How do the multi-branching and small diameter of capillaries help the exchange of substances between the capillaries and tissue?
The multi-branching of capillaries (there are around 60,000 capillaries per square centimeter of cross-section) as well as their small diameter (capillaries are often not much larger than the blood cells they contain) help the exchange of substances between blood and tissue in several ways. The multi-branching insures that no cell is far away from a blood supply and creates a great surface area through which the exchange of substances can occur. Further, due to the physical law that states as the total cross section of a system increases, the rate of flow decreases, by increasing the total cross section of the system, the branching slows blood flow through the capillaries to allow more time for exchange to occur. The small diameter of the capillaries slows blood flow, too, by increasing the frictional resistance exerted by the vessels on the blood flow.
