In this article we will discuss about the concept of total body water and its distribution.
Water is the most vital and at the same times the most abundant component of the human body. It constitutes about 70 percent of the total body weight and within which the major cations like sodium, potassium, calcium, hydrogen, magnesium and anions like chloride, bicarbonate and protein of the body are dissolved.
Without water there would be no form of life and it forms the intracellular medium within which metabolic reactions characteristics of living substances take place. Water-deprivation brings about death more earlier than that of food-deprivation. If water is given instead of food, life may continue for several weeks by the loss of most of the body fat and 50 percent, of tissue protein.
Total body water in an average human being, weighing about 70 kg is 40 litres to 45 litres. In human being it is about 65% of the body weight in males and about 10% less in females. But the above values vary mostly with the relative degrees of leanness and fatness of the individual. In lean person, the value is higher than that of in obese person. In general, woman contains more fat than man. The total body water content can be determined most accurately by the process of desiccation.
In 1863 Bischoff determined the water content of an executed criminal by the method of desiccation. Mitchell and his associates (1945), Wid-dowson and his co-workers (1951) have also determined the water content of the human beings by direct method. The average water content in different tissues of the body has been presented in Table 5.1.
It has been observed after studying thoroughly the water content of the body in man as well as in different animal species that the total water content in man is similar to that of in other animals. Besides this, the relative distribution of water in the various organs and tissues is mostly same in man as well as in other species.
The percentage of water in various tissues and the proportion of total weight of the body which each tissue represents, have been presented in Table 5.3.
The water of the body can be considered to be distributed within two main compartments—the extracellular and the intracellular. The distribution of body water in different compartments has been presented schematically in Fig. 5.1. The cell membrane actually provides the boundary in between the extracellular and the intracellular compartments.
1. Extracellular Compartment:
The extracellular fluid compartment is a compartment containing heterogenous collections of fluids and not a continuous fluid phase. Edelman and Leibman (1959) have studied thoroughly the distribution pattern of body water by dilution technique and also by tissue analysis. It is postulated that 55% of water is present in the intracellular space and the rest in the extracellular space.
The extracellular fluid phase can be divided into following sub compartments:
i. Transcellular water- 2.5 percent
ii. Dense connective tissue and cartilage water- 7.5 percent.
iii. Plasma water that is confined within the vascular system- 7.5 percent.
iv. Interstitial fluid and lymph- 20 percent.
v. Inaccessible bone water- 7.5 percent
The term transcellular was introduced by Edelman and associates (1952) in order to designate the extracellular fluid having been separated from the other extracellular fluid by an epithelial membrane.
This transcellular fluid includes:
(a) Cerebrospinal fluid,
(b) Joint or synovial fluid,
(c) Intra-ocular fluid,
(d) Fluids of the pleural, pericardial and peritoneal cavity,
(e) Fluids within the ducts of the digestive gland,
(f) Mucous membranes of the nasorespiratory tract, gastro-intestinal tract and genitalia, and
(g) Intraluminal fluid of gastro-intestinal system.
2. Intracellular Fluid Compartment:
It is neither a continuous nor a homogeneous phase and represents the sum of the fluid contents of all the cells of the body. In a cell there are many anatomic subdivisions and for this reason there is a striking difference in water content and ionic composition in between the cytoplasm, nucleus, mitochondria and microsomes of various cell types. This intracellular fluid contains about 30-40% of the body weight and holds about 55% of the whole body water.
Measurement by Dilution Techniques:
Total body water and extracellular water can be measured by dilution technique with varying degree of precisions. Volume of water present in each compartment cannot be measured directly and thus indirect method—the dilution technique has been adopted for its determination.
In this technique the amount of dye used, the final concentration of the dye in the solution is made, are considered for determining the volume of distribution. E.g., if a known quantity of dye—Q is taken and the final concentration is achieved as C, then the volume of distribution V will be; V = (Q/C).
If a beaker of unknown capacity is taken, then its volume can be determined by mixing uniformly a known amount of dye in the volume of water present in the beaker. If the final concentration of the dye is determined by the calorimeter then the volume capacity of the beaker can be determined. Suppose 35 mg of dye has been added and the final concentration that has been achieved to be 0.07 mg per ml, then the volume of the beaker will be;
The result will be valid only when the drug will be mixed thoroughly.
In vivo determination of body fluid compartments by the dilution principle, certain points are generally considered. The dye injected in the body must be evenly distributed and confined to the body fluid compartment to be measured. If the dye is excreted or lodged in other compartments or metabolised then those amount should be determined and subtracted from the quantity administered.
So the equation will be:
Total Body Water:
Total body water is generally determined by using antipyrine. The antipyrine is distributed evenly throughout all the body water compartments and thus diffuses readily across the cell membrane. It is not bound to any intracellular and extracellular compartments. It is also slowly excreted and slowly metabolised.
Tritiated water (H3O or HTO) and deuterium oxide (D2O) -the two isotopes are often used for the determination of total body water. D2O and H3O are distributed in the body exactly like water. These are excreted in the urine, faeces and respiratory gases and also evaporated through the skin.
As for example of using the D2O or HTO for the measurement of total body water, suppose 100 ml of D2O in isotonic saline solution is injected intravenously to a man of weighing about 75 kg. After an equilibrium period of 2 hours, the plasma sample is analysed and D2O concentration is found to be 0.0023 ml per ml. During the period of equilibrium it is found to have a loss (through respiratory, urinary and circulatory pathways) of average 0.5% of the quantity administered.
So the volume of distribution will be:
Extracellular Fluid Volume:
The extracellular fluid volume is not determined so precisely only due to lack of substances that may diffuse to cross the capillary walls readily, enter the cell interstices easily but do not permeate through the cell membrane. Besides this, the substance must be non-toxic and the rate of excretion must be very low in comparison with the rate of distribution in extracellular compartment. There is no such ideal substance available but several substances that have been used are insulin, raffinose, sucrose, mannitol thiosulphate, radiosulphate, thiocyanate, radiochloride and radiosodium.
Intracellular Fluid Measurement:
There is no direct method has yet been developed. It can be determined by subtracting the value of the extracellular compartment from the value of the total body water.