The tunica intima is the innermost layer. It has an endothelium and a thin layer of areolar connective tissue. The tunica media is the middle layer of the vessel wall. It has circularly arranged layers of smooth muscle cells. The tunica externa is the outermost layer. It has connective tissue, including elastic and collagen fibers.
Arteries conduct blood away from the heart toward capillary beds, while veins drain capillaries and conduct blood toward the heart. The lumen of an artery is narrower than that of a corresponding vein. In contrast, the thickness of the artery wall is greater than that of a corresponding vein. Arteries must withstand high blood pressure, while veins are not subject to high blood pressure. The tunica media is thicker in the artery than in the companion vein, while the tunica externa is thicker in the vein than in the companion artery.
Elastic arteries (conducting arteries) are the largest arteries. They have a large proportion of elastin throughout all three tunics, especially in the tunica media. The abundant elastin allows the artery to stretch when a ventricle contracts and ejects blood. Examples of elastic arteries include the aorta, pulmonary trunk, and brachiocephalic trunk. Muscular arteries (distributing arteries) are medium-sized arteries.The elastin in muscular arteries is confined to two circumscribed rings: the internal elastic lamina and the external elastic lamina. Muscular arteries have a proportionately thicker tunica media with multiple layers of smooth muscle fibers.The greater amount of muscle and lesser amount of elastic tissue result in less elasticity but better ability to vasoconstrict and vasodilate. Examples of muscular arteries include the brachial, anterior tibial, and coronary arteries. Arterioles are the smallest arteries and generally have less than six layers of smooth muscle in their tunica media. Larger arterioles have all three tunics, whereas the smallest arterioles may have a thin layer of endothelium surrounded by a single layer of smooth muscle fibers.
The thin wall and the narrow vessel diameter of capillaries are optimal for the function of diffusion of gases, nutrients, and wastes between blood in the capillaries and body tissues. The three basic kinds of capillaries are continuous capillaries, fenestrated capillaries, and sinusoids. Continuous capillaries have endothelial cells that form a complete, continuous lining and are connected by tight junctions. Materials can pass through endothelial cells or the intercellular clefts via simple diffusion or by pinocytosis. Fenestrated capillaries have holes within each endothelial cell. These fenestrations measure between 10 and 100 nanometers in diameter. The basement membrane remains continuous. Fenestrated capillaries are found where a great deal of fluid transport across the blood vessel wall occurs. Sinusoids have larger gaps than fenestrated capillaries, and they have a discontinuous or absent basement membrane. Sinusoids tend to be wider, larger vessels, and their large openings allow for transport of larger materials, such as proteins or formed elements, through vessel walls.
Blood pressure is greater in arteries than in veins. Hypertension causes functional and structural changes in the blood vessel walls, making them more prone to further injury. As damage increases, the blood vessels are more prone to atherosclerosis. Hypertension causes undue stress on arterioles, resulting in thickened arteriole walls and reduced luminal diameter (a condition called arteriolosclerosis). Furthermore, hypertension is a major cause of heart failure owing to the extra workload placed on the heart.
Three main arterial branches emerge from the aortic arch: (1) The brachiocephalic trunk bifurcates into the right common carotid artery supplying arterial blood to the right side of the head and neck, and the right subclavian artery supplying the right upper limb and some thoracic structures. (2) The left common carotid artery supplies the left side of the head and neck. (3) The left subclavian artery supplies the left upper limb and some thoracic structures.
Both the upper and lower limbs are supplied by a main arterial vessel (subclavian artery for the upper limb, femoral artery for the lower limb). The artery bifurcates at the elbow or knee. Arterial and venous arches are found in both the hand and the foot. Finally, both the upper limb and the lower limb have a superficial and deep network of veins.
The arteries of the systemic circulation carry oxygenated blood from the left side of the heart to body tissue capillary beds, and the veins carry deoxygenated blood from these capillary beds back to the right side of the heart. The pulmonary circulation is responsible for carrying deoxygenated blood in its arteries from the right side of the heart to the lungs, and then returning the newly oxygenated blood to the left side of the heart in its veins.
When the umbilical cord is clamped after birth, the umbilical vein and umbilical arteries constrict and become nonfunctional. They turn into the round ligament of the liver and the medial umbilical ligaments, respectively. After birth, since there is no blood going through the umbilical vein and the ductus venosus, the ductus venosus ceases to function and constricts to become the ligamentum venosum. Since pressure is now greater on the left side of the heart, the two flaps of the interatrial septum close off the foramen ovale. The only remnant of the foramen ovale is a thin, oval depression in the wall of the septum called the fossa ovalis. Within 10–15 hours after birth, the ductus arteriosus closes and becomes a fibrous structure called the ligamentum arteriosum.
As adults age, the heart and blood vessels become less resilient. Many of the elastic arteries are less able to withstand the forces of the pulsating blood. Systolic blood pressure may increase with age, exacerbating this problem. As a result, older individuals are more prone to developing an aneurysm, whereby part of the arterial wall thins and balloons out. In addition, aging increases the incidence and severity of atherosclerosis.
