Measurement of Total Body Water. Radioactive water (tritium, 3H2O) or heavy water (deuterium, 2H2O) can be used to measure total body water. These forms of water mix with the total body water within a few hours after being injected into the blood, and the dilution principle can be used to calculate total body water (Table 1). Another substance that has been used to measure total body water is antipyrine, which is very lipid soluble and can rapidly penetrate cell membranes and distribute itself uniformly throughout the intracellular and extracellular compartments.
Table1. Measurement of Body Fluid Volumes
Measurement of Extracellular Fluid Volume. The volume of extracellular fluid can be estimated using any of several substances that disperse in the plasma and interstitial fluid but do not readily permeate the cell mem brane. They include radioactive sodium, radioactive chloride, radioactive iothalamate, thiosulfate ion, and inulin. When any one of these substances is injected into the blood, it usually disperses almost completely throughout the extracellular fluid within 30 to 60 minutes. Some of these substances, however, such as radioactive sodium, may diffuse into the cells in small amounts. Therefore, one frequently speaks of the sodium space or the inulin space, instead of calling the measurement the true extracellular fluid volume.
Calculation of Intracellular Volume. The intracellular volume cannot be measured directly. However, it can be calculated as
Intracellular volume = Total body - water Extracellular volume
Measurement of Plasma Volume. To measure plasma volume, a substance must be used that does not readily penetrate capillary membranes but remains in the vascular system after injection. One of the most commonly used substances for measuring plasma volume is serum albumin labeled with radioactive iodine (125Ialbumin). Also, dyes that avidly bind to the plasma proteins, such as Evans blue dye (also called T-1824), can be used to measure plasma volume.
Calculation of Interstitial Fluid Volume. Interstitial fluid volume cannot be measured directly, but it can be calculated as
Interstitial fluid volume = Extracellular fluid volume - Plasma volume
Measurement of Blood Volume. If one measures plasma volume using the methods described earlier, blood volume can also be calculated if one knows the hematocrit (the fraction of the total blood volume composed of cells), using the following equation:
For example, if plasma volume is 3 liters and hematocrit is 0.40, total blood volume would be calculated as
Another way to measure blood volume is to inject into the circulation red blood cells that have been labeled with radioactive material. After these mix in the circulation, the radioactivity of a mixed blood sample can be measured and the total blood volume can be calculated using the indicator-dilution principle. A substance frequently used to label the red blood cells is radioactive chromium (51Cr), which binds tightly with the red blood cells.
