The three types of blood vessel are shown in the diagram. We do refer to arterioles and venules, which are the tiniest of arteries and veins on each side of the capillary networks.

The arteries have a thick wall with a good deal of elastic tissue and smooth muscle. Their lumen is relatively small compared to their overall diameter and they are lined by a substantial endothelium. They  carry blood at high pressure and the smooth muscle contracts in a peristaltic wave, the pulse, to push the blood away from the heart and maintain a high pressure.

The vein has a much thinner wall with a small amount of elastic tissue and very little smooth muscle. The vein’s lumen is large compared to its diameter and the vein is lined by endothelium. They carry blood at low pressure and have valves along their length so that blood must flow in the direction of the heart. The contraction of skeletal muscles compresses the veins helping to force blood from one section, through the valves into the next section.

The capillary has a wall composed of a single layer of endothelial cells held together somewhat loosely by a basement membrane of microscopic fibres and this allows the capillaries to lose fluid as the pressurised blood enters them, thus facilitating exchange of materials and the formation of tissue fluid or lymph. As fluid is lost from the capillaries and the pressure falls some fluid moves back into the capillaries by osmosis.