Structure of Blood Vessels: Arteries, Veins & Capillaries

Structure of Blood Vessels: The heart is widely recognised for pumping blood to all parts of the body. But how does this blood get to all of the body’s different parts? It is carried to all regions of the body by an artery, which is a type of blood vessel that transports oxygenated blood. The blood pressure is extremely high in this area. But how could these arteries withstand such strain?

Even when blood pressure is low, how do most veins transport deoxygenated blood from body tissues back to the heart? To understand all of this, we must first understand the structure of blood vessels.

The structure of blood vessels, such as arteries, veins, and capillaries, will be discussed in this article.

Blood Vessels

1. Blood circulates throughout the body continuously through a series of arteries, veins and capillaries.
2. Blood vessels are elastic and muscular tubes that carry blood in animals.
3. The study of blood vessels is called angiology. (angeion = vessel, logos = study)

Arteries

1. Arteries refer to the blood vessels which carry blood from the heart to the various parts of the body.
2. All the arteries carry oxygenated blood, except the pulmonary artery.
3. Anastomosis: If more than one artery is supplying to one organ, then branches of these arteries unite to form a network called anastomosis.

Learn The Concept of Blood Vessels

Veins

1. Veins refer to the blood vessels which carry blood from various parts of the body to the heart.
2. All the veins carry deoxygenated blood, except the pulmonary vein.

Capillaries

1. Arteries and veins are joined together by capillaries in the tissues.

Structure of Blood Vessels

Arteries and veins are made of three layers, i.e.,
1. Tunica Interna
a. It is the innermost and the thinnest lining of the blood vessel.
b. It surrounds the hollow space in the blood vessel called the lumen and comes in contact with the blood.
c. It is the inner lining of the squamous epithelium, which consists of two layers, the inner endothelium resting on a basement membrane and the outer elastic membrane of yellow elastic tissue.
d. The outer elastic membrane separates the tunica interna from tunica media.
e. Endothelium provides a smooth surface that minimizes resistance to blood flow.

2. Tunica Media
a. It is the middle layer of the artery and veins.
b. It is the thickest layer in the artery. But in veins, the elastic membrane of tunica media is relatively thinner than artery media.
c. It consists of elastic fibres are arranged in a circular manner. In contrast, muscle fibres and elastic fibres are fewer in tunica media of veins.
d. Therefore, veins have a thinner and less elastic wall but a wider cavity than an artery of the same diameter.
e. It is rich in smooth muscle mixed with a network of elastic fibres.

3. Tunica Externa
a. Also known as tunica adventitia (additional coat).
b. It is the outermost layer of the arteries and veins.
c. It surrounds the tunica media.
d. It is made of collagen and elastic fibres.
e. Blood vessels are anchored to nearby organs with the help of collagen present in the outer layer, tunica externa.
f. External elastic lamina surrounds the tunica externa of arteries.
g. Functions: This layer does not allow the arteries to expand much and gives structural support to the blood vessels.
h. Scurvy is a vitamin C deficiency disease that is related to this tunica externa.
i. Vitamin C is important to maintain the collagen fibres, and its deficiency makes the outer layer devoid of collagen, thus leading to rupturing of the layer.

Fig: Structure of Blood Vessels

Valves
1. Valves are present in veins except for pulmonary vein and not in arteries except pulmonary artery and aorta in which valve is there.
2. It prevents the backflow of blood in veins, i.e., ensures the flow of blood in one direction.

Fig: Artery and Vein

Capillaries
1. It is the junction between arterioles and venules.
2. Arteries divide into still smaller vessels called the arterioles. While capillaries unite to form larger vessels, called the venules, which in turn join to form still larger vessels, the vein.
3. Capillaries possess about 7% of total body blood and are present near almost all cells of the body in the intercellular spaces.
4. Capillaries are surrounded by cells of connective tissue called pericapillary cells. Some of these cells are contractile and phagocytic in nature and are called Rouget cells or pericytes.
5. Capillaries consist solely of endothelium. So, it is involved in the exchange of water and dissolved substances between the blood and body cells.

Fig: Capillaries

6.  Capillaries are of three types-
a. Continuous capillaries: It has a continuous endothelial lining, due to which only smaller molecules can pass through it. These are present in organs such as lungs, muscles, connective tissues and brain tissues.
b. Fenestrated capillaries: It has pores and not continuous endothelial lining. So it allows smaller but limited amounts of molecules to diffuse through it. These are present in endocrine glands, intestinal villi, cavities of the brain, kidney, and ciliary body of the eye.
c. Sinusoidal capillaries: It is also called discontinuous capillaries, as, with pores, it has irregular spaces. So it allows larger cells like red and white blood cells to pass through it. Found in the liver, spleen, bone marrow, parathyroid, pituitary gland, etc.

Fig: Types of Capillaries

7. Depending on functions, the capillary can also be divided as:
a. Arteriolar capillary: It supplies nutrition, respiratory gases etc., to the body cells.
b. Veinular capillaries: It collects the metabolic wastes from the body cells.

Difference between Arteries and Veins

Summary

Arteries, veins and capillaries are the types of blood vessels. Arteries and veins consist of three layers, tunica externa, tunica media and tunica interna. In arteries, tunica media is thicker than in veins. The elastic membrane of the tunica interna of arteries is also thicker than the vein. Capillaries consist solely of the endothelium. So, it is involved in the exchange of water and dissolved substances between the blood and body cells. Blood flows from arteries to arterioles, then to capillaries, to venules to veins. Valves are present in veins and not in arteries to check the backflow of blood.

Frequently Asked Questions

We have provided some frequently asked questions here:

Q.1. Which type of blood vessel is the smallest?
Ans: Capillaries are the smallest blood vessel.

Q.2. What is the difference between artery and vein?
Ans: The difference between arteries and veins is-
a. An artery carries oxygenated blood except for the pulmonary artery, while veins carry deoxygenated blood except for pulmonary veins.
b. Tunica media in the artery is thick while in the vein is thin.
c. Arteries carry blood away from the heart, while veins carry blood towards the heart.

Q.3. What is blood vessels function?
Ans: Functions of blood vessels are:-
1. Transports blood
2. Arteries carry oxygenated blood and veins deoxygenated blood except for the pulmonary artery and pulmonary vein, respectively.
3. Arteriolar capillary supplies nutrition, respiratory gases etc., to the body cells.
4. Veinular capillaries collect the metabolic wastes from the body cells.

Q.4. What are the 3 types of blood vessels?
Ans: Blood vessels are:
a. Arteries
b. Veins
c. Capillaries

Q.5. What is the structure of arteries and veins?
Ans: Arteries and veins are made of three layers- tunica externa, tunica media and tunica interna. Tunica media is thick in arteries while it is thin in veins.

Study The Concept Of Blood

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