Blood Vessels

BLOOD VESSELS

Blood vessels are the important components of the circulatory system that transport blood throughout the human body. They form a closed network through which blood continuously circulates between the heart and body tissues.

Blood is carried through the body only via blood vessels, which vary in size, structure, and function depending on their role in circulation.

An artery is a blood vessel that carries blood away from the heart. As arteries move farther from the heart, they divide repeatedly into smaller vessels.

The smallest branches of arteries are called arterioles. Arterioles further branch into extremely thin vessels known as capillaries, where the exchange of nutrients, gases, and waste materials takes place.

After exchange occurs in the capillaries, blood enters small vessels called venules. Venules unite to form veins, which are larger blood vessels that return blood back to the heart.

The walls of blood vessels contain varying amounts of fibrous tissue, elastic tissue, and smooth muscle, depending on the type of vessel and the pressure of blood flowing through it.

Although arteries and veins differ in structure and function, both possess three basic layers of tissue.

Layers of Blood Vessel Wall

1.     Tunica interna (Intima)

o    The innermost layer

o    Made of a smooth epithelial lining

o    Provides a frictionless surface for blood flow

2.     Tunica media

o    The middle layer

o    Composed of smooth muscle and elastic connective tissue

o    Responsible for regulating blood vessel diameter

3.     Tunica externa (Adventitia)

o    The outermost layer

o    Made of connective tissue

o    Provides strength and support to the vessel

Types of Blood Vessels

Structure of an Artery and a Vein

Blood vessels differ in structure, size, and function, and are classified into arteries, arterioles, capillaries, venules, and veins.

Both arteries and veins consist of three layers of tissue, but their thickness and composition vary based on blood pressure and function.

Arteries have thicker walls to withstand high pressure, whereas veins have thinner walls because blood flows through them at lower pressure.

Relationship Between the Heart and the Different Types of Blood Vessels

Arteries and their smaller branches, arterioles, carry blood away from the heart.

Arterioles divide into a vast network of thin-walled capillaries. These capillaries allow oxygen, nutrients, and water to diffuse into tissues, while carbon dioxide and waste products diffuse into the bloodstream.

Capillaries merge to form small venules, which further unite to form large veins. These veins carry blood back to the heart, completing the circulatory loop.

Arteries

Arteries are the main blood vessels that carry oxygenated (oxygen-rich) blood from the heart to various parts of the body (except pulmonary arteries).

They are the strongest blood vessels with thick, muscular, and elastic walls designed to withstand high blood pressure.

Arteries consist of three distinct layers and are generally located deep within the body. They appear red in colour due to oxygen-rich blood.

Blood flows through arteries under high pressure, moving in a downward direction from the heart to body tissues.

Arterioles

Large arteries divide into medium-sized muscular arteries, which further divide into smaller arteries known as arterioles.

Approximately 400 million arterioles are present in the body, with diameters ranging from 15 µm to 300 µm.

Arterioles play a vital role in:

• Delivering blood to capillaries
• Regulating blood flow
• Controlling blood pressure

The terminal part of an arteriole, called the metarteriole, tapers toward the capillary junction.

Arterioles regulate blood flow by controlling vascular resistance, which is the opposition to blood flow caused by friction between blood and vessel walls.

Capillaries

Capillaries are the smallest blood vessels in the body, with diameters of approximately 5–10 µm.

They form U-shaped networks that connect arterial outflow to venous return.

Capillaries are the primary sites of exchange, where:

• Oxygen and nutrients move into tissues
• Carbon dioxide and waste products move into blood

They connect the arterial system to the venous system and play a crucial role in maintaining tissue health.

Venules

Venules drain blood from capillaries and initiate the return of blood toward the heart.

They are the smallest veins, measuring about 10 µm to 50 µm in diameter.

Venules collect deoxygenated blood from capillaries and deliver it to larger veins.

Veins

Veins are blood vessels that return blood to the heart at low pressure.

The walls of veins are thinner than those of arteries, but they still consist of the same three tissue layers.

Veins vary in size from 0.5 mm in small veins to about 3 cm in large veins such as the superior and inferior vena cava.

They are thinner because they contain less smooth muscle and elastic tissue in the tunica media, as veins carry blood at lower pressure than arteries.

Comparison of Arteries and Veins