What are the Blood Vessels

Blood vessels are tubular structures that comprise of arteries and arterioles, veins and venules, and capillaries and sinusoids. Arteries and arterioles carry blood away from the heart. Veins and venules transport blood towards the heart, and capillaries and sinusoids form a network between the arterioles and the venules. 

Arteries and Arterioles 

These vessels transport oxygenated blood away from the heart and into the cells of the body. They vary in size, arterioles being the smallest arteries, and they consist of three layers of tissue.

Tunica Adventitia

This is the outer layer of the arterial wall and is made up of fibrous tissue. 

Tunica Media

Comprising of smooth muscle and elastic tissue, the tunica media makes up the middle layer. 

Tunica Intima

A lining of squamous epithelium known as endothelium, this is the innermost layer of the wall. 

Although arteries and arterioles are made up of the same structures, the amount varies depending on the size of the blood vessel. In large arteries the tunica media has more elastic tissue and less muscle. Within the arterioles the tunica media is made up of almost all smooth muscle and very little elastic tissue.

Arteries carry oxygenated blood and nutrients from the heart into the organs and tissues within the body. The largest artery is the aorta which stems from the heart’s left ventricle. It forms an upward arch before descending downwards. The arteries that ascend from the aortic arch supply blood to the head, neck and arms. The descending aorta and the arteries that branch from it supply blood to the rest of the body.

The pulmonary arteries differ in that they carry deoxygenated blood into the lungs. The artery trunk leaves the heart at the right ventricle and divides into the right and left pulmonary arteries. The left pulmonary artery travels to the left lung where it branches into two, one for each lobe of the lung. The right pulmonary artery supplies blood to the right lung where it also divides into two parts. The largest of these supplies blood to the lower and middle lobe of the lung, and the smaller artery branch transports blood to the upper lobe. 

Veins and Venules 

These vessels transport deoxygenated blood towards the heart, with the exception of the pulmonary veins that take oxygenated blood into the heart from the lungs. Two pulmonary veins leave each lung and transport oxygenated blood into the heart’s left atrium. 

Vein walls are thinner than those of arteries, but possess the same three layers. There are three types of vein known as superficial veins, deep veins and anastomosing veins. The anastomosing veins are also known as perforator veins because they perforate the fascia that surrounds muscles. 

Superficial veins are located below the surface of the skin and are visible. Deep veins travel through muscle and are not visible. The anastomosing veins transport blood from the superficial veins to the deep veins. The venules make up the smallest veins in the network. 

Some veins possess valves and it is these that ensure the blood travels towards the heart. Valves are mostly located within the limbs and in particular the legs.


Capillaries are the smallest of the blood vessels measuring approximately 7 microns in diameter. They form a network and link between the smallest arterioles and venules, they help to maintain body temperature, and they transfer the required substances into the appropriate cells.

The capillary wall consists of a single layer of endothelial cells. These allow for substances with tiny molecules, such as water, to pass through the walls.

Substances such as nutrients, hormones and oxygen are transported within the blood vessels via the blood plasma; this is extracellular fluid that surrounds the blood cells. When substances reach the capillaries they are then transported from the plasma through the capillary wall and into the extracellular fluid, and from here the substances are transported into the interior of the cell.

The capillaries located in the skin’s dermis help to maintain normal body temperature. As body temperature rises the capillaries dilate, this then allows for the blood to be cooled as it circulates.


Sinusoids clear waste materials from the blood. They are wider than capillaries although the wall is much thinner. The wall is also made up of endothelial cells but in some sinusoids there is space between cells. Sometimes other types of cells, such as macrophages, are found among the endothelial cells. Sinusoids are located in bone marrow, in endocrine glands, the spleen and the liver. 

The blood vessels along with the heart make up the circulatory system and this consists of pulmonary circulation, portal circulation, and systemic circulation. 

Pulmonary circulation pertains to the blood circulated between the heart and the lungs. Portal circulation is particular to the hepatic portal system. The stomach, spleen, pancreas, the intestines, and the liver make up this system. The veins from these organs form a network that ultimately transports the blood into the liver. Portal circulation is responsible for gastrointestinal blood transportation into the liver via the gastric vein. The necessary nutrients can then be absorbed and stored. Toxins and other waste materials are transported out of the liver by the portal vein and are ultimately eliminated from the body. Systemic circulation, also known as general circulation, refers to blood circulation between the heart and the rest of the body.