To calculate the intracellular fluid volume subtract the extracellular fluid volume from the total fluid volume. To calculate the interstitial (fluid not in the cells and not in the blood) fluid volume, subtract the plasma volume from the extracellular volume. To measure the extracellular fluid volume, use a cell inpermeant marker substance such as inulin or mannitol that will equilibrate everywhere except in the cells (it is possible to make inulin and mannitol radioactive). One such marker is Evan's Blue, a dye which binds to plasma proteins. Therefore, to measure the volume of the blood plasma fluid compartment, you need a marker which equilibrates throughout the blood supply and nowhere else. M U is usually calculated from C U, the concentration of marker lost in the urine and V U, the volume of the urine thus: M U = C U. Where V is the volume of the body fluid compartment, M is the mass of marker injected, M U is the mass of marker lost in the urine during equilibration and C is the measured concentration of the marker. As this is not possible (the kidney will excrete everything dissolved in the bloodstream) the calculation must correct for excretion. To be an absolutely perfect marker, the substance should also not be excreted. Therefore: if you know the mass of marker injected into the body and are able to measure the marker concentration once equilibration is complete, you can calculate the volume of the compartment occupied by the marker. Human beings are mostly water, ranging from about 75 percent of body mass in infants to about 5060 percent in adult men and women, to as low as 45 percent in old age. Given that concentration (C) = mass (M) / volume (V) it should be obvious that: Water content varies in different body organs and tissues, from as little as 8 percent in the teeth to as much as 85 percent in the brain. Furthermore, it must be possible to measure the concentration of the marker once equilibration is complete.Tritiated ( 3H) water is a good marker for the whole body fluid compartment because it diffuses throughout the body, it is chemically identical to normal water and it is easy to measure the equilibrium concentration because 3H water is radioactive. To be a perfect marker a substance must also not be metabolised. This water is distributed unevenly between the intracellular (33) and extracellular (27) compartments, with extracellular fluid further split between plasma volume, interstitial and lymph fluid, dense connective tissue and bone, and adipose tissue. To measure the volume of any fluid compartment within the body you must inject or infuse a marker substance that will equilibrate (diffuse freely to a uniform concentration) throughout this compartment. The volume of total body water is 60 in male and 50 in female patients, and declines with age. The methods that have been employed for the measurement of total body water may be considered under three general headings: desiccation procedures, body specific gravity measurements, and dilution techniques.ĭESICCATION The first estimates of body water content were accomplished with desiccation procedures.Body fluid compartments calculations Body Fluid compartments Measuring Body Fluid Compartments Cellular function itself is affected by abnormal fluctuations in water content. Water content varies in different body organs and tissues, from as little as 8 percent in the teeth to as much as 85 percent in the brain. It furnishes the means of transport of virtually all enzymes, substrates, metabolites, gases, electrolytes, antibodies, and other substances that must be moved to and from the cell. Water Content of the Body’s Organs and Tissues. Its presence is essential to the great mass of chemical reactions on which life depends. Water furnishes the preponderant single molecular element in the fabrication of tissue. This is not surprising when one pauses to consider the overwhelming importance of water in the body economy. A satisfactory method for the measurement of total body water in the living subject has long been sought.
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