In this article we will discuss about:- 1. Definition and Sources of Tissue Fluid 2. Composition of Tissue Fluid 3. Functions 4. Aggregation.
Definition and Sources of Tissue Fluid:
Tissue fluid is formed from the plasma by process of diffusion and filtration. This fluid occupies the intracellular space and forms the connecting link in the transport of nutrition, gases and the metabolic end products between blood capillaries, tissue cells and the lymph. It constitutes the internal environment of the body, which surrounds tissue cells.
Tissue fluid is derived from two sources:
1. Blood Capillaries:
The amount of tissue fluid formed from blood depends upon:
(a) Capillary permeability,
(b) The difference of pressure between the capillary and the tissue fluid, and
(c) The difference of colloidal osmotic pressure of blood and tissue fluid.
It is obvious that anything that increases the capillary permeability will increase the amount of tissue fluid formed. Regarding blood pressure and osmotic pressure, it is known that at the arterial end of capillaries, the average blood pressure is about 32 mm of Hg and at the venous end, 10 mm of Hg.
The colloidal osmotic pressure at both ends is same (25 mm of Hg on the average). At the arterial end, the net filtration pressure which is the difference between the two is 7 mm of Hg towards the tissue (interstitial) fluid. At the venous end due to fall in blood or hydrostatic pressure, the filtration pressure is 15 mm of Hg to the opposite side, i.e., from tissue fluid to the capillary (Fig. 5.2).
2. Tissue Activities:
The amount of tissue fluid formed from the tissue cells depends upon the degree of metabolic activity of the cells. Tissue cells produce water as an end product of metabolism. This metabolic water is added to the already existing tissue fluid. More the degree of activity more will be the metabolic water formed and consequently the amount of tissue fluid will increase.
Two important exceptions to the capillary pressure are:
(a) In capillaries of the lungs, hydrostatic pressure about 6 mm of Hg, and
(b) In capillaries of the kidneys, glomerular hydrostatic pressure about 60 to 70 mm of Hg.
If the hydrostatic pressure is increased within capillaries, then it will interfere the return of materials to the lymphatic’s or capillaries and will result in excess accumulation of tissue fluid (i.e., oedema).
Composition of Tissue Fluid:
It is very difficult to obtain a pure sample of tissue fluid; hence, its exact composition is not known. It is believed that its composition is same as that of lymph, excepting that its protein content is negligible; and as such, its colloidal osmotic pressure is very low.
The composition and volume of tissue fluid is regulated by constant interchange with blood and lymph. It has been mentioned above that filtration of tissue fluid takes place at the arterial end of the capillaries. At the venous end of the capillary the blood pressure is very low—about 10 mm of Hg and the colloidal osmotic pressure is much higher. These two factors help in drawing away just as much fluid comes out from the arterial side. As we know that water content of tissue fluid is derived from two sources—blood and tissue cells.
The amount of water that goes out of blood is drawn in again at the venous side of the capillaries. But vascular capillaries cannot draw away the amount of metabolic water formed by the tissue cells. It is for the drainage of this excess water that the lymphatic system has developed. Thus it will be seen that blood and lymph remain as if on two sides of tissue fluid and try to keep it constant in volume and composition by continuous interchange.
Specific gravity of the tissue fluid is about 1.015 to 1.023. It may contain a few erythrocytes. But regarding the white cells, the tissue fluid contains a good number of lymphocytes and a small number of granulocytes. Blood proteins and nutrient contents of it are very low. It does not contain platelets and may also clot, but with a very slow process. It contains higher concentration of waste products but glucose, salt and water contents are more or less same as those are present in blood.
Functions of Tissue Fluid:
i. It constitutes the internal medium in which the tissue cells are bathed. The cells draw in oxygen and nutrition from the tissue fluid and excrete their metabolites into it. Hence, tissue fluid may be regarded as the medium which supplies all the immediate requirements of the cell.
ii. It acts as a great reservoir of water, salts, nutrition, etc. This function is very important. Under any condition, in which the blood volume is increased or diminished, physical forces are set up by which the blood volume is kept constant with the help of the tissue reserve. For example in haemorrhage, the capillary pressure becomes very low and goes below the colloidal osmotic pressure in the capillary which remains same.
Due to this higher O.P in the capillaries, water is drawn in from the tissue spaces, so that blood volume is restored. When water is drawn away from blood, such as due to diuresis, excessive sweating or diarrhoea, blood volume and blood pressure will be lowered, but the plasma proteins will be more concentrated. This will increase the colloidal O.P. of blood. This increased osmotic pressure of plasma and reduced blood pressure will increase the rate of absorption from the tissue fluid, and thus blood volume will be kept constant.
On the other hand, when blood volume increases, as for instance, by intravenous injection of large quantities of isotonic saline, fluid will pass out into the tissue spaces due to two causes:
a. Saline will dilute the colloids and reduce the colloidal osmotic pressure.
b. Increased volume of blood will raise the blood pressure and cause more filtration. Both these factors will cause more fluid to run out into the tissue spaces, until blood volume comes back to the original level.
Aggregation of Tissue Fluid:
Swelling or oedema observed sometimes in different parts of the body is due to the aggregation of the tissue fluid.
This might result from several factors:
i. Increased capillary permeability resulting from dilated, damaged or inflammated capillary.
ii. Increase in the capillary pressure which might be due to changes in posture (in lower extremities it is due to continued standing), obstruction to veins or rise in the venous pressure as observed in the cardiac failure.
iii. Blockage of lymphatic nodes or vessels, as a result of inflammation of the node or blockages by very small worms like that of Filaria.
iv. Loss of the plasma proteins whether due to malnutrition or excessive loss resulting from the renal damage, causes decrease in plasma osmotic pressure and excessive aggregation of the tissue fluid.
v. Renal disease causes impairment of excretion of urine and the resulting water retention causes increase in the tissue fluid.
vi. Unfamiliar exercise might cause swelling due to accumulation of metabolites.
vii. Ingestion of a large amount of salts results in retention of water. Adrenal cortical extract also produces similar effects.