The blood in human body flows from the heart to the arteries which branch into arterioles and then smaller capillaries, which then merge to form venules that merge into larger veins that carry blood back to the heart. This process is called the circulatory system.
Capillaries act as a bridge between arterioles and venules, surrounded by interstitial fluid. This means a change in pressure and osmosis occur at either end of the capillary while there is an exchange of substances between tissues and the blood with the interstitial fluid. This capillary structure enables the blood flow away from the heart passing through the arterioles to bridge through to the venules to close the circulatory system.