The vascular system comprises an extensive network of arteries, capillaries, and veins. The vascular system can be broadly divided into the blood and lymphatic systems. Typically, blood vessels can be categorized into three histological regions: tunica intima, tunica media, and tunica adventitia. The tunica intima consists of a single layer of endothelial cells attached to the basal lamina. Underlying the basal lamina is a connective tissue layer and an elastic lamina that gives stability and flexibility to the endothelial lining. Endothelial cells remain in close contact with the multipotent perivascular cells called pericytes. These pericytes help endothelial cells modulate blood flow by providing contractile strength. Below the tunica intima lies the tunica media composed of smooth muscle cells and elastin fibers. They mainly offer structural support to the endothelial lining of blood vessels. The outermost layer is formed by fibro-elastic connective tissue called the tunica adventitia.
The lymphatic system is composed of capillaries and large vessels called collecting vessels lined by endothelial cells. The lymphatic endothelial cells arise from blood endothelial cells during embryonic development. While the blood endothelial cells are mainly involved in transporting nutrients and oxygen, the lymphatic vascular system is essential for transporting immune cells, fat mobilization, and drainage of interstitial fluid. Although functionally distinct, the blood and the lymphatic vascular system interact closely at lymph nodes and regulate fluid homeostasis of the body.
