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.^[1]

By body fluid compartment

Volume contraction is more or less a loss of

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.^[2][3] 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.^{[citation needed]} Volume contraction is sometimes even used synonymously with hypovolemia.^{[citation needed]}

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.

Estimation

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.

1. To calculate an estimation, the total amount of substance in the body before the loss is first estimated:

${\displaystyle n_{b}=Osm_{b}\times TBW_{b}}$

where:

• n_b = Total amount of substance before fluid loss
• Osm_b = Body
  osmolarity before loss (almost equal to plasma osmolality of 275-299 milli-osmoles per kilogram^[4]
  )
• TBW_b =
  Total body water
  before loss (approximately 60% of body weight, or using tritiated water or deuterium)

• V_{ICF a} = Intracellular fluid volume after fluid loss
• n_{ICF a} = Amount of substance in ICF after fluid loss
• Osm_a = Body osmolarity after loss
• V_{ICF b} = Intracellular fluid volume before fluid loss (approximately 40% of body weight, or subtracting ECF from TBW)
• Osm_b = Body
  osmolarity before loss (almost equal to plasma osmolality of 275-299 milli-osmoles per kilogram^[4]
  )
• n_{lost K+} = Amount of substance of lost potassium

• V_{ECF a} = Extracellular fluid volume after fluid loss
• n_{ECF a} = Amount of substance in ECF after fluid loss
• V_{ECF b} = Extracellular fluid volume before fluid loss (approximately 20% of body weight, or by using inulin)
• Osm_b = Body
  osmolarity before loss (almost equal to plasma osmolality of 275-299 milli-osmoles per kilogram^[4]
  )
• n_{lost K+} = Amount of substance of lost potassium

• Contraction alkalosis, the increase in blood pH that occurs as a result of fluid losses

References