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Blood vessels are the transport system for the blood from the heart to the rest of the body and back again. It is important to know how this works so we then know how to manage to bleed and understand how shock affects the body.

Blood vessels are divided into different categories because of their shape and function. It is important to remember that they are all linked in a continuous loop. One type of blood vessel will gradually be split or linked to and from another type of blood vessel.

There are broadly three types of vessels that differ in construction and size according to their function and position in the body. These are arteries, capillaries and veins. These blood vessels are responsible for transporting blood to and from the heart and thereby delivering nutrients to and from the tissues.

Arteries are muscular tubes with thick walls which can contract, like all muscles, to squeeze blood along the passageways away from the heart. The large artery that leaves the left ventricle of the heart is called the aorta. This divides and subdivides gradually becoming arterioles. The smooth muscle tissue that surrounds the artery and arteriole walls are thicker and more powerful than that surrounding the walls of veins.

As blood is ejected powerfully from the heart, the arterial walls are required to stretch passively to receive the blood under high pressure and then immediately contract as in a recoil action to assist in propelling the blood further on down the line to the body.

Unlike the veins, there are no valves in the arteries, other than those at the exit points of the ventricles to prevent backflow. Arteries and arterioles predominantly carry oxygenated blood around the body. The exception to this rule is the pulmonary arteries and arterioles which carry de-oxygenated blood to the lungs to be re-oxygenated

The arteries branch off into smaller arterioles and these become smaller and thinner until they are described as capillaries. These capillaries have extremely thin walls, approximately one cell thick, and spread to all parts of the body even the smallest area of tissue.

As the walls of these blood vessels are so thin they allow the diffusion of nutrients and gases through the walls and into the tissue cells. Food and oxygen pass through the walls from the blood and into the individual tissue cells to be used, likewise, the waste products such as carbon dioxide and lactic acid passed back into the blood to be carried away and excreted.

Blood flows through the capillary bed slowly to allow for this exchange to take place. There are a greater number of capillaries than of any other type of blood vessel.

Once the capillaries have passed through the tissue, be it muscle or alveoli, they gradually link together to form progressively larger blood vessels called venules. These venules then eventually become larger veins.

Veins have thinner walls in comparison to arteries with a little ability to contract. Veins carry blood from the tissues back towards the heart. The smooth muscle in the walls of the veins contracts automatically in a wavelike action to assist in blood flow.

Veins and venules predominantly carry deoxygenated blood which is therefore high in carbon dioxide. The exception to this rule is the pulmonary vein and venules, which are carrying oxygenated blood from the lungs back to the heart. The flow of blood back to the heart is called Venous return.

Unlike arteries, veins carry blood under low pressure which makes it harder for the blood to flow. All veins, therefore, have a series of one-way valves, that work against gravity to prevent backflow of the blood as it passes back towards the heart. This will also help to prevent the blood from going the wrong way and pooling of blood in the veins.

There are contributing factors that assist with Venous return and they are:

- Gravity - this will assist in the return of blood from anywhere above the heart such as the head and shoulders.
- Non-return valves in the veins prevent backflow of blood and are one of the biggest factors assisting venous return.
- The diaphragm is a large dome-shaped muscle used in respiration which produces a suction effect on the veins below the hearts right atrium to suck the blood back.
- Skeletal muscle contraction, which is where veins are assisted by squeezing action of the nearby skeletal muscles.

Blood circulation is a closed system, in which the pressure varies constantly.

Long periods of inactivity can slow the blood from the lower legs which can result in blood pooling. This pooling could produce a blood clot or Deep Vein Thrombosis (DVT), blocking the blood vessels.