Blood and Blood Vessels

Blood

Blood is made up of two parts; a solid portion consisting of red blood cells, white blood cells and platelets, and a liquid portion consisting of plasma. 

The function of red blood cells (transporting oxygen & carbon dioxide), white blood cells (defence and immunity) and platelets (blood clotting) is discussed more in the 'major functions of the cardiovascular system' page, so here we’ll focus on the liquid portion (plasma) that makes up the majority (approximately 55%) of the bloods volume.

Plasma is primarily made up (about 90%) of water and it is in plasma that nutrients such as glucose, amino acids (proteins) and lipids (fats) are absorbed from the small intestine and transported to various parts of the body.

Plasma also transports hormones to their target tissues and takes waste products such as urea from the liver to the kidneys where it becomes urine for expulsion from the body. 

Blood vessels

The blood vessels make up the body’s internal ‘road’ network linking the heart to all the organs and tissues of the body that the blood needs to reach.  There are five types of vessels, they are:

1. Arteries
2. Arterioles
3. Capillaries
4. Venules
5. Veins

The blood vessels differ in their makeup based on what they do. 

All blood vessels have an inner lining layer called the endothelium.  This layer is a simple sheet of tissue that reduces friction in the heart and vessels, allowing blood to flow easily. 

Other layers in blood vessels change depending on the role of the vessel so we will look at the types of blood vessels in more detail now.

The adjacent image shows the different types of blood vessels and what they are made up of.

Arteries carry blood away from the heart at high pressure.

Because of the high pressure exerted on arteries they have thick walls of smooth muscle covered by a tough outer layer of fibrous connective tissue.  Arteries also have layers of elastic fibres that control the diameter of the artery.

Blood flows through the arteries in surges of pressure caused by each heartbeat.  The elastic layers within the artery walls expand with each surge.

This surge can be felt as a pulse in arteries near the surface of the skin, such as the carotid pulse in the neck (carotid artery) or the radial pulse in the wrist (radial artery). 

Arteries typically carry oxygenated blood, which is bright red in colour.  The exception is the pulmonary artery which carries deoxygenated blood from the heart to the lungs.

Arteries divide into arterioles which are of a similar structure, however the walls are not as thick as in arteries as the pressure within the vessels has lessened by the time blood reaches them.  The arterioles then divide further into capillaries.

Capillaries are the smallest blood vessels and their walls are only one cell thick which allows diffusion between the blood and cells to occur. 

Nutrients and oxygen from the blood diffuses through the capillary walls into the tissues that need it.  This is aided by the relatively high blood pressure that also squeezes the oxygen and nutrients through the capillary wall and into the cells of the tissue.

At the same time carbon dioxide and other waste products re-enter the capillary in order to be processed and removed from the body.

This is aided by a process called osmotic attraction, which is where elements move from an area of high concentration (CO₂ in the cells) to an area of low concentration (CO₂ in the blood).

Once in the blood the waste products and CO₂ now start travelling back to the heart.  Flowing first from the capillaries into the venules (small veins), before entering the main veins.

Veins are the largest blood vessels that carry de-oxygenated blood back to the heart, which is dark red in colour (but shown as ‘blue’ in diagrams simply to show differentiation between the bright red arterial blood). 

Veins have a thick outer layer made of collagen and below this are thin bands of smooth muscle and elastic tissue, with the innermost layer being made up of endothelium cells.

Veins are a lot further from the pumping action of the heart than arteries and as a result have much thinner walls because they operate under much less pressure. 

As there is less pressure in the veins and no ‘pump’ to force blood back to the heart, veins have some special features to assist venous return (the return of blood to the heart from the veins). 

The skeletal muscles surrounding veins expand and contract which presses on veins causing a pumping effect.  The surge of pressure in adjacent arteries also provides a similar effect. 

The adjacent diagram also illustrates valves that are located at regular intervals in large veins.  These valves ensure that blood only flows in one direction towards the heart, as they are automatically closed by the backflow of blood, similar to the valves in the heart.