8.1.9—Tissue fluid

Tissue fluid is formed from plasma at capillary beds, bathes cells for exchange, and is returned either directly to capillaries or, when excess remains, through lymphatic drainage back to the blood.

1. Arterial-end filtration: blood hydrostatic pressure is relatively high at the arterial end of a capillary → water and small dissolved substances are forced through gaps in the capillary wall → tissue fluid forms around nearby cells. Large plasma proteins and blood cells remain in the blood because they cannot pass through the gaps.
2. Cell exchange: tissue fluid bathes the cells → oxygen and solutes can move from the blood towards cells, while carbon dioxide and other waste products move from cells towards the capillary for removal.
3. Venous-end reabsorption: pressure falls along the capillary → the water-potential effect of plasma proteins becomes relatively more important → water moves back into the capillary by osmosis, so much of the tissue fluid is reabsorbed.
4. Lymphatic return: not all filtered fluid returns directly at the venous end → excess tissue fluid enters lymph vessels as lymph → lymph is eventually returned to the bloodstream, helping prevent fluid accumulation in tissues.

Force boundary: hydrostatic pressure pushes fluid out, whereas the protein-related water-potential gradient draws water back in. Changes such as high blood pressure or low blood-protein content can leave more fluid in the tissues.

Tissue fluid is not whole blood: cells and most large plasma proteins remain in the vessels. Lymph is excess tissue fluid within lymph vessels on its return route; this card stops at fluid balance and does not enter later immune functions. Staff-only visual brief: capillary with arterial filtration, venous reabsorption and a lymphatic side-return arrow; do not generate or bind an image.