In invertebrates, each cell is either in communication with the blood or is close enough to the blood sinuses such that the diffusion of substances across several intervening cells is feasible. A closed blood vascular system efficiently carries nutrients, gases, wastes, hormones, metabolites to and from distant actively metabolizing tissues. The blood vascular system consists of arteries , capillaries and veins . Arteries Arteries carry the blood away from the heart and are called efferent vessels. These vessels branch and become smaller as they carry the blood to the tissues.
Arteries have thicker walls than other vessels of the same size.
In very large arteries another external elastic membrane may be present between the media and collagen-rich adventitia. The copious elastic tissue present in the adventitia, media and intima of large vessels enable them to withstand the high blood pressures seen during contraction of the heart and the elastic recoil acts to maintain arterial pressure. Capillaries Capillaries are thin walled highly branched vessels that feed the tissues and collect wastes to be carried to the lungs, liver or kidneys for elimination. They are composed of a single layer of flattened cells surrounded by a porous basal lamina. The cells are elongated in the direction of blood flow, held together by zona occludens and gap junctions , and the nuclei bulge into the lumen of the capillary.