What is the composition of tissue fluid?How is it formed?

Question

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Answer 1

Interstitial fluid (or tissue fluid, or intercellular fluid) is a solution which bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid.

On average, a person has about 11 litres (2.4 imperial gallons) of interstitial fluid providing the cells of the body with nutrients and a means of waste removal. This constitutes 27.5% of the total amount of fluid present within the body, 65% (25 litres) of which is intracellular fluid, and 35% (15 litres) of which is extracellular fluid.

Formation of tissue fluid

Hydrostatic pressure is generated by the pumping force of the heart. It pushes water out of the capillaries.

The water potential is created due to the inability of large solutes to pass through the capillary walls. This buildup of solutes induces osmosis. The water passes from a high concentration (of water) outside of the vessels to a low concentration inside of the vessels, in an attempt to reach an equilibrium. The osmotic pressure drives water back into the vessels. Because the blood in the capillaries is constantly flowing, equilibrium is never reached.

The balance between the two forces is different at different points in the capillaries. At the arterial end of the vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement (see net flux) favors water and other solutes being passed into the tissue fluid. At the venous end, the osmotic pressure is greater, so the net movement favours substances being passed back into the capillary. This difference is created by the direction of the flow of blood and the imbalance in solutes created by the net movement of water favoring the tissue fluid.

Composition

Interstitial fluid consists of a water solvent containing amino acids, sugars, fatty acids, coenzymes, hormones, neurotransmitters, salts, as well as waste products from the cells.

The composition of tissue fluid depends upon the exchanges between the cells in the tissue and the blood. This means that tissue fluid has a different composition in different tissues and in different areas of the body.

Not all of the contents of the blood pass into the tissue, which means that tissue fluid and blood are not the same. Red blood cells, platelets and plasma proteins cannot pass through the walls of the capillaries. The resulting mixture that does pass through is essentially blood plasma without the plasma proteins. Tissue fluid also contains some types of white blood cell, which help combat infection.

Lymph is considered a part of the interstitial fluid. The lymphatic system returns protein and excess interstitial fluid to the circulation.