Corpus: Blood vessel

A blood vessel is a hollow organ responsible for transporting blood. It is a tubular structure consisting of at least one, but usually several, wall layers surrounding an internal space known as the lumen.

Blood vessels collectively form the anatomical foundation of the circulatory system, enabling the delivery of oxygen and nutrients to the body. They transport blood to the body's periphery and return it to the heart.

Blood vessels are categorized as follows:

• Arteries
• Arterioles
• Capillaries
• Venules
• Veins

The aorta and vena cava are sometimes classified separately due to their distinct anatomical features.

The walls of larger blood vessels typically consist of three layers:

• Intima coat
• Middle coat (media)
• Adventitia

Capillaries, however, have a simpler structure. They are made up of a thin endothelial layer with numerous branched pericytes on the exterior. Unlike other blood vessels, capillaries are permeable, allowing the passage of specific molecules and blood cells.

4.1. Intima coat

The intima coat is composed of a single layer of endothelial cells aligned along the vessel's longitudinal axis. It facilitates the exchange of gases, fluids, and substances between the blood and surrounding tissues.

4.2. Middle coat

The middle coat contains a layer of smooth muscle and/or elastic connective tissue, with its composition varying by vessel type. For instance, arteries near the heart (elastic arteries) have more elastic tissue to accommodate the pulsatile blood flow (Windkessel function), while arteries further away (muscular arteries) have a thicker muscle layer for more precise regulation of blood flow.

4.3. Adventitia

The adventitia is the outermost layer of connective tissue surrounding the vessel. In larger vessels with thick walls, it may also contain small nourishing blood vessels called the vasa vasorum.

Blood vessels can adjust their diameter by contracting or relaxing the muscle layer in their walls, thereby influencing blood flow. This is particularly significant in arteries but occurs to a lesser extent in veins.

The diameter of blood vessels is regulated by fibers of the autonomic nervous system, which can initiate two opposing processes:

• Vasodilation (widening of the vessel)
• Vasoconstriction (narrowing of the vessel)

This mechanism allows the body to control the distribution of blood volume, optimizing oxygen delivery or thermoregulation in specific body regions.

Blood vessels maintain physiological blood pressure, which ranges from 80 to 120 mm Hg in arteries and up to 10 mm Hg in veins (central venous pressure).