Tissue Fluid (7.6)

• Tissue fluid is a watery liquid that contains glucose, amino acids, fatty acids, ions in solution and oxygen. Tissue fluid supplies all of these substances to tissues.
• In exchange for the materials it delivers, tissue fluid receives carbon dioxide and other waste materials from the tissues.
• Tissue fluid is formed from blood plasma and the composition is controlled by various homeostatic systems. Tissue fluid is therefore able to provide a fairly constant environment surrounding cells.
• The first stage in forming tissue fluid involves blood from the heart passing through the arteries, arterioles and finally the capillaries. Pumping from the heart creates hydrostatic pressure at the arterial end of capillaries.
• The second stage in forming tissue fluid occurs once hydrostatic pressure causes tissue fluid to move out of the blood plasma.
• The outward hydrostatic pressure in tissue fluid formation is opposed by two other forces. These include hydrostatic pressure of the tissue fluid from outside the capillaries, resisting the outward movement of liquid, and the lowered water potential of the blood causing water to move back into the blood from the capillaries (due to plasma proteins).
• The combined effect of all three forces acting in tissue fluid formation produces an overall pressure which pushes tissue fluid out of the capillaries at the arterial end.
• Ultrafiltration occurs during tissue fluid formation, since the outward pressure created is only enough to force small molecules out of the capillaries, leaving all cells and proteins inside the blood because they are too large to cross the membranes.
• Once tissue fluid has exchanged metabolic materials with the cells it bathes, it is returned to the circulatory system. Most tissue fluid returns to the blood plasma directly via the capillaries.
• (1) Tissue fluid is returned to the circulatory system when the loss of tissue fluid from the capillaries reduces their hydrostatic pressure.
• (2) Once the blood has reached the venous end of the capillary network, the hydrostatic pressure is lower than the tissue fluid outside it. As a result, tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outside them.
• (3) In addition, the plasma has lost water and still contains proteins. It therefore has a lower water potential than the tissue fluid. As a result, water leaves the tissue via osmosis down a water potential gradient.
• Not all tissue fluid can return to the capillaries ; the remainder is carried back to the lymphatic system.
• The lymphatic system is a network of vessels that begin in the tissues. Initially, they resemble capillaries, but gradually merge into larger vessels throughout the body. These larger vessels drain their contents back into the bloodstream via two ducts that join veins closer to the heart.
• The contents of the lymphatic system (lymph) are not moved by the pumping of the heart, but instead by two different mechanisms. Firstly, by hydrostatic pressure of the tissue fluid that has left the capillaries. Secondly, contraction of the body muscles that squeeze the lymph vessels (containing valves) so that lymph travels in the direction of the heart.