COMPOSITION OF BODY FLUIDS

COMPOSITION OF BODY FLUIDS

Body fluids contain water and solids. Solids are organic and inorganic substances.

ORGANIC SUBSTANCES

Organic substances are glucose, amino acids and other proteins, fatty acids and other lipids, hormones and enzymes.

INORGANIC SUBSTANCES

Inorganic substances present in body fluids are sodium, potassium, calcium, magnesium, chloride, bicarbonate, phosphate and sulfate. ECF contains large quantity of sodium, chloride, bicarbonate, glucose, fatty acids and oxygen. ICF contains large quantities of potassium, magnesium, phosphates, sulfates and proteins. The pH of ECF is 7.4. The pH of ICF is 7.0.

MEASUREMENT OF BODY FLUID VOLUME

Total body water and the volume of different compartments of the body fluid are measured by indicator dilution method or dye dilution method.

INDICATOR DILUTION METHOD

Principle

A known quantity of a substance such as a dye is administered into a specific body fluid compartment.

These substances are called the marker substances or indicators. After administration into the fluid compartment, the substance is allowed to mix thoroughly with the fluid. Then, a sample of fluid is drawn and the concentration of the marker substance is determined. Radioactive substances or other substances whose concentration can be determined by using colorimeter are generally used as marker substances .

Formula to Measure the Volume of Fluid by Indicator Dilution Method

Quantity of fluid in the compartment is measured using the formula:

Correction factor

Some amount of marker substance is lost through urine during distribution.

Uses of Indicator Dilution Method

Indicator dilution or dye dilution method is used to measure ECF volume, plasma volume and the volume

of total body water.

Characteristics of Marker Substances

Dye or any substance used as a marker substance should have the following qualities:

1. Must be nontoxic 2. Must mix with the fluid compartment thoroughly within reasonable time

3. Should not be excreted rapidly

4. Should be excreted from the body completely within reasonable time

5. Should not change the color of the body fluid

6. Should not alter the volume of the body fluid.

MEASUREMENT OF TOTAL BODY WATER

Volume of total body water (fluid) is measured by using a marker substance which is distributed through all thecompartments of body fluid.

Deuterium oxide and tritium oxide mix with fluids of all the compartments within few hours after injection.

Since plasma is part of total body fluid, the concentration of marker substances can be obtained from sample of plasma. The formula for indicator dilution method is applied to calculate total body water.

Antipyrine is also used to measure total body water. But as it takes longer time to penetrate various fluid

compartments, the value obtained is slightly low.

MEASUREMENT OF EXTRACELLULAR FLUID VOLUME

Substances which pass through the capillary membrane but do not enter the cells, are used to measure ECF volume. These substances remain only in ECF and do not enter the cell (ICF). When any of these substances is injected into blood, it mixes with the fluid of all subcompartments of ECF within 30 minutes to 1 hour. Indicator dilution method is applied to calculate ECF volume. Since ECF includes plasma, the concentration of marker substance can be obtained in the sample of plasma. Some of the marker substances like sodium, chloride, inulin and sucrose diffuse more evenly throughout all subcompartments of ECF. So, the measured volume of ECF by using these substances is referred as sodium space, chloride space, inulin space and sucrose space.

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