Roughly describe the exchange of substances between blood (plasma) and the interstitial fluid
The capillaries have a permeable membrane with pores through which small particles (vitamins, hormones, Erys – but not blood proteins) pass.
In the arterial thigh (beginning of the capillaries) there is an effective filtration pressure (which is dependent on the blood pressure)
This pressure forces the plasma with its small components into the interstitial space: approx. 0.5% = results in 20 l per day
In the venous thigh, on the other hand, there is the colloid osmatic pressure which counteracts the filtration pressure – the filtration pressure and blood pressure decrease further away from the high pressure system and thus the pressure reverses in the opposite direction: effective reabsorption pressure.
This acts like a suction: The previously squeezed out liquid is pulled back: 90% of it = 18l per day gets back into the venous system and thus to the right heart
The remaining 2l enter the lymphatic system including all the substances that are to be removed