Forces in Capillaries

• Forces in Capillaries pt1

  • Blood pressure within the capillaries is called hydrostatic pressure and it is essentially a filtrating pressure i.e. forces fluid out of the capillary into the interstitial space
  • hydrostatic pressure is higher at the arterial end & so that’s where you’d see most filtration occurring
• Forces in Capillaries pt2

  • However, there is a secondary force – osmotic pressure, which has the opposite action to hydrostatic pressure and we need to consider this in order to understand the direction of water movement and the volume of water being shifted – i.e. whether water leaves or enters a capillary and how much fluid moves depends entirely on the interaction of these 2 opposing forces – hydrostatic versus osmotic pressure
• Forces in Capillaries pt3

  • Unlike hydrostatic pressure, osmotic pressure tries to “hold on to” or attract water
  • It’s produced by ions and plasma proteins (albumin, globulins, fibrinogen, prothrombin etc..) which are normally trapped within capillaries simply because they are too big to squeeze through the pores between capillary cells to escape
  • Since, normally these proteins can’t escape, osmotic pressure remains constant along the length of a capillary from arterial to venous end
• Forces in Capillaries pt4

  • So, within the capillary there is a power struggle between 2 opposing forces i.e
  • Hydrostatic pressure, which seeks to expel water into the tissue spaces
  • Osmotic pressure, which seeks to retain water
  • These opposing forces are described as Starlings forces. Basically, the differences between these 2 opposing forces determine:
  • Whether fluid moves out into the interstitial space
  • How much fluid moves out
  • And how much fluid comes back in again