Tissue fluid is the medium in which cells are bathed. It is otherwise known as interstitial fluid. It forms about 20% of extracellular fluid (ECF).


Because of the capillary membrane, there is no direct contact between blood and cells. And, tissue fluid acts as a medium for exchange of various substances between the cells and blood in the capillary loop. Oxygen and nutritive substances diffuse from the arterial end of capillary through the tissue fluid and reach the cells. Carbon dioxide and waste materials diffuse from the cells into the venous end of capillary through this fluid.


Formation of tissue fluid involves two processes:

1. Filtration.

2. Reabsorption.


Tissue fluid is formed by the process of filtration. Normally, the blood pressure (also called hydrostatic

pressure) in arterial end of the capillary is about 30 mm Hg. This hydrostatic pressure is the driving force

for filtration of water and other substances from blood into tissue spaces. Along the course of the capillary, the pressure falls gradually and it is about 15 mm Hg at the venous end. Capillary membrane is not permeable to the large molecules, particularly the plasma proteins. So, these proteins remain in the blood and exert a pressure called oncotic pressure or colloidal osmotic pressure. It is about 25 mm Hg.

Osmotic pressure is constant throughout the circulatory system and it is an opposing force for the filtration of water and other materials from capillary blood into the tissue space. However, the hydrostatic pressure

in the arterial end of the capillary (30 mm Hg) is greater than the osmotic pressure. And, the net filtration pressure of 5 mm Hg is responsible for continuous filtration.

Starling Hypothesis

Determination of net filtration pressure is based on Starling hypothesis. Starling hypothesis states that the

net filtration through capillary membrane is proportional to the hydrostatic pressure difference across the membrane minus the oncotic pressure difference. These pressures are called Starling forces.


Fluid filtered at the arterial end of capillaries is reabsorbed back into the blood at the venous end of capillaries. Here also, the pressure gradient plays an important role. At the venous end of capillaries, the hydrostatic pressure is less (15 mm Hg) and the oncotic pressure is more (25 mm Hg). Due to the pressure gradient of 10 mm Hg, the fluid is reabsorbed along with waste materials from the tissue fluid into the capillaries. About 10% of filtered fluid enters the lymphatic vessels. Thus, the process of filtration at the arterial end of the capillaries helps in the formation of tissue fluids and the process of reabsorption at the venous end helps to maintain the volume of tissue fluid.

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