Transfer of Materials Between Circulatory System & Cells

This section describes how substances move from body capillaries into cells.

Tissue Fluid

"Understand how the interchange of substances occurs through the formation and reabsorption of tissue fluid, including the effects of hydrostatic pressure and oncotic pressure."
Capillary walls are permeable, allowing materials to leave the blood and form a tissue fluid which surrounds and bathes cells.
  • Oncotic pressure - the negative water potential of the blood because of proteins (remains roughly the same throughout).
  • Hydrostatic pressure - the pressure from the heartbeat that causes material to move out of the capillary (reduces as you move away from the arteriole end).
  • Arteriole end - hydrostatic pressure is greater than oncotic pressure causing material to move out of the capillaries.
  • Venous end - hydrostatic pressure is lower than oncotic pressure causing material to move back into the capillaries.


"Know that tissue fluid that is not reabsorbed is returned to the blood via the lymph system."
However, not all of this tissue fluid is reabsorbed into the capillaries. Some is removed through lymphatic vessels which have one way valves. These vessels drain away waste tissue fluid and return it back to the bloodstream close to the heart.