Distribution

Body Water Compartments

The water in the human body is said to be distributed in different compartments. There are two major compartments: intracellular fluid (ICF) and extracellular fluid (ECF).

This is called the 60-40-20 rule. ICF is 2/3 of total body water, and since total body water is approximately 60% of body weight,

ICF is$\left(\frac{2}{3}\cdot60\%\ \right)=\ 40\%$ of body weight. Similarly, ECF is $\frac{1}{3}\cdot60\%\ =20\%$ of body weight.

Know that these compartments are dynamically interacting and water is exchanged freely between them, usually through some "barrier". In this case, ICF and ECF are separated by cellular membranes.

The ECF compartment can be further subdivided into interstitial fluid (ISF) (3/4 of ECF) and plasma (1/4 of ECF). That is, ISF is $\frac{3}{4}\cdot20\%\ =\ 15\%$of body weight, and plasma is the remaining 5%.

Plasma is the aqueous component of blood: the fluid in which the blood cells are suspended. Typically, plasma constitutes 55% of blood volume, whereas the remaining 45% is composed on red blood cells (majority), white blood cells and platelets. When whole blood is centrifuged, plasma can be seen clearly as the yellowish fluid on top. Serum is plasma minus clotting factors.

The percent of blood volume occupied by red blood cells is called hematocrit. Normal ranges can be seen above and vary between men and women, with males having higher hematocrit values on average.

Interstitial fluid is considered an ultrafiltrate of plasma: that is, plasma minus plasma proteins and blood cells. This is because ISF is separated from plasma by the capillary walls: this is the barrier that separates ISF and plasma. Capillary walls essentially filter the plasma retaining proteins and blood cells in circulation.

There are yet another two minor subcompartments of ECF: transcellular fluid and lymph.

Transcellular fluid is a collection of small fluid volumes secreted by cells into certain body cavities for a specific function. Examples: pleural and pericardial fluid (lungs and heart), peritoneal (abdominal cavity), synovial (joints), intraocular (eyes), etc.

Lymph is the ISF that gets into the lymphatic system. After the plasma is filtered by the capillaries into ISF, this fluid has to go somewhere to be returned to the circulation: this is lymph. The lymphatic system is a network of blind tubules that coalesce into larger vessels and finally return lymph into the circulation.

Indicator Dilution Method

Method to determine compartment volumes based on a known amount of a test substance introduced and its measure concentration from blood. The volume is determined by using $V\ =\frac{Q}{c}$, where V is the compartment volume, Q is the known quantity of indicator and c is its concentration in plasma. Different indicators should be used for different volumes of interest:

1. Total body water: Antipyrine, $D_2O$ or $T_2O$
2. ECF: Inulin, sucrose or mannitol
3. Plasma: Evan's Blue or $I^{131}$-Albumin

Remember: Total Body Water = ICF + ECF = ICF + (ISF + Plasma)