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Capillaries allow exchange of substances with body tissues through their thin walls.

As blood travels at high pressure in the arteries towards the capillaries, pressure filtration occurs which results in plasma passing through the capillary wall into the tissue fluid which surrounds the cell.

Tissue fluid provides cells with useful substances such as glucose and oxygen and waste products are passed out of the cells into the tissue fluid to be removed.

Most of the tissue fluid is returned to the blood. Any excess tissue fluid is absorbed by lymphatic vessels which return it to the circulatory system as lymph.

Plasma and tissue fluid are composed of similar substances except for plasma proteins which are only found in plasma. They are too large to be filtered through the capillary walls.