Volume contraction

Volume contraction is a decrease in the volume of body fluid, including the dissolved substances that maintain osmotic balance. 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 and/or intracellular fluid.

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 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 and potassium 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:
where:

n_b = Total amount of substance before fluid loss
Osm_b = Body osmolarity before loss
TBW_b = Total body water before loss

2. The total amount of substance in the body after the loss is then estimated:
where:

n_a = Total amount of substance after fluid loss
n_b = Total amount of substance before fluid loss
n_{lost Na+} = Amount of substance of lost sodium
n_{lost K+} = Amount of substance of lost potassium

3. The new osmolarity becomes:
where:

Osm_a = Body osmolarity after loss
n_a = Total amount of substance after fluid loss
TBW_b = Total body water before loss
V_lost = Volume of lost fluid

4. This osmolarity is evenly distributed in the body, and is used to estimate the new volumes of ICF and ECF, respectively:
where:

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
Osm_b = Body osmolarity before loss
n_{lost K+} = Amount of substance of lost potassium

In homologous manner:
where:

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
Osm_b = Body osmolarity before loss
n_{lost K+} = Amount of substance of lost potassium

5. The volume of lost fluid from each compartment:
where:

V_{I/ECF b} = Intra/Extra-cellular fluid volume before fluid loss
V_{I/ECF a} = Intra/Extra-cellular fluid volume after fluid loss