Blood Vessels: Do you know how the blood is carried throughout our body? Blood is carried by the blood vessels that form one of the components of the circulatory system. The heart pumps blood throughout the body in a network of blood vessels. Blood vessels carry the blood to deliver the nutrients to and remove wastes from our trillions of cells. Blood, heart and blood vessels together form the cardiovascular system.

Blood vessels are divided into three types based on their structure and functions namely, arteries, veins, and capillaries. The study of blood vessels is often referred to as angiology. In this article, we will study the definition of blood vessels, their structure, types, functions, and more. Scroll down to continue reading!

Definition of Blood Vessels

The blood vessels are an intricate network of hollow tubular structures carrying blood throughout the body. They transport blood cells, nutrients and oxygen and carry away carbon dioxide and waste materials from the tissues and organs. The study of blood vessels is called angiology.

Blood Vessels Structure or Anatomy

The walls of the blood vessels (except capillaries) are made up of three layers from outside to inside, surrounding the central blood carrying canal called a lumen. They are:

1. Tunica Adventitia (Tunica Externa): They form strong outer covering of arteries and veins composed of loose connective tissues with longitudinally arranged white and yellow fibres. These fibres allow the arteries and veins to stretch to prevent overexpansion due to the pressure that is exerted on the walls by blood flow.
2. Tunica Media: This layer consists of circular smooth muscles and elastic fibres, and is thicker in arteries than in veins. It provides support to the vessel and also changes the diameter of the vessel to regulate blood flow and blood pressure.
3. Tunica Intima (Tunica Interna): This layer has inner endothelium of long, thin cells of squamous epithelium resting on a basement membrane along with an outer elastic membrane of yellow elastic tissue. It forms the inner layer of arteries and veins.

Fig: Anatomy of Artery and Vein

Types of Blood Vessels

Blood vessels can be divided into three types based on their structure and functions. Below we have listed the types of blood vessels:

1. Arteries
2. Veins
3. Capillaries

1. Arteries

a. Arteries are elastic vessels that carry blood from the heart to different parts of the body.
b. All arteries carry pure or oxygenated blood, with the exception of the pulmonary artery that carries the blood from the heart to the lungs for oxygenation.
c. As the walls of the arteries are thick and non-collapsible, the pressure inside them is very high.
d. Valves are absent in arteries.
e. The largest artery is the aorta, which originates from the heart and branches out into smaller arteries.
f. Arteries are divided into fine branches, known as arterioles, which are further divided to form finer branches called capillaries.
g. The arterioles play an important role in regulating blood flow into the tissue capillaries.
h. About \(10\% \) of the total blood volume is in the arterial system at any given time.

2. Veins

a. Veins are elastic vessels that bring blood from different body parts to the heart, i.e., they carry blood towards the heart.
b. The walls of veins have fewer smooth muscles and connective tissues compared to arteries. Thus, the walls of the veins are thinner than that of the arteries.
c. The blood in the veins has less pressure than in the arteries.
d. All veins carry deoxygenated blood except the pulmonary vein that carries oxygenated blood from lungs to the heart.
e. Veins are provided with valves (in the tunica intima layer) to prevent the backward flow of blood.
f. Veins can hold more blood as their walls are thinner and less rigid compared to arteries.
g. The smallest veins in the body are called venules, which receive blood from the arteries via arterioles and capillaries.
h. From the venules, the blood flows progressively into larger and larger veins until it reaches the heart.
i. About \(70\% \) of the total blood volume is in the veins at any given time.

  Fig: Path of blood flow through the blood vessels

3. Capillaries

a. Capillaries are the smallest and most numerous blood vessels located within the tissues of the body.
b. They form a connection between the vessels that carry the blood away from the heart (i.e., arteries) and the vessels that return blood to the heart (i.e., veins).
c. They transport blood from the arteries to the veins.
d. They have no muscular wall and are made up of a single layer of flat endothelial cells.
e. They are permeable to water and small solutes, but not to proteins and other macromolecules.
f. The main function of capillaries is the exchange of materials between the blood and tissue cells.
g. The distribution of capillaries varies with the metabolic activity of the body tissues.
h. Tissues such as skeletal muscles, liver and kidneys have an extensive network of capillaries as they are metabolically active and require more oxygen and nutrient supply.
i. Connective tissues have a less abundant supply of capillaries.
j. The epidermis of the skin and the lens and cornea of the eye lacks a completely capillary network.
k. Only about 5% of the total blood volume is contained in the capillaries. Another 10% is in the lungs.
l. The smooth muscle cells of the arterioles where they branch to form the capillaries regulate the flow of blood from the arterioles into the capillaries.

Difference between Arteries, Veins and Capillaries

Arteries	Veins	Capillaries
Arteries carry blood away from the heart.	Veins carry blood towards the heart.	Capillaries link arteries to veins.
The lumen of arteries are small.	The lumen of veins are large.	The lumen of capillaries are small.
Semilunar valves are absent.	Semilunar valves are present.	Semilunar valves are absent.
The pressure of blood is high in arteries.	The pressure of blood is low in veins.	Pressure is falling in capillaries.
Arteries carry oxygenated blood.	Veins carry deoxygenated blood.	They supply oxygen and nutrient-rich blood to tissues.
Situated very deep into the skin.	Situated superficially on the skin.	Situated in the terminals of arteries or veins.

Fig: Different Types of Blood Vessels

Blood Vessels Physiology

1. The arteries and veins transport the blood in two different circulations, i.e., systemic circulation and pulmonary circulation.
2. The systemic arteries provide oxygen rich blood to the body tissues. The blood returned through the systemic veins has less oxygen content because much of the oxygen has been delivered to the cells.
3. In pulmonary circulation, the arteries carry blood low in oxygen from the heart to the lungs for oxygenation. Pulmonary veins then return the oxygenated blood from the lungs to the heart, so that it can be again pumped back into systemic circulation.

Fig: Physiology of Blood Vessels

Functions of Blood Vessels

Blood vessels perform various functions. Below we have provided some of the functions of Blood vessels:

1. They transport the blood throughout the human body.
2. They also transport blood cells, nutrients and other chemicals to various tissues of the body.
3. They take away the waste materials from various tissues to the kidney for excretion.
4. The capillaries help in the process of gaseous exchange.
5. They help in maintaining homeostasis and health.
6. They provide an ideal network for immune system surveillance and distribution.
7. The blood vessels present near the skin play an important role in thermoregulation by vasoconstriction and vasodilation process.

Summary

Our large and complex bodies require blood to deliver nutrients and remove wastes from trillions of cells. The heart pumps the blood throughout the body in a network of blood vessels. Thus, blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules and veins before returning to the heart. Furthermore, blood vessels take away the waste materials from our tissues to the kidney for excretion. It also performs a significant role in offering an ideal network for immune system surveillance and distribution. It also performs a vital role in thermoregulation of the skin by vasoconstriction and vasodilation process.

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FAQs on Blood Vessels

Q.1. What are the 5 types of blood vessels?
Ans: The 5 types of blood vessels are arteries, arterioles, capillaries, venules and veins.

Q.2. What are blood vessels?
Ans: The blood vessels are an intricate network of hollow tubular structures carrying blood throughout the body. They transport blood cells, nutrients and oxygen to and take away carbon dioxide and waste materials from the tissues and organs.

Q.3. What are the functions of blood vessels?
Ans: The functions of blood vessels are as follows:
a. They transport the blood throughout the human body.
b. They also transport blood cells, nutrients and other chemicals to various tissues of the body.
c. They take away the waste materials from various tissues to the kidney for excretion.
d. The capillaries help in the process of gaseous exchange.

Q.4. What are the three layers present in the wall of the blood vessels?
Ans: The name of three layers present in the walls of blood vessels are tunica interna, tunica media and tunica externa.

Q.5. What are the three types of blood vessels?
Ans: The three types of blood vessels are arteries, veins and capillaries.

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