Blood Vessels

Blood vessels

Mammals have three types of blood vessels: arteries, veins, and capillaries.

Arteries

Arteries are thick-walled, muscular, and elastic vessels that carry oxygenated blood from the heart to various parts of the body. The pulmonary artery is the exception, carrying deoxygenated blood to the lungs.

Cross-section of an artery

The arterial wall consists of three layers:

1. Endothelium – The innermost layer, one cell thick, forming a smooth lumen.
2. Muscular Layer – Smooth muscle and elastic fibers, allowing contraction and relaxation for efficient blood flow.
3. Fibrous Layer – Connective tissues, such as collagen, enabling the artery to withstand high pressure from the heart.

The arteries expand and contract with each heartbeat, creating the pulse, which reflects the heart rate. An average adult human pulse rate is around 72 beats per minute.

Arteries branch into arterioles, which then lead to capillaries. These smaller vessels join into venules, which eventually form veins.

Veins

Veins are blood vessels that transport blood to the heart. They are less muscular and have a larger lumen than arteries because blood in veins flows at low pressure. Veins contain valves that prevent blood from flowing backward, and surrounding muscles help move the blood towards the heart. The pulmonary vein is an exception, transporting oxygenated blood from the lungs to the heart.

Cross-section of a vein

Functioning of a valve

Capillaries

Capillaries are the smallest blood vessels, consisting of a single layer of endothelial cells. They form a network in tissues for the efficient exchange of substances. Their thin walls facilitate the diffusion of oxygen and nutrients from the blood to the cells, and the removal of waste products like carbon dioxide. Capillaries join to form venules, which then form veins.

Cross-section of a capillary

Differences between arteries, veins, and capillaries

Feature	Arteries	Veins	Capillaries
Lumen Size	Narrow, smooth	Wide, irregular	Narrow, smooth
Wall Thickness	Thick, muscular	Thin, less muscular	One cell thick
Valves	None, except at heart	Present at regular intervals	None
Pressure	High	Low	Low
Function	Transport blood from the heart	Transport blood to the heart	Exchange gases and nutrients
Blood Type	Oxygenated, except pulmonary	Deoxygenated, except pulmonary	Either oxygenated or deoxygenated