Blood vessel

Blood vessels are the tubular structures that form a circulatory system in animals. They serve as a transport network for blood cells, nutrients, and oxygen. This system also moves waste products and carbon dioxide away from body tissues. Some tissues are considered avascular, meaning they lack a blood vessel supply. This includes the epithelium, cartilage, and the cornea and lens of the eye.

There are five primary types of blood vessels that function in a specific sequence. Arteries carry blood away from the heart, while arterioles are smaller branches of these vessels. Capillaries are the smallest vessels, where the exchange of chemicals and water occurs between blood and tissues. Following the capillaries, venules collect the blood to lead it toward the veins. Finally, veins transport the blood back toward the heart. Some specialized vessels called sinusoids exist in the liver, spleen, and bone marrow.

Most arteries and veins are organized into three distinct structural layers. The innermost layer is the tunica intima, which consists of a single layer of flat cells called simple squamous epithelium. This layer is surrounded by subendothelial connective tissue and an internal elastic lamina. The middle layer is the tunica media, which contains elastic fibers and connective tissue. In arteries, the tunica media is especially thick and contains vascular smooth muscle. This muscle allows the vessel to regulate its caliber, or diameter.

The outermost layer is known as the tunica adventitia. This layer is composed entirely of connective tissue. In larger blood vessels, the adventitia contains nerves and nutrient capillaries called vasa vasorum. While arteries have both an internal and external elastic lamina, veins only possess an internal one. The tunica media is also significantly thicker in arteries than it is in veins. To prevent the backflow of blood against gravity, some veins utilize valves.

Blood vessels regulate flow through processes called vasoconstriction and vasodilation. Vasoconstriction occurs when vascular smooth muscle contracts, narrowing the vessel's cross-sectional area. This process is managed by the autonomic nervous system and various agents like epinephrine or angiotensin. Conversely, vasodilation widens the vessel, often mediated by nitric oxide. These antagonistic processes help the body manage thermoregulation and blood flow to specific organs. The diameter of these vessels varies greatly, from the 25–30 millimeter aorta to 5 micrometer capillaries.

The movement of blood relies on pressure generated by the heartbeat. Oxygen-rich blood enters the left side of the heart through pulmonary veins. It then moves into the aorta to reach the rest of the body. In most arteries, hemoglobin is 95% to 100% saturated with oxygen. In most veins, the oxygen saturation of hemoglobin is about 75%. In the pulmonary circulation, these values are reversed as blood moves to the lungs.

Blood flow faces resistance from three main factors: viscosity, vessel length, and vessel radius. Blood viscosity refers to the thickness of the blood, which is 92% water by weight. As the total length of a vessel increases, the resistance caused by friction also increases. Additionally, a smaller vessel radius increases resistance because more blood makes contact with the vessel wall. Pressure in the arterial system is typically around 120 mmHg systolic and 80 mmHg diastolic. In contrast, venous pressure is much lower and rarely exceeds 10 mmHg.

Understanding blood vessels is vital for studying many medical conditions. Atherosclerosis, which involves plaque formation, causes about 85% of cardiovascular disease deaths. This can lead to coronary artery disease, contributing to significant mortality rates. For example, coronary artery disease caused over 919,000 deaths in the United States in 2023. Furthermore, cancer relies on angiogenesis, which is the formation of new blood vessels, to meet its metabolic demands. Maintaining the health of the endothelium is essential to prevent inflammation and hemorrhage.