Structure of Blood Vessels
The arteries and veins have the same structure and they are composed of three layers (see Fig. 6.8) :
- • Tunica intima: It consists of simple squamous endothelium, the subendothelial layer, and the internal elastic membrane. The tunica intima is in the innermost layer, as well as the thinnest layer.
- • Tunica media: The tunica media is the middle layer, which is a thick layer of elastic fibers and smooth muscle. The tunica media is thicker in the arteries than it is in the veins. The second and third layer is separated by another thick elastic band called external elastic lamina.
- • Tunica adventitia: The tunica adventitia is made of connective tissue that’s mostly collagen fibers, which contains nerves that supply the vessel and nutrient capillaries in the larger blood vessels.
Capillaries are extremely small vessels located within the tissues of the body, which contain a layer of endothelium and occasional connective tissue.
Blood is circulated through the body by the cardiovascular system, which contained the circulatory system and the heart. Oxygen-bearing blood is brought by the cardiovascular system to all the tissues of the body. The cardiovascular system pushes the blood out into two cycles when the heart contracts. The systemic loop begins when the oxygen-rich blood coming from the lungs flows into the left atrium. The
Fig. 6.8 Schematic diagram of blood vessel structures. An intact endothelium is contained in the lumen of the blood vessel. The blood contains cells such as platelets, leukocytes, and erythrocytes and plasma. The construction of the vessel lies on its location and size within the vasculature. Smaller vessels are surrounded by pericytes, which are regarded as the source of MSCs and preserve vessel maturity. On the other hand, larger vessels are surrounded by concentric layers of SMCs in order to maintain EC quiescence and vascular tone  (Copyright permission from ElsevierScience Ltd)
mitral valve will open when the chamber fills and the blood will flow into the left ventricle. Then the blood on the left flows into the aorta when the ventricles contract. Through the network of smaller arteries and capillaries, the blood brings oxygen to all organs. Through the network of veins, the used blood from the body come back to the heart .