Blood vessel

Tiny tubes carry blood through your body.

These tubes move food and the air you breathe to your parts. They also take waste away.

Some tubes move blood away from your heart. These are called arteries. Other tubes carry blood back to your heart. These are called veins.

There are also very small tubes. These are called capillaries. They are as thin as a hair.

Your heart pushes the blood through these tubes. This keeps the blood moving all the time.

It is a very busy job. These tubes help you stay well!

Blood vessels are tiny tubes in your body.

They act like a road system for your blood. These tubes carry oxygen and food to your cells. They also take waste away.

There are five main types of vessels. Arteries carry blood away from your heart. Arterioles are smaller versions of arteries. Capillaries are the smallest type. They are very thin. This is where blood swaps water and chemicals with your body. Venules are small tubes that lead to veins. Finally, veins carry blood back to your heart.

Most vessels have three layers. The middle layer is called the tunica media. This layer is thick in arteries. It has muscle that can make the tube wider or narrower. This helps control blood flow.

Some parts of your body have no blood vessels. These parts are called avascular. This includes the lens of your eye. Your blood moves because your heart pumps it. This creates pressure to push the blood along.

Some veins have valves. These are small flaps. They stop blood from flowing backward. This helps blood move against gravity.

Blood vessels are the tubular structures that make up a circulatory system. They act like a vast network of paths for your blood. These vessels carry important things like oxygen and nutrients to your tissues. They also pick up waste products and carbon dioxide to move them away. Some parts of the body are avascular, which means they have no blood vessels. Examples of these include the lens and cornea of the eye.

There are five main types of blood vessels that work together. Arteries carry blood away from the heart, while arterioles are smaller versions of them. Capillaries are the smallest type of vessel. This is where the exchange of water and chemicals happens between blood and tissues. Venules are small vessels that lead into the larger veins. Finally, veins carry the blood from the capillaries back toward the heart.

Most blood vessels are built with three distinct layers. The inner layer is the tunica intima, which is a single layer of flat cells. The middle layer is the tunica media, and it is the thickest in arteries. This middle layer contains smooth muscle that can change the vessel's size. This process is called vasoconstriction when the vessel narrows. It is called vasodilation when the vessel becomes wider.

Scientists have studied how these vessels function for a long time. A Danish physiologist named August Krogh made early estimates about capillary length. He thought the total length of capillaries in human muscles could be very large. The largest artery in the body is called the aorta. It can have a diameter of about 25 to 30 millimeters. In contrast, capillaries are tiny, measuring only about 5 micrometers across.

Your heart provides the power to keep this system moving. The heart creates pressure to propel blood through the arteries and arterioles. In the lungs, capillaries allow blood to receive oxygen and release carbon dioxide. This happens through tiny air sacs. In the rest of the body, arteries usually carry blood that is 95% to 100% full of oxygen. Veins carry blood that is about 75% full of oxygen.

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.