Volume contraction

Volume contraction is a decrease in the volume of body fluid, including the dissolved substances that maintain osmotic balance (osmolytes). The loss of the water component of body fluid is specifically termed dehydration.

Volume contraction is more or less a loss of extracellular fluid (ECF) and/or intracellular fluid (ICF).

Volume contraction of extracellular fluid is directly coupled to and almost proportional to volume contraction of blood plasma, which is termed hypovolemia. Thus, it primarily affects the circulatory system, potentially causing hypovolemic shock.

ECF volume contraction or hypovolemia is usually the type of volume contraction of primary concern in emergency, since ECF is approximately half the volume of ICF and is the first to be affected in e.g. bleeding. Volume contraction is sometimes even used synonymously with hypovolemia.

Volume contraction of intracellular fluid may occur after substantial fluid loss, since it is much larger than ECF volume, or loss of potassium (K⁺) see section below.

ICF volume contraction may cause disturbances in various organs throughout the body.

Na⁺ loss approximately correlates with fluid loss from ECF, since Na⁺ has a much higher concentration in ECF than ICF. In contrast, K⁺ has a much higher concentration in ICF than ECF, and therefore its loss rather correlates with fluid loss from ICF, since K⁺ loss from ECF causes the K⁺ in ICF to diffuse out of the cells, dragging water with it by osmosis.

When the body loses fluids, the amount lost from ICF and ECF, respectively, can be estimated by measuring volume and amount of substance of sodium (Na⁺) and potassium (K⁺) in the lost fluid, as well as estimating the body composition of the person.

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