sodium in the ECF causes increased excretion of water due to inhibition of the release of ADH. A decline in osmolality decreases the GFR with a subsequent increase in reabsorption of sodium and water. Many sodium deficient animals have a strong behavioural drive to salt to replace the deficiency of sodium. This is called as salt hunger and is commonly seen in ruminants.

An increase in angiotensin II stimulates the salt hunger. A decrease in the plasma sodium and BP stimulates the release of aldosterone from the adrenal cortex via the Renin- Angiotensin system. Aldosterone enhances the reabsorption of sodium from the renal tubule. Aldosterone also increases the secretion of potassium ions from the renal tubules into the lumen.

Urinary excretion of Sodium

Sodium is filtered at the glomerulus and most of it is reabsorbed in the renal tubules. If the plasma sodium or the GFR increases, the amount of sodium filtered into the tubule will also increase but the reabsorption of sodium will not increase.

An increase in the arterial BP results in the release of Atrial Natriuretic factor (ANF) from the atria of the heart. This hormone inhibits renin and aldosterone release which results in increased excretion of sodium and inhibition of reabsorption of sodium. Along with sodium, water will also be lost thereby bringing the BP back to normal level.

2. Potassium

Potassium is the major cation of intra cellular fluid and about 89% of the total body content of potassium is present in the ICF. Potassium is important for the functioning of excitable cells and in the regulation of fluid levels with in  the cells.

Potassium output in usually equal to the potassium input. Almost nearly all the potassium filtered by the kidney glomeruli is reabsorbed by proximal convoluted tubule and is secreted in the distal tubule and the collecting ducts. Potassium reabsorption by the tubular cells occurs by active transport of potassium into the cells in exchange for sodium through the Na/K ATPase system.

Potassium concentration is regulated by the aldosterone. An increase in ECF potassium results in the aldosterone release, which increases the potassium excretion in the urine and returning of the plasma potassium to its normal level. Potassium is secreted in exchange for the sodium ion in the tubular cells. Hydrogen ion secretion competes with K⁺ for Na⁺ with which it exchanges. Therefore,  an increased H⁺ secretion depresses K⁺ secretion. Likewise an increased reabsorption of Na⁺ facilitates increased K⁺ secretion. An excess of aldosterone results in hypokalemia whereas deficiency of aldosterone causes hyperkalemia.

3. Chloride and bicarbonate

The sodium ions are balanced electrically with the chloride and bicarbonate ions. The chloride ions are regulated secondarily to sodium and bicarbonate ions.

The excretion or reabsorption of sodium ions is accompanied by chloride ions. Similarily, chloride ions are excreted along with the bicarbonate ions to maintain electroneutrality in the ECF. The bicarbonate ion is unique in that it is formed or removed rapidly by the body. Bicarbonates are formed and removed by carbon dioxide.