We know that at the arteriolar end of the capillary,important materials required for tissue diffuse out to interstitial space and at the venular end waste materials come out from interstitium to enter venules.
This exchange process is governed by Sterling forces.
There are 4 types of forces in this case,some helps in passing of substances from capillary to interstitium and some the reverse.
- Capillary Hydrostatic pressure.
This pressure is equals to blood pressure,which pushes blood against capillary wall and as a result tries to send materials out from the capillary to interstitium. But as we progress from arteriolar end of the capillary to the venular end,its value decreases as a lot of substances present in blood has diffused out at the arteriolar end.
- Plasma Oncotic pressure
This is a pulling force by proteins present in plasma of blood. Protein has a tendency to retain water along with it,as a result this force acts against capillary Hydrostatic pressure i.e it prevents movement of water from capillary to interstitium.
- Interstitial Hydrostatic pressure
Very clearly this is the pressure given by the fluid present in interstitium,so it pushes substances towards capillary lumen from interstitium
- Capillary Oncotic pressure
This is the force due to proteins present in interstitium but as proteins present in interstitium is normally low,this force which tries to retain water in the interstitium,is not very important until inflammation has happened where protein leaks out from capillary to interstitium and this pressure increases.
Also,in the interstitium lymph vessels work quickly to take substances whatever come to interstitium away from there,so this partial vacuum created enables entry of substances from arteriolar end.
So, clearly if we take the direction of flow of fluid from capillary to interstitium positive, the net force helping in this process becomes
Now,at the arteriolar end of the capillary
So,if a person is suffering from high blood pressure,