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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.
By body fluid compartment
Volume contraction is more or less a loss of extracellular fluid (ECF) and/or intracellular fluid (ICF).
ECF volume contraction
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.
ICF volume contraction
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.
Dependence on lost solutes
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.