The average man has approximately six litres of blood in his body. This blood is circulated throughout the body by several different types of blood vessels, each of which are specialised to fulfill their own unique role. The three primary categories of vessel are arteries, veins and capillaries, and this article will examine the structures and functions of each in turn. Generally speaking, vessel walls can be divided into three sections; the tunica intima (innermost), tunica media, and tunica adventitia.
As a whole, the arterial system takes oxygenated blood from the heart and delivers it to the capillaries, where oxygen and nutrient exchange can occur. There are four main types of artery in the body, each with their own distinct structure and function which will be examined in more detail below.
Large elastic arteries are the largest of the arteries and are found closest to the heart, functioning to 'conduct' blood from the heart to other areas of the body. The primary elastic arteries include most of the named vessels surrounding the heart, such as the aorta and pulmonary arteries.
From the large elastic arteries, blood enters smaller distributing arteries, which distribute the blood to sub-regions of the body. Medium muscular arteries are similar in structure to large elastic arteries.
Arterioles are part of the microcirculation and carry blood from the muscular arteries to the metarterioles. Arterioles have a diameter of less than 0.1mm and generally consist of three layers of smooth muscle cells, with the internal elastic lamina being absent. The external elastic lamina is only present in larger arterioles.
Arteries that supply capillary beds are known as metarterioles. Unlike other vessels, metarterioles do not contain a continuous layer of smooth muscle cells. Instead, intermediate rings of smooth muscle known as precapillary sphincters are located at certain points which contract to control blood flow to the capillary bed.
The contraction of precapillary sphincters is important for the regulation of tissue perfusion. When certain bodily activities take place, these structures are able to restrict blood flow to certain regions and encourage it to others.
The arterial and venous systems are the main conduits for the movement of blood around the body. Though the two systems differ, they are equally important for maintaining a healthy circulatory system. In this article, we will look more closely at the structure and function of the different types of blood vessels, and their role in transporting blood around the body.
The arterial system transports oxygenated blood away from the heart, to the capillaries, where gas and nutrient exchange can occur. This section will discuss the four main types of artery in the body.
Medium Elastic (Muscular) Arteries: These are medium-sized arteries that branch off of the large elastic arteries, to supply blood to different tissues in the body. They are 'muscular' arteries, as their walls are thicker and contain an abundance of circular smooth muscle. Examples of these include the brachial artery, carotid artery, and renal artery.
Small Muscular Arteries: Also known as distributing arteries, these are the smallest arteries in the body, and are responsible for the delivery of blood to the capillaries. Examples include the interlobular arteries and arterioles.
Capillaries: These are the smallest vessels in the circulatory system, and are packed with thin walled endothelial cells. There are three main types of capillaries; continuous, fenestrated, and sinusoidal. Their walls contain small gaps that act as a sieve, allowing molecules and structures to leave the capillary at different rates, depending on their size. For example, in continuous capillaries (located in skeletal muscle) only water and certain ions can leave, whereas in sinusoidal capillaries (located in the liver) larger structures such as cells and proteins are able to exit.
The venous system takes deoxygenated blood from the capillaries, and delivers it to the heart (with the exception of the pulmonary veins). We shall look at each type of vessel structure in more detail, in order of ascending size.
Postcapillary Venules: These receive blood from capillaries and empty into venules. They have a diameter of 10-20 micrometres, and their walls also contain a thin layer of endothelial cells, pericytes, and connective tissue. As the pressure in the venules is lower than that of the capillaries or tissue, surrounding tissue fluid tends to drain into them. As they are relatively permeable, they are the preferred sites of white blood cell migration.
Venules: These are continuous with the post-capillary venules, and function to transport blood away from the capillary beds. They can have a diameter of up to 1mm, and their walls are made up of endothelial cells, associated with pericytes or thin smooth muscle cells. Venules contain valves that press together to restrict retrograde transport of blood.
Veins: These are the major vessels of the venous system, and the final step in the return of blood to the heart. Veins have a larger diameter and a thinner wall than the accompanying artery, with more connective tissue, and less elastic and muscle fibres. Veins also contain valves that primarily prevent the back-flow of blood, and act together with muscle contractions to squeeze the veins and propel blood towards the heart.
Venae Comitantes: These are deep paired veins, wrapped together with an artery in one sheath. The pulsations of the artery promote venous return within the paired veins.
The arterial system is responsible for taking oxygenated blood away from the heart and transporting it to the organs and tissues throughout the body. It is made up of various vessel structures, each with different functions and structures.
The main structures of the arterial system are the large elastic arteries, medium muscular (distributing) arteries, arterioles, metarterioles, and capillaries. These will be examined in order of size (ascending, as we move away from the capillaries).
The large elastic arteries are the very first vessels that receive blood from the heart. They comprise of three main layers - a tunica intima, a tunica media, and a tunica adventitia.
The Tunica Intima is the innermost layer, consisting of an endothelium, a subendothelial layer and a thick elastic lamina. The Tunica Media is the thickest part of the artery. It consists of 40-70 fenestrated elastic membranes with smooth muscle cells and collagen between them. Lastly, the Tunica Adventitia is a thin layer of connective tissue containing lymphatics, nerves, and vasa vasorum (blood vessels supplying blood to the artery).
From the large elastic arteries, blood enters the smaller distributing arteries. They distribute the blood to sub-regions of the body. Medium muscular arteries are similar in structure to large elastic arteries, but are smaller and have a thinner wall.
The innermost layer, known as the Tunica Intima, consists of an endothelium, a subendothelial layer and a thick elastic lamina. The Tunica Media consists of around 40 layers of smooth muscle connected by gap junctions to facilitate coordinated contraction. The outermost layer of connective tissue is the Tunica Adventitia, containing minor vasa vasorum, lymphatics, and nerve fibres.
Arterioles are part of the microcirculation. They carry blood from the muscular arteries to the metarterioles. Arteries with a diameter of less than 0.1mm are classed as arterioles. Generally these have around 3 layers of smooth muscle, with neither an internal nor external elastic lamina present.
Metarterioles are arteries supplying capillary beds. At certain points, they have intermediate rings of smooth muscle rather than a continuous layer, known as precapillary sphincters. The precapillary sphincters can contract to control blood flow to the capillary bed. This is an important process as it allows blood to be diverted to certain regions as needed, such as in the case of running when more blood is sent to the skeletal muscles.
Precapillary sphincters play an important role in the control of tissue perfusion. During certain activities, such as running, they relax to increase blood flow to the skeletal muscles. In this way, the body is able to respond accordingly to the needs of the various organs and tissues.
Capillaries consist of a single layer of endothelium and its associated basement membrane. They are specially adapted to provide a short diffusion distance for nutrient and gaseous exchange with the tissues they supply.
There are three types of capillaries, each varying in size. The Continuous capillaries, located in skeletal muscle, only allow water and certain ions to exit. The Fenestrated capillaries, found in the kidney, allow for the passage of large molecules such as proteins. Lastly, in the Sinusoidal capillaries, located in the liver, even larger structures such as cells can leave.
The venous system takes deoxygenated blood from the capillaries and delivers it to the heart, with the exception of the pulmonary veins. From the heart, the blood can be pumped to the lungs to reoxygenate. Like the arterial system, it consists of different vessel structures, each one with different functions.
Postcapillary venules receive blood from capillaries and empty into venules. Furthermore, the surrounding tissue fluid tends to drain into them, as their pressure is lower than that of the capillaries or tissue.
During inflammation the pressure in the venules often becomes higher than that of the surrounding interstitium. This allows fluid to leak into the site of inflammation along with inflammatory cytokines and white blood cells.
Venules are continuous with the postcapillary venules. They carry the blood away from the capillary beds and many venules join together and form a vein.
Veins are the major vessels of the venous system, and are the final step in the return of blood to the heart. They generally have a larger diameter and a thinner wall than the accompanying artery. The vessel wall contains more connective tissue, with less elastic and muscle fibres.
Veins vary slightly in structure according to their size. Small and medium veins have a well-developed tunica adventitia and a thin tunica intima and media, while large veins have diameters greater than 10mm and a thicker tunica intima. These large veins have a well-developed longitudinal smooth muscle in the tunica adventitia, and the media has circular smooth muscle which is usually not particularly prominent, except when it comes to the veins found in the legs.
Veins also contain valves that press together to restrict retrograde transport of blood and to help propel blood towards the heart. This is aided by muscle contraction in the veins, which squeezes them to help move the blood along.
Venae comitantes are deep paired veins that are wrapped together with an artery in one sheath. The pulsations of the artery encourage venous return within the paired veins.
These paired veins also have the added benefit of preventing back-flow of blood, as veins contain valves that press together and restrict retrograde transport of blood. This system allows the veins to move blood to the heart quickly and efficiently, and is an important part of the circulatory system.
The veins of the circulatory system act as a pathway for transporting blood back to the heart, away from the tissues of the body. This blood contains oxygen that has been passed from the lungs to the tissues, as well as metabolic waste products that need to be cleared from the body.
The veins are able to transport blood since the walls of the veins are flexible and can stretch when necessary. This allows the veins to accommodate larger volumes of blood and also to reduce the pressure on them as the blood moves past. The venae comitantes contribute to this process due to their paired construction, as the pulsations of the artery help to encourage venous return within the paired veins.
The valves inside the veins also aid in the return of blood to the heart. The valves act together with muscle contraction to help squeeze the veins and propel the blood back towards the heart. This also helps to prevent any back-flow of blood and keeps the blood moving in the correct direction.
In conclusion, veins are a vital part of the circulatory system, with their large diameter and thin walls helping to transport blood away from the tissues and back to the heart. The valves found in the veins also help to restrict any back-flow of blood and move the blood in the right direction. Finally, the venae comitantes are important since the pulsations of the artery help to encourage venous return within the paired veins.
