How is tissue fluid formed and absorbed?
How is tissue fluid formed and absorbed? – The tissue fluid is formed by the action of a variety of forces. Fluid is forced out of the blood vessels and capillaries into the tissue fluid by hydrostatic pressure – the great increase in blood pressure is produced every time the heart beats. …
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Question
Explain how the formation and absorption of tissue fluid occurs, referring to the processes of hydrostatic and osmotic pressure in a capillary bed.
Answer
Although the circulatory system transports oxygen, metabolites and solutes to all the tissues in the human body, the blood never directly comes into contact with any of the tissues other than the interiors of the cardiovascular system (except in case of trauma). Instead, the tissues are bathed in tissue fluid (sometimes called interstitial fluid), formed from, and very similar in composition to, the plasma of the blood.(How is tissue fluid formed and absorbed?)
The tissue fluid is formed by the action of a variety of forces. Fluid is forced out of the blood vessels and capillaries into the tissue fluid by hydrostatic pressure – the great increase in blood pressure is produced every time the heart beats. This pressure forces water out of the capillaries through the semi-permeable membranes of the cells that line them (and in some vessels, through intercellular clefts, tiny membrane-sealed gaps between cells). These cells are also involved in facilitated diffusion of a wide variety of solutes and other molecules between the two fluid bodies. Metabolites diffuse from the plasma to the tissue fluid, and cellular waste products diffuse in the opposite direction.(How is tissue fluid formed and absorbed?)
Fluid is returned to the blood vessels by osmotic pressure. Large macromolecules (notably serum albumin) are unable to pass across the cellular membranes that line the vessels, and so they remain in circulation, lowering the osmotic pressure in the blood vessels (water flows across osmotic gradients from high to low osmotic pressure). Interaction between the two forces causes more fluid to leave at the arterial end of the capillary bed, and return at the venous end, as described by the Starling Equation.(How is tissue fluid formed and absorbed?)
Excess tissue fluid is removed by the lymphatic system, a series of vessels that carry fluid towards the heart, where it drains into the left subclavian vein. Failure of effective lymph transport results in a build-up of tissue fluid known as 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.(How is tissue fluid formed and absorbed?)
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.(How is tissue fluid formed and absorbed?)
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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.
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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.
Related FAQs
1. Where does filtration of tissue fluid take place in the body?
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.
2. What controls the formation and absorption of tissue fluid?
A brief description of the balance of hydrostatic and osmotic forces that control tissue fluid formation and absorption. Explain how the formation and absorption of tissue fluid occurs, referring to the processes of hydrostatic and osmotic pressure in a capillary bed.
3. What is the difference between tissue fluid and blood plasma?
Although the tissue fluid is formed from the blood plasma, both fluids differ in composition. Tissue fluid mainly lacks large plasma proteins (e.g., albumin) and red blood cells present in the blood plasma.
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