Q - How does tissue fluid return to the circulatory system?
In my textbook, it says the main reason is that it re-enters the capillaries by the loss of hydrostatic pressure within them, so by the time the blood gets to the venous end of the capillary the hydrostatic pressure is less within the capillary than outside them in the tissue fluid, so the tissue fluid is forced back into the blood in the capillary. However, when I look at mark schemes it says that it mainly re-enters by osmosis? How does it move in by osmosis? (I know that the remaining moves back into the bloodstream via the lymphatic system)
Answer
As the blood moves through the capillary it is getting further from the heart and pressure is being split between branches. This means hydrostatic pressure, and the force that attempts to push the fluid through the walls, decreases, However, osmotic pressure (basically how big the gradient of water potentials is) must get higher than hydrostatic pressure to ‘over power’ it. With nutrients moving out with the water, this means water potential would hardly be affected. However, there are special proteins in the plasma of the blood called plasma proteins. Being soluble (unlike may proteins) they alter water potential and being large (like many proteins) they are unable to cross the semi-permeable cell membranes of the capillary wall. As water moves out, the concentration of these proteins increases, and the water potential in the capillaries decreases. The gradient of water potential from outside to inside gets higher, as does the osmotic pressure. Eventually, osmotic pressure is higher than hydrostatic pressure, and fluid is forced back in! Any excess is returned via the lymphatic system, entering dead-ended lymphatic capillaries. This system of vessels eventually drains the excess fluid back into the circulatory system.
