Capillaries are the sites of diffusion of respiratory gases--among other things-- between the blood and interstitial fluid. Interstitial fluid in active tissues becomes oxygen poor and carbon dioxide rich; the reverse is true of inactive tissues. Also, fluid shifts, due to an interplay between hydrostatic and osmotic pressures, play a major role in water balance.
Capillaries are not always open; flow through them is controlled by precapillary sphincters shown as a donut in the map. The relationship between vasoconstriction and flow is inverse (dashed arrow); an increase in vasoconstriction causes a decrease in flow into the capillary.
Vasoconstriction is under the control of carbon dioxide (CO2) and oxygen (O2) in the surrounding interstitial fluid. The relationship between carbon dioxide concentration and vasoconstriction is inverse (dashed arrow). An increase in carbon dioxide (CO2) causes a decrease in vasoconstriction (VCN). The reverse is true for oxygen concentration; the relationship is direct (solid arrow). An increase in oxygen (O2) causes an increase in vasoconstriction (VCN).
When blood enters porous capillaries, hydrostatic pressure (HP) forces fluid (HP block arrow) into the interstitial space leaving larger plasma proteins (PP) in the vessel. This direct relationship is shown by a solid arrow pointing from the HP block arrow to PP in the capillary. If hydrostatic pressure was the only force at work, the interstitial fluid would become increasingly dilute and the volume of plasma would decrease while becoming increasingly concentrated.
The force that prevents this from happening is called osmotic pressure (OP). A clear understanding of diffusion is necessary to understand how this force works.
If a substance is concentrated in one place it will tend to 'spread out' into adjacent regions where it is less abundant. This is because of its random motion. Eventually, the concentration of the substance will be uniform throughout the region. While it is 'spreading out' it is said to be diffusing. Once the concentration is uniform throughout random motion is still occurring but diffusion is not.
Blood flows through capillaries passing through oxygen poor tissues; oxygen diffuses from the blood into the interstitial fluid (block arrow labeled D). Oxygen poor tissues have a high concentration of carbon dioxide; carbon dioxide diffuses from this fluid into the blood (block arrow labeled D).
If the substance under consideration happens to be water it will behave just like any other substance. It will spread out (i.e., diffuse), along its gradient, away from a place where it is highly concentrated into adjacent regions where it is less abundant. This will continue until the concentration of water is uniform throughout the region. Because water is so abundant and its diffusion is so critical to life the diffusion of water is given a special name...osmosis.
Increasing plasma proteins (PP) in the capillaires means there is a shortage of water there; water was forced out by hydrostatic pressure. A water gradient now exists between these two adjacent fluid compartments and is symbolized by the solid arrow pointing from PP to the OP block arrow. Accordingly, water will diffuse--osmosis-- back into the capillaries. This return of water prevents a significant decrease in blood volume or pressure indicated by the OP block arrow pointing to the blood pressure acronym, BP. It is helpful to think of osmotic pressure (OP) as the re-established blood pressure due to this return of water to the blood.
Last update: 7/15/2005