pressure filtration

• 1. at the start of the capillary bed near to arteries, the hydrostatic pressure in the capillaries is more than in the tissue fluid this difference forces fluid out the capillaries and into the spaces around the cell forming tissue fluid
• 2) as fluid leaves the capillaries the hydrostatic pressure reduces so its much lower at the end of the capillary bed nearest to the venules
• 3) another working pressure is oncotic pressure - generated by plasma proteins present in the capillaries which lower the water potential.

  • at the venule end of capillary bed the water potential in capillaries is lower than in the tissue fluid due to the fluid loss from the capillaries and the high oncotic pressure
  • meaning some water re-enters the capillaries from the tissue fluid at the end venule by osmosis
• as blood enters the capillaries from arterioles, the small diameter results in a high hydrostatic pressure, water, glucose, amino acids, fatty acids, ions, oxygen are forced out - ultrafiltration
• reabsorbed - large molecules remain in the capillaries which creates a lowered water potential. towards venule end of capillaries, hydrostatic pressure is lowered due to loss of fluid, water potential lowers and water re enters capillaries through osmosis