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Body fluid is body water and its dissolved solutes ¹.

Compartments of body fluid

Fluid found within the cells is called intracellular fluid (ICF) and that found outside cells is called extracellular fluid (ECF). The extracellular fluid is further divided into that which is found as blood plasma within blood vessels, and that which is found in the microscopic spaced between cells called interstitial fluid.

Approximately 2/3 of body fluid is intracellular and 1/3 is extracellular. Of the ECF approximately 80% is interstitial fluid and 20% is blood plasma¹.

There are some special fluid and compartments including: lymph; cerebrospinal fluid; synovial; aqueous humour/vitreous body of the eyes; endolymph/perilymph in the ears; pleural, pericardial and peritoneal fluid between serous membranes; and glomerular filtrate in the kidneys.

Selectively permeable membranes separate body fluids into distinct compartments. Plasma membranes of individual cells separate ICF from ECF and blood vessel walls separate blood plasma from interstitial fluid.

The major components of these fluids include water and solutes. The solute is mostly comprised of electrolytes: inorganic compounds that dissociate into ions. Electrolytes include cations (positively charged atoms – sodium, potassium, calcium, magnesium, carbonate) and anions (negatively charged atoms – chloride, sulphide, phosphate, bicarbonate).

Regulation of Fluid Gain

The term fluid balance defines the state where a body’s required amount of water is present and proportioned normally among the various compartments; this state is inseparable from electrolyte balance.

Under normal conditions water loss equals water gain and a body’s water volume remains constant. Avenues for water loss include the kidneys, skin, lungs, feces, and menstruation. Water is sourced mostly from dietary intake; this is called preformed water. However, metabolic processes such as cellular respiration and dehydration synthesis reactions generate a small component. Approximately 1.6 litres per day is sourced from ingested water, 0.7 litres per day from ingested food, and 0.2 litres from metabolic processes.

Water is not produced by the body to maintain homeostasis; metabolic water production is simply a by-product of cellular respiration. The body regulates water intake via the thirst reflex which stimulates us to drink. When water loss is greater than water gain the body reaches a state of dehydration, and dehydration stimulates the thirst reflex in three ways:

1. the level of saliva drops resulting in a dry mucosa in the mouth and pharynx;
2. there is an increase in blood osmotic pressure which stimulates osmoreceptors in the hypothalamus;
3. there is a drop in blood volume, which leads to the renin/angiotensinII pathway stimulating the thirst centre in the hypothalamus.

All three mechanisms stimulate the thirst centre in the hypothalamus resulting in the sensation of thirst and causing us to drink to increase fluid volume. Drinking inhibits the thirst centre by stretching the stomach and intestines and reducing the osmotic pressure of the blood ¹.

Typically, if the thirst centre is activated dehydration has already occurred to some extent, and it is noted that this reflex is not always reliable in young children, the elderly, or those in a confused mental state ¹.

Regulation of Fluid Loss

If fluid loss regulation is considered as the maintenance of the volume of fluid already present in the body, then three hormones play a key role: Antidiuretic Hormone (ADH), Aldosterone, and Atrial Natriuretic Peptide (ANP).

A drop in body fluid results in an increase in blood tonicity and a decrease in blood volume which in turn causes the release of renin in the kidneys and stimulation of osmoreceptors in the hypothalamus. The hypothalamus causes ADH to be released from the posterior pituitary gland. ADH targets the kidneys, and sudoriferous glands reducing fluid loss through each, and it also causes arterioles to constrict. Renin is released by the juxtaglomerular cells in the kidneys, it acts on angiotensinogen a plasma protein produced in the liver to form angiotensin I. Angiotensin I is converted in the lungs to the active hormone angiotensin II by the imaginatively named angiotensin converting enzyme (ACE). Angiotensin II causes: vasoconstriction of the arterioles; stimulation of the release of aldosterone by the adrenal cortex; stimulation of the thirst centre in the hypothalamus; and stimulation of the release of more ADH. Aldosterone increases the retention of sodium and chloride ions as well as water by the kidneys. These factors combined result in an increase in the level of body fluid and an increase in blood pressure.

On the other hand an increase in body fluid results in an increase in blood volume. This causes stretching of the right atrium, which stimulates the release of ANP a hormone that increases the loss of fluid in the urine. At the same time the level of ADH and renin is decreased under negative feedback.

Other factors that control fluid loss include:

• severe dehydration which results in a decrease in blood pressure and a decrease in the glomerular filtration rate with a resultant decrease in the loss of water in urine;

• water overload will increase the blood pressure and the glomerular filtration rate resulting in an increased loss of water in the urine;

• hyperventilation increases fluid loss via water vapour in the lungs;

• vomiting and diarrhea increase fluid loss via the GIT;

• fever, heavy perspiration, and skin loss (burns) increase fluid loss via the skin.

Movement of body fluids

Fluid moves between the compartments of the body through various mechanisms.

Substances leave and enter capillaries via three mechanisms: vesicular transport, diffusion, and bulk flow. Vesicular transport and diffusion are associated with the movement of solutes whereas bulk flow is the most important process for the maintenance of relative volumes of blood and interstitial fluid¹.

Bulk flow is the movement of both solvent and solute into the interstitial space. Pressures acting to move substances out of the capillary include blood hydrostatic pressure (BHP) and interstitial fluid osmotic pressure (IFOP). Blood colloid osmotic pressure (BCOP) and interstitial fluid hydrostatic pressure act to push substances into the capillary. At the arterial end of the capillary the sum of the outward moving pressures is dominant and substances move into the interstitial fluid (filtration). At the venous end the inward pressure is dominant and the substances move into the capillary (reabsorption).

The exchange of interstitial and intracellular fluid is controlled mainly by the presence of the electrolytes sodium and potassium. Potassium is the chief intracellular cation and sodium the chief extracellular cation. Because the osmotic pressure of the interstitial space and the ICF are generally equal water typically does not enter or leave the cell. A change in the concentration of either electrolyte will cause water to move into or out of the cell via osmosis. A drop in potassium will cause fluid to leave the cell whilst a drop in sodium will cause fluid to enter the cell. Aldosterone, ANP and ADH regulate sodium levels within the body, whilst aldosterone can be said to regulate potassium.

Electrolytes

Fluid balance is implicitly linked to electrolyte balance. Electrolytes establish osmotic pressure and are largely responsible for the movement of fluids. Five of the main electrolytes found in the body are summarised in table 1 below.

Table 1 Five key electrolytes found in the body.

References

1.) Tortora, G.J., Grabowski, S.R., Principles of Anatomy and Physiology - 8^th Edition, Harper Collins, NY, 1996.