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blood vessel
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Diagram of the circulatory system showing a red artery carrying blood from the heart, branching into smaller arterioles and capillaries, then merging into blue venules and a vein returning blood to the heart. Zebra fish brain vasculature
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Diagram of the circulatory system showing a red artery carrying blood from the heart, branching into smaller arterioles and capillaries, then merging into blue venules and a vein returning blood to the heart.
Basic concepts of circulation Blood flows from the heart through arteries and into capillaries. It then returns to the heart through veins.

blood vessel

anatomy
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Top Questions

What are the main types of blood vessels?

What is the function of capillaries?

How do veins help move blood against gravity?

What is angiogenesis?

What are some common blood vessel-related conditions?

blood vessel, a tubular structure that carries blood throughout the body as part of the circulatory system.

The different types of blood vessels
  • Arteries: Carry blood away from the heart to the body’s tissues. Their thick, elastic walls withstand the high pressure generated by the heartbeat.
  • Arterioles: Small branches of arteries that regulate blood flow into capillary networks by constricting or dilating.
  • Capillaries: Microscopic vessels where oxygen, nutrients, hormones, and waste products are exchanged between the blood and surrounding tissues.
  • Venules: Small vessels that collect blood from capillaries and channel it into veins.
  • Veins: Return blood to the heart. Their thinner walls and one-way valves help move blood against gravity and prevent backflow.

The vessels that carry blood away from the heart are called arteries, and their very small branches are arterioles. Very small branches that collect the blood from the various organs and parts are called venules. Venules unite to form veins, which return the blood to the heart. Capillaries are minute thin-walled vessels that connect the arterioles and venules; it is through the capillaries that nutrients and wastes are exchanged between the blood and body tissues.

Zebra fish brain vasculature
Zebra fish brain vasculatureConfocal microscope image of the brain vasculature of a live zebra fish larva. The brain vasculature plays an important role in maintaining brain health and function.

The inner surface of every blood vessel is lined by a thin layer of cells known as the endothelium. The endothelium is separated from the tough external layers of the vessel by the basal lamina, an extracellular matrix produced by surrounding epithelial cells. The endothelium regulates the movement of nutrients, gases, hormones, immune cells, and waste products between the bloodstream and surrounding tissues. It also helps control blood clotting, inflammation, and the widening or narrowing of blood vessels. Under certain circumstances, tissues may grow new blood vessels, a process known as angiogenesis. Angiogenesis is essential for normal growth, wound healing, and tissue repair but also contributes to diseases such as cancer by supplying growing tumors with oxygen and nutrients.

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In humans, the function and structure of blood vessels can be affected by a number of different diseases and conditions. Common conditions include atherosclerosis, in which fatty deposits accumulate within artery walls; hypertension, or persistently high blood pressure, which places excess strain on blood vessels and the heart; and vasculitis, inflammation of blood vessels that can restrict circulation and damage organs. .

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Kara Rogers.