AQA AS Biology help


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.

Answers
Tissue fluid is a mixture of different substances (blood plasma) and is forced out of the capiliaries at the arterial end when under greater pressure. This would leave the remaining blood with a much higher concentration of solutes and less water. At the venous end of the capillary, does the blood still have a higher concentration of solutes (lower water potential) than the tissue fluid or not? The blood is also at this point at a low hydrostatic pressure. Different substances can be exchanged between tissue fluid and surrounding cells by the expected modes of transport - lipid diffusion, ions by facilitated diffusion, active transport and of course water by the process of osmosis. Consider at the venous end why aqueous tissue fluid is likely to reenter the capillary by osmosis - think water potential, solute concentration and pressure potential. Hope this helps!
hellouk
12 April 2012
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 potential's 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.
hywel
28 August 2015
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