What are the two major factors that regulate the movement of water and electrolytes from one fluid compartment to the next?

Summary

Electrolytes are minerals in your body that have an electric charge. They are in your blood, urine, tissues, and other body fluids. Electrolytes are important because they help:

• Balance the amount of water in your body
• Balance your body's acid/base (pH) level
• Move nutrients into your cells
• Move wastes out of your cells
• Make sure that your nerves, muscles, the heart, and the brain work the way they should

Sodium, calcium, potassium, chloride, phosphate, and magnesium are all electrolytes. You get them from the foods you eat and the fluids you drink.

The levels of electrolytes in your body can become too low or too high. This can happen when the amount of water in your body changes. The amount of water that you take in should equal the amount you lose. If something upsets this balance, you may have too little water (dehydration) or too much water (overhydration). Some medicines, vomiting, diarrhea, sweating, and liver or kidney problems can all upset your water balance.

Treatment helps you to manage the imbalance. It also involves identifying and treating what caused the imbalance.

Distribution of body water and sodium

To address the regulation of water and electrolyte balance, the processes which govern the distribution of body water must be considered. Body water constitutes approximately 60% of total body weight in adults (a higher proportion in infants and children).

Movement of water between compartments: osmotic pressure

The concept of osmotic pressure can be explained by considering two water-containing compartments, separated by a membrane that is permeable to water but not to solute. Random motion of water molecules results in movement across the membrane (diffusion). The presence of a solute on one side of the membrane decreases random movement on that side owing to intermolecular forces. There is, therefore, overall movement of water molecules into the solute-containing compartment (osmosis). This process

Fluid compartments

Body water is distributed between two principal compartments: intracellular and extracellular. Water is able to pass freely between these compartments and the distribution of water is therefore determined by osmotic pressure. The extracellular com-partment is subdivided into the interstitial fluid and the intravascular compartment (plasma).

Each of the compartments contains a principal solute, which is confined largely to that compartment and therefore acts as the main osmotic agent. Potassium

Osmoregulation

Osmoregulation can be considered an essential mechanism to maintain cell volume. This is well illustrated by considering the consequences of an abrupt rise or an abrupt fall in plasma osmolality. Rapid onset of severe hyponatraemia causes water to move into cells, which swell. Within the confined space of the skull, uncontrolled cerebral oedema results in seizures, coma and death. Conversely, rapid development of severe hypernatraemia causes cells to shrink with potential for permanent

Volume regulation

In contrast, volume regulation is brought about largely through changes in sodium excretion. As the principal solute within the extracellular compartment, sodium balance is intimately related to body water content.

Volume regulation is an essential requirement to maintain perfusion of tissues. Although osmoregulation is managed by a single sensory arm, volume regulation is governed by multiple receptors reflecting the potential for variation in perfusion of different regions of the vasculature.

Anti-diuretic hormone

ADH is a nine-amino-acid peptide that increases the permeability of the renal collecting ducts to water. Water is thus reabsorbed without salt, producing a more concentrated urine. ADH is the principal regulator of so-called free water excretion, with urine osmolality ranging between extremes of approximately 50 mOsmol/kg and 1200 mOsmol/kg under the control of ADH.

ADH is produced in the supraoptic and paraventricular nuclei of the hypothalamus, and then migrates along the axons of these

Thirst

Like ADH release, thirst can be stimulated independently by either hyperosmolality or hypovolaemia. Increased water intake driven by thirst, together with water preservation driven by ADH release, returns elevated osmolality to normal or, if it is volume driven, helps to correct volume depletion.

The response to ADH can take several hours to restore normality. It is, therefore, appropriate that thirst tends to be satisfied quickly by consumption of water but recurs in bursts. If this were not

Regulation of salt balance and effective circulating volume

Sodium is the principal solute acting to preserve water within the extracellular compartment. Total body water and extracellular volume are dependent on total body sodium. Consequently, maintenance of sodium balance is central to volume regulation.

Changes in sodium balance lead to changes in plasma volume and are sensed principally through changes in the circulation. It is, therefore, not surprising that the systems regulating sodium excretion are closely integrated with those regulating blood

Renin–angiotensin system

The RAS has several functions, but is principally concerned with maintenance of pressure and volume. The RAS plays a central role in regulation of salt excretion and thus in maintenance of extracellular fluid volume.

Renin is a proteolytic enzyme that is released from the juxtaglomerular cells in afferent arterioles of the kidney. It cleaves angiotensinogen to produce the decapeptide angiotensin I. This is converted enzymatically to an octapeptide, angiotensin II, primarily by

Aldosterone

Aldosterone, which is synthesized in the zona glomerulosa of the adrenal cortex, is a steroid hormone with mineralocorticoid activity. Unlike the other adrenal cortical hormones, aldosterone is not regulated by adrenocorticotropic hormone, but by the RAS and also by plasma potassium.

Natriuretic peptides

Sodium loading results in an appropriate increase in sodium excretion. Experiments inhibiting the effects of salt loading on glomerular filtration rate (GFR) and aldosterone secretion indicate the presence of additional factors promoting sodium excretion. The physiological role of the various natriuretic peptides is not known.

Pressure natriuresis

Changes in blood volume directly alter cardiac output and blood pressure. This results in increased renal excretion of sodium and water, independent of neural and humoral mechanisms. Pressure natriuresis might explain why patients with excessive inappropriate aldosterone secretion (primary hyperaldosteronism) are not usually severely volume overloaded.

The steady state

The discussions so far have dealt principally with responses of the regulatory systems for water and salt balance to acute perturbations. These systems must also be able to accommodate changes in salt intake. This is achieved by reaching a new steady state.

An abrupt and maintained increase in sodium intake causes extracellular volume to rise, owing to a rise in osmolality. This stimulates ADH secretion and thirst; a fall in renin secretion and aldosterone concentration and a rise in ANP ensue,

The response to diuretics

The concept of the steady state is helpful in examining the actions of diuretics and the timescale of the response. Diuretics decrease sodium reabsorption, resulting in increased excretion of sodium and water and a fall in extracellular volume.

This stimulates renin secretion, leading to an increase in circulating aldosterone and sodium reabsorption in the distal nephron. Thirst is also stimulated. A new steady state is achieved over the course of several days, after which net salt and water

Renal potassium handling

Most filtered potassium is reabsorbed in the proximal tubule. Only approximately 5% of the filtered load reaches the distal nephron, but this is the principal site at which potassium excretion is controlled.

Aldosterone has a major role in potassium balance, stimulating potassium secretion from the luminal membrane of the principal cells of the cortical collecting duct. ADH also stimulates distal tubular potassium excretion.

Magnesium balance

Bone is the main reservoir of magnesium, but there is almost no exchange with circulating magnesium. Regulation of magnesium balance is unusual in that no hormones influence magnesium excretion. Daily intake is approximately 15 mmol, of which only approximately 30% is absorbed. Because there is no appreciable exchange with bone stores, balance is maintained by renal excretion of the 5 mmol absorbed.

Unlike other electrolytes, filtered magnesium is reabsorbed principally in the thick ascending

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