8.1.5

Tissue Fluid

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Tissue Fluid

Substances carried in the capillaries are transported to the cells via the tissue fluid. Tissue fluid is a combination of oxygen, water and nutrients. The fluid surrounds the cells in the body.

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Pressure filtration

  • In order for the substances carried in the bloodstream (e.g. oxygen) to diffuse into the cells, they must first move out from the capillaries.
  • Substances move into tissue fluid from the capillaries in a process called pressure filtration.
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Pressure gradient

  • There is high hydrostatic pressure in the capillaries at the entrance to a capillary bed.
  • The high hydrostatic pressure is created by a high volume of blood being forced through narrow capillaries.
  • The high pressure creates a pressure gradient between the inside of the capillaries and the outside.
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Movement of fluid

  • The fluid in the capillaries flows down the pressure gradient into the surrounding space, where there is a lower pressure.
    • This is pressure filtration.
  • The fluid that has moved out of the capillaries creates the tissue fluid.
  • Substances in the tissue fluid can now diffuse or be transported into the cells.
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Decreasing pressure

  • The movement of fluid out of the capillaries causes the hydrostatic pressure inside the capillaries to decrease.
  • This means that the hydrostatic pressure in the capillaries at the exit of the capillary bed is lower than the pressure at the entrance to the capillary bed.
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Water potential gradient

  • When fluid moves out of the capillaries, plasma proteins remain inside the capillaries.
  • The plasma protein concentration inside the capillaries increases and the water potential decreases.
  • A water potential gradient is established and water diffuses via osmosis back into the capillaries from the tissue fluid.
  • Oncotic pressure pulls water back into the capillaries and is created by the increased plasma protein concentration. It is the opposite to hydrostatic pressure, which forces water out of the capillaries.
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The lymphatic system

  • Excess tissue fluid flows into the lymphatic system.
  • The lymphatic system recycles the excess tissue fluid into the bloodstream.

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19Genetic Technology (A2 Only)

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