Distribution of Fluids in the Human Body

In this article we will discuss about the Distribution of Fluids in the Human Body:- 1. Fluid Concentration in Human Body 2. Extracellular Fluid 3. Intracellular Fluid 4. Exchanges between Fluid Compartments.

Fluid Concentration in Human Body:

1. Water makes up 50 to 70 per cent of the weight of the adult human body and var­ies inversely as the fat content.

2. This mentioned amount of water is dis­tributed throughout the body as the major component of the intracellular and extra­cellular fluids.

3. The intracellular fluids amount to about 50 per cent of the body weight in a lean individual and much less in an obese per­son.

4. The extracellular fluids represent about 20 per cent of the body weight.

5. Of the extracellular fluids, interstitial flu­ids amount to some 15 per cent and blood plasma about 5 per cent of the body weight.

6. Relatively small volumes are represented by specialized fluids such as cerebrospi­nal fluid, ocular fluid, lymph, and syno­vial fluids, etc.

Extracellular Fluid:

1. All body cells exist in an environment of fluid collectively designated extracellu­lar fluid. This includes the blood plasma, interstitial fluid and lymph.

2. 7 per cent protein is present in plasma, slightly less in hepatic lymph and 0.1 per cent protein in subcutaneous interstitial fluid.

3. They are solutions mainly of Nacl and NaHCO₃, with small amounts of Ca, Mg, K, H, phosphate, sulphate and organic acid ions, some nonelectrolytes (glucose, urea, lipids, etc.) and with pH values ranging from 7.35 to 7.45 under normal conditions.

4. The total concentration of the ionic con­stituents is about 310 m mol per litre of plasma.

5. Much of the intracellular magnesium is not in the ionic form but is bound to pro­tein and other smaller organic molecules.

6. Several components of the extracellular fluid are important in preserving osmotic, anion-cation balance and hydrogen ion regulation.

7. The cations (K⁺, Ca⁺⁺, Mg⁺⁺ and H⁺), al­though present in comparatively very low- concentrations, exert profound influences on physiological processes.

8. The interstitial fluid contains a higher total concentration of diffusible anion and a lower concentration of cation than does the plasma.

Intracellular Fluid:

1. Less amounts (5-10 m mol/litre) of sodium are present in the intracellular fluid which also contains little but extremely biologi­cally important calcium.

2. The chief cations of this fluid are potas­sium (about 160 m mol/litre) in muscle and magnesium (about 26 m mol/litre) in muscle.

3. The intracellular fluids contain much more phosphate and sulphate ions as well as proteins than the extracellular fluid. Chlo­ride ion is practically absent from this fluid except in the case of erythrocytes, and cells of the kidney tubules, stomach and intestines. Both Na and K ions are able to cross the membrane more freely under cer­tain physiological and pathological con­ditions.

4. Much of the magnesium is present as un-dissociated compounds of protein and organic phosphate and, therefore, is not in ionic form.

Exchanges between Fluid Compartments:

Many of the substances entered into the body and produced by the cells are distributed to other tis­sues or excreted.

The exchange systems are out­lined as follows:

1. Alveolar Air – Blood Plasma:

This system provides for entrance of oxygen into and loss of CO₂ and water from the body.

2. Plasma – Erythrocytes:

This system pro­vides for ready exchange of oxygen, CO₂, water and certain anions (particularly CI^– and HCO₃^–) in both directions. Cations are exchanged very slowly.

3. Plasma – Interstitial Fluid:

These two me­dia are separated by the capillary walls, which are permeable to water, inorganic ions and small organic molecules (glucose, amino acids, urea, etc.) but not to large organic molecules such as proteins.

4. Interstitial Fluid – Intracellular Fluid:

These two compartments are separated by the cell membranes across which gases in solu­tion, water and small unchanged mol­ecules can diffuse. The small molecules, e.g., glucose, is not subjected to simple dif­fusion but is carried across cell membrane by active transport processes.

The perme­ability of electrolytes follows biological pump mechanisms. These membranes are also relatively impermeable to large mol­ecules such as proteins, except in special situations, namely the liver.