The vascular system is a closed network of tubes that transports blood throughout the human body. It acts as the body's highway system, delivering oxygen and nutrients to tissues while removing metabolic waste products. This continuous circuit begins and ends at the heart, branching into two main pathways: the pulmonary circulation, which takes deoxygenated blood to the lungs, and the systemic circulation, which supplies oxygenated blood to the rest of the organs. Understanding this layout helps nurses trace how cardiovascular diseases affect distant parts of the body.
Arteries carry blood away from the heart under high pressure. Because they face significant force with every ventricular contraction, their walls are thick, muscular, and elastic. As arteries travel farther from the heart, they branch into smaller vessels called arterioles. Arterioles regulate blood flow into capillary beds by changing their diameter through vasoconstriction and vasodilation, earning them the title of resistance vessels.
Capillaries are the microscopic vessels where the actual exchange of gases, nutrients, and wastes occurs between the blood and tissue cells. Their walls consist of a single layer of endothelial cells resting on a basement membrane, which minimizes the diffusion distance. Bed networks weave closely around nearly every cell in the body, though metabolic demand dictates their density in specific tissues.
| Layer | Primary Tissue Composition | Main Function |
|---|---|---|
| Tunica Intima | Simple squamous epithelium (endothelium) | Provides a smooth, friction-reducing lining |
| Tunica Media | Smooth muscle and elastic fibers | Controls vessel diameter and withstands pressure |
| Tunica Externa | Collagen and connective tissue | Anchors and protects surrounding structures |
Blood flow is driven by pressure gradients and opposed by vascular resistance. Blood moves from areas of high pressure to areas of low pressure. Resistance is determined by blood viscosity, vessel length, and vessel radius. Even minor changes in vessel radius through vasoconstriction or vasodilation dramatically alter resistance and flow, making the arterioles the primary controllers of systemic blood pressure.
Pathologies affecting blood vessels frequently present in clinical settings. Atherosclerosis involves the buildup of fatty plaques in the arterial wall, narrowing the lumen and restricting blood flow. Varicose veins occur when superficial vein walls weaken and valves fail, causing blood to pool and veins to dilate. Aneurysms represent abnormal ballooning of weakened arterial walls, carrying a high risk of rupture and internal hemorrhage.