What are Blood Vessels? Types, Structure, & Functions
Blood vessels are hollow tubes that circulate blood throughout the body. They carry oxygen, nutrients and hormones to the different parts of the body.
The blood that is pumped out by the heart is circulated throughout the body. The circulated blood is then returned back to the heart. The blood vessels help in the smooth and continuous circulation of the blood throughout the body.
The body has roughly around 62,000 miles of blood vessels. The length of the blood vessels when placed end to end will go around the earth two and half times.
Types of Blood Vessels
There are three types of blood vessels -
They form an important part of the circulatory system.
Structure and Function of Arteries
Arteries carry oxygenated blood from the hearts to different parts of the body. The arteries divide into smaller branches called arterioles and the arterioles further break up into smaller branches called capillaries.
Each artery is a tube that is lined by three layers of tissues.
- tunica adventia – the outermost layer, made up of connective tissues that attach the artery to the adjacent tissues
- tunica media – is the middle layer that helps the arteries to withstand the high pressure of blood pumped out by the heart and is made up of muscles
- tunica intima – the innermost layer called the endothelium that is lined by smooth endothelial cells
The largest artery is the aorta that originates from the left ventricle of the heart and carries oxygenated blood. The aorta further branches off into the artery, arterioles and capillaries that supply blood to the surrounding tissues.
Another artery called the pulmonary artery carries deoxygenated blood from the heart to the lungs to be replenished with oxygen.
All arteries carry oxygenated blood. The only exception is the pulmonary artery that carries deoxygenated blood.
The pulmonary artery branches off from the right ventricle and further divides into the left and right pulmonary arteries that enter the left and the right lungs respectively.
Structure and Function of Veins
Veins are blood vessels that carry deoxygenated blood from the different parts of the body back to the heart.
The deoxygenated blood appears to be a darker shade of red due to the lack of oxygen in the blood that flows through the veins.
The largest vein in the body is called the vena cava. Vena cava in Latin means a hollow vein. Vena cava starts and ends with the right ventricle of the heart.
The vena cava branches off into two sections -
- the superior vena cava
- the inferior vena cava.
Superior vena cava - the superior vena cava runs above the heart and brings back deoxygenated blood from the head, neck, chest and hands back to the heart.
Inferior vena cava - the inferior vena cava runs below the heart and brings back blood from all the other parts of the body back to the heart.
The structure of a vein is similar to the structure of the artery. The vein has three layers just like the artery -
- tunica adventia – the outermost layer, made up of loose fibrous connective tissues that attach the artery to the adjacent tissues
- tunica media – the middle layer is made up of smooth muscles
- tunica intima – the innermost layer lined by smooth endothelial cells
The difference is that the wall of the veins are not elastic in nature and are thinner when compared to the arteries. This is because the veins do not face the pressure of the blood pumped out by heart.
When the blood is returned to the heart through the veins, the blood has to flow against gravity. The valves present in the veins prevent the blood from flowing backwards,
The vein further divides into small branches called venules.
What are capillaries?
Capillaries are the smallest of all the blood vessels running through the body. They form the network between the arteries and veins.
The capillaries have thin walls so that the surrounding tissues can easily absorb the oxygen and other substances from the blood.
The thin wall also helps the capillaries to absorb deoxygenated blood and other waste materials from the surrounding tissues.
Function of the capillaries -
- to distribute oxygenated blood from the arterioles to the surrounding tissues
- to collect deoxygenated blood from the venules and feed it to the veins to be carried back to the heart
Types of Capillaries
There are three types of capillaries in our body –
Continuous capillaries have cells that have smooth lining without any perforations. These capillaries are tightly packed and allow only small molecules to enter and exit the cells. These capillaries are found in the central nervous system, muscles and skin.
Fenestrated capillaries have tiny pores called fenestra. These capillaries allow small molecules along with a few nutrients to enter and exit. Fenestrated capillaries are found in the kidneys, pancreas and intestines.
Sinusoidal capillaries do not have the regular cylindrical shape like the other capillaries. They are irregular in shape and are found in organs such as the liver, spleen, bone marrow and some parts of the endocrine system. The sinusoidal capillaries can also have perforations on their wall.
Blood vessels transport blood, oxygen, hormones and other nutrients throughout the body. The blood vessels are of three different types - arteries, veins and capillaries.
The artery divides into small branches called arterioles, and the veins divide into small branches called venules.
The arteries carry oxygenated blood along with hormones and other nutrients to the different parts of the body.
The veins carry deoxygenated blood along with waste materials from the different parts of the body back to the heart.
The capillaries form the network between the arteries and veins. The capillaries distribute oxygenated blood from the arterioles to the surrounding tissues. They also transfer the deoxygenated blood from the surrounding tissues into the venules so that it can be sent back to the heart through the veins.
The blood vessels ensure the smooth and continuous distribution of blood throughout the body.