What Are Human Blood Vessels?

Blood vessels are small tubes made up of three distinct layers:

• The inner layer, tunica intima, consists of the endothelium, which is made up of a thin layer of elongated endothelial cells. The endothelium plays a crucial role in regulating the passage of nutrients and wastes through the blood vessel.
• The middle layer, tunica media, consists of circular layers of smooth muscle cells that are made up of elastic collagen fibers. This layer is the thickest in the arteries.
• The external layer, tunica externa or tunica adventitia, is made up of loose fibrous connective tissue and extracellular matrix produced by epithelial cells. This layer is thicker in veins.

Based on the structure and function, blood vessels are classified into arteries, capillaries, and veins.

Arteries

Arteries carry blood away from the heart. They are muscular, elastic, and thicker than veins.

All the arteries, except for the pulmonary artery, carry oxygenated blood and nutrients. The pulmonary artery transports deoxygenated blood from the right ventricle (the right lower heart chamber) to the lungs.

Systemic arteries transport oxygenated blood, which is saturated with 95-100% oxygen, from the left ventricle of the heart to the body tissues. Aorta is the largest artery and arises from the left ventricle.

Blood from the ventricles is pumped into arteries and propelled through them by the high pressure generated by the pumping of the heart. The middle layer of the artery wall, tunica media, is usually thicker and provides support and alters the vessel diameter to regulate blood pressure and blood flow.

Arteries branch off repeatedly into smaller arteries called arterioles. Arterioles carry blood with oxygen and nutrients into the smallest blood vessels called the capillaries.

Capillaries

Capillaries form the connection between the arteries and the veins. They are the smallest and most numerous of the blood vessels, forming a network of meshes of varying sizes throughout the body tissues.

Capillaries are very small and can be seen only under a microscope. The diameter of a capillary is very narrow, about 8-10 microns, which is just sufficient to let a single row of red blood cells pass through.

Through the thin walls of the capillaries, the exchange of nutrients, gases, and wastes takes place between the blood and tissue cells. The walls are permeable, allowing oxygen to move into the cells of the tissues and organs from the capillary and carbon dioxide to move from the cells into the capillaries.

Veins

Veins carry blood toward the heart. Blood passes from the capillaries and enters the smallest veins called venules and then flows into progressively larger veins.

In general, all veins except the pulmonary vein, carry deoxygenated blood from the tissues back to the heart.

The pulmonary veins transport oxygenated blood from the lungs to the left atrium of the heart. Systemic veins transport deoxygenated blood from the body’s tissues into the right atrium of the heart.

The superior vena cava and inferior vena cava, also called the central veins, are the largest veins. The superior vena cava carries blood from the brain and arms to the right atrium of the heart whereas the inferior vena cava carries blood from the legs and abdominal cavity to the right atrium.

Veins are often found closer to the skin and are less muscular and thinner than arteries. About 70% of the total blood volume is found in the veins. Veins have venous valves, which prevent the backward flow of blood in response to gravity.

Blood vessels play crucial roles within the body. Their functions are:

• Transporting blood with oxygen and nutrients (glucose, amino acids, and lipids) throughout the body.
• Removing waste products from the cells of the tissues and organs.
• Aiding in gas exchange.
• Maintaining homeostasis (a state of balance and equilibrium) and health.
• Working as a network for white blood cells to allow their circulation in the body when the immune response is activated against infection or injury.
• Blood vessels near the skin play a key role in thermoregulation by increasing or decreasing blood flow.