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Capillary function
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The capillary exists for one thing: to negotiate with the tissue around it. It hands over the nutrients that reached it from the arteriole, trades fluid and metabolites with that tissue, and collects the waste the cells left behind as they worked. That exchange is not driven by anyone's decision, but by a tug of war between two pressures, and together they are called the Starling forces.
One is hydrostatic pressure: the force with which the fluid inside pushes against the wall, like water inside a full bag. It drives fluid out into the tissue, and that outward movement is called filtration. The other is oncotic pressure, which pulls the opposite way and brings fluid back into the capillary; that return is called reabsorption. At the start of the capillary, the pressure pushing outward wins. As blood moves along the capillary, hydrostatic pressure falls further and oncotic pressure then prevails, so fluid and waste move back in. And the fluid left outside still has a way out: the lymphatic vessels can pick it up.
In brief
The capillary's job is to exchange fluid and metabolites with the interstitium: it delivers the nutrients arriving from the arteriole and carries away metabolic waste.
That movement is dictated by the Starling forces, driven mainly by the difference in hydrostatic and oncotic pressure between capillary and interstitium. Hydrostatic pressure pushes fluid into the tissue, filtration; oncotic pressure pulls it back, reabsorption.
By the end of the capillary, hydrostatic pressure has fallen further, oncotic pressure prevails and fluid returns; whatever is left over can be absorbed by the lymphatics.
Sources (2)
- National Center for Biotechnology InformationHistology, Capillary. StatPearls
- OpenStax, Rice UniversityAnatomy and Physiology 2e, 20.3 Capillary ExchangePublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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