Starlings Forces

• • 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
• What happens physiologically in the normotensive situation:

  • If you look at the arterial end of the capillary where blood comes in from the arterial system, hydrostatic pressure is greater than the constant osmotic pressure exerted by those trapped plasma proteins i.e. filtration pressure of capillary > water retention pressure of the capillary so water leaves the capillary & enters the interstitial space

  
• What happens physiologically in the normotensive situation:

  • at the venous end of the capillary the relationship between osmotic (retentive) and hydrostatic (filtration) pressures are quite different. At the venous end, hydrostatic pressure (out) is significantly less than osmotic pressure (in) meaning that at the venous end:
  • Water no longer leaves the capillary & enters the interstitial space
  • Water which has just been pushed out into the spaces re-enters the capillary

  
• What does this all mean and in terms of blood pressure regulation?

  • Hydrostatic pressure fluctuates with BP, therefore, when systemic BP rises, hydrostatic pressure rises & when BP falls hydrostatic pressure falls
  • if BP rises in the arterial system, hydrostatic pressure will rise in the capillaries
  • However, there is no cause here to change osmotic pressure
  • So, the difference between pressures at the arterial end of the capillary is increased i.e. hydrostatic >> osmotic pressure and compared to normal more fluid leaves the capillary and enters the tissue spaces
• What does this all mean and in terms of blood pressure regulation?

  • At the venous end of the capillary, hydrostatic pressure would still be higher than normal and therefore closer than normal to the osmotic pressure
  • This higher than normal hydrostatic pressure at the venous end counters some of the reclaiming / attracting force of capillary osmotic pressure and less fluid than normal re-enters the capillary
  • This means more fluid ends up sitting in the interstitial space
• What does this all mean and in terms of blood pressure regulation?

  • Now it doesn’t sound like we are talking about much fluid movement here just a bit more than normal – but most of the blood vessels we have are capillaries
  • If you mentally scale up and imagine a little bit more fluid than normal being moved out of the blood system in thousands of capillaries, you can see we have the capacity to fairly rapidly move relatively large volumes of fluid out of the capillaries – lowering blood volume and helping to reduce and normalise BP