Tiny blood vessels are called capillaries. The capillaries main function is to work as a bridge between the larger arteries.
As the blood passes through the capillaries, the oxygen is passed on to the tissue. The blood enters and leaves the capillaries through venules which are very small veins. After passing through the venules, the blood enters the body's main veins that lead to the heart. Arteries contain thick muscular walls as they have a higher blood pressure. Veins and venules, on the other hand, have thin muscular walls as they contain a much lower amount of blood pressure within them.