24.2: Structure of Blood Vessels
Blood is circulated throughout the human body through a network of blood vessels called the circulatory system. This system includes arteries that transport blood from the heart to various body parts. These arterial pathways divide into smaller vessels until they reach the arterioles, which further split into capillaries. It is within these minuscule capillaries that the exchange of nutrients and waste products takes place. After this exchange, the blood is collected by venules, which fuse to form veins, returning the blood to the heart.
Certain commonalities can be discerned by examining the structures of these various types of vessels. Arteries and arterioles have thicker walls due to their proximity to the heart and the high-pressure blood they receive. Their lumens, or interior passageways through which blood flows, are smaller than those of veins, assisting in maintaining blood pressure. In contrast, the initial pressure exerted by heart contractions is significantly reduced when blood reaches the venules, having passed through capillaries. As a result, venules and veins experience lower blood pressure, have thinner walls, and larger lumens, facilitating more effortless blood flow. Many veins, especially those in the limbs, contain valves that help direct blood toward the heart. This counteracts slowing blood flow in extremities due to lower pressure and gravity.
Artery and vein walls consist primarily of living cells and their byproducts, such as collagenous and elastic fibers. These cells require nourishment and produce waste. As blood travels quickly through larger vessels, the opportunity for nourishment exchange is limited. Furthermore, large vessel walls are too thick for nutrients to permeate entirely. Larger arteries and veins contain vasa vasorum, minute vessels within the vessel walls that aid in nourishment and waste exchange. The comparatively high pressure within arteries necessitates the positioning of vasa vasorum in the outer layers of the vessel, preventing collapse due to blood pressure. However, lower pressure within veins allows the vasa vasorum to be situated nearer to the lumen. The prevalence of arterial diseases over venous diseases can be attributed to the outer positioning of the vasa vasorum, complicating nourishment delivery and waste removal. Additionally, tiny nerves, known as nervi vasorum, are found within the walls of both types of vessels and are responsible for regulating smooth muscle contraction and dilation.
The general structure of the blood vessels includes three layers. The innermost layer, the tunica intima (conventionally termed tunica interna), is composed of epithelial and connective tissues. The innermost lining consists of a specialized squamous epithelium known as the endothelium, which extends throughout the vascular system, including the heart's internal chambers. The disruption of this lining, which exposes blood to the underlying collagen fibers, significantly contributes to blood clot formation. Initially perceived merely as a demarcating layer between the vessel walls and the lumen, contemporary research accentuates the critical physiological role of the endothelium in activities like capillary exchange regulation and blood flow alteration. The endothelium produces local chemicals known as endothelins, which can constrict the muscular component of the vessel walls, thereby elevating blood pressure. Prolonged endothelin overproduction may lead to hypertension and cardiovascular disorders.
Adjacent to the endothelium lies the basement membrane or basal lamina, which effectively links the endothelium to the connective tissue. This membrane ensures strength and flexibility and allows material passage. The thin tunica intima's outer layer houses a slight amount of areolar connective tissue, mainly comprising elastic fibers for additional flexibility and collagenous fibers for strength.
The tunica media, the middle layer of the vessel wall, is the thickest layer in arteries and is notably thicker in arteries than in veins. Composed of smooth muscle layers reinforced by connective tissue, primarily elastic fibers, it contributes to the contraction and relaxation of the circular muscles, influencing the diameter of the vessel lumen. Regulated by the nervi vasorum, vasoconstriction and vasodilation alter blood flow and pressure. Hormonal and local chemical factors also monitor blood vessels, adjusting blood flow according to body conditions.
The tunica externa, the outermost tunic, forms a substantial sheath primarily composed of collagenous fibers with some elastic fibers. This is the thickest tunic, particularly in veins, and may surpass the tunica media in some larger arteries. This tunic merges with the surrounding connective tissue, stabilizing the vessel's position.
