Every cell in the body is surrounded and bathed by a liquid called tissue fluid. Cells are not in direct contact with the blood, so all exchange of substances between the blood and cells takes place through the tissue fluid.
Tissue fluid is formed from blood plasma that is pushed out of the capillaries through the small gaps between the endothelial cells. It therefore contains many of the same substances as plasma, including water, glucose, amino acids, fatty acids, ions and oxygen. However, it contains no red blood cells and very few plasma proteins, because these are too large to pass through the capillary wall. A few white blood cells can squeeze out into the tissue fluid to fight infection.Useful substances such as glucose and oxygen diffuse from the tissue fluid into the cells. Waste products such as carbon dioxide and urea diffuse from the cells into the tissue fluid. In this way the tissue fluid provides cells with a stable environment of fairly constant composition.Tissue fluid is continually being formed and drained. Most of it returns into the capillaries, carrying waste products back into the blood. The rest drains into lymph vessels. The balance between fluid leaving and returning to the capillaries depends on two pressures: oncotic pressure and hydrostatic pressure, which are covered in the next lessons.If more tissue fluid forms than is drained away, it builds up in the tissues, causing swelling called oedema.
Tissue fluid is formed from blood plasma that is pushed out of the capillaries through the small gaps between the endothelial cells. It therefore contains many of the same substances as plasma, including water, glucose, amino acids, fatty acids, ions and oxygen. However, it contains no red blood cells and very few plasma proteins, because these are too large to pass through the capillary wall. A few white blood cells can squeeze out into the tissue fluid to fight infection.Useful substances such as glucose and oxygen diffuse from the tissue fluid into the cells. Waste products such as carbon dioxide and urea diffuse from the cells into the tissue fluid. In this way the tissue fluid provides cells with a stable environment of fairly constant composition.Tissue fluid is continually being formed and drained. Most of it returns into the capillaries, carrying waste products back into the blood. The rest drains into lymph vessels. The balance between fluid leaving and returning to the capillaries depends on two pressures: oncotic pressure and hydrostatic pressure, which are covered in the next lessons.If more tissue fluid forms than is drained away, it builds up in the tissues, causing swelling called oedema.