The function of the blood vessels through the body is to deliver oxygen and nutrients to the cells, carry away waste products, and maintain blood pressure—the blood vessels make up two closed circuits that begin and end in the heart. The pulmonary vessels make up one system, which transports blood from the heart's right ventricle to the lungs and back to the left atrium of the heart. The systemic system carries blood from the left ventricle to the tissues in all parts of the body and then returns the blood to the right atrium. The two systems create a closed helical circuit where the blood travels with the heart in the centre. The amount of blood vessels in the body is remarkable and, if stretched out into a single line, would cover 100000 km for an average grown adult.  There are three main types of blood vessels in the body: arteries, capillaries and veins. 

-Arteries

The role of the artery is to carry blood away from the heart. The pulmonary arteries superior to the heart transports deoxygenated blood from the right ventricle and branches to the left and right lungs. Once the blood is fully oxygenated, the systemic arteries pump the blood from the left venatical to all body tissues. From both ventricles, blood is pumped into large elastic arteries that branch out repeatedly and decrease in size, getting smaller until the branching results in microscopic arteries called arterioles. The walls of arteries are much thicker than veins because of the high blood pressures in them. The arteries and arterioles play a key role in controlling the blood pressure by contracting and opening their diameter. They also regulate the blood flow into the tissue for the different needs of the body. At any given time, within the systemic arterial system, there is 10% of the total blood volume. 

Even though the walls of arteries are much thicker than veins because of the high blood pressure,  arteries and the veins consist of three layers. 

- Layers of the blood vessels 

The innermost layer, the tunica intima, is made up of a simple tissue called the squamous epithelium, which embryologically derives from the mesoderm (middle body). The squamous epithelium tissue is surrounded with elastic fibres by connective tissue and a basement membrane, which forms an anatomical barrier between the cells and the connective tissue. The basement membrane relays important signals to differentiate and maintain tissue.

The middle layer, the tunica media, is made primarily of smooth muscle and normally the thickest layer. As well as providing support to the vessel, it also is responsible for the changes in the diameter of the vessel to regulate blood flow and blood pressure. 

The outermost layer, the tunica external or tunica adventitia, attaches the vessel to the surrounding tissue. This layer is connective tissue with varying amount of elastic and collagenous fibres. The connective tissue in this layer changes from being quite dense at the tunic media edge to being loose near the periphery of the vessel. 

- Capillaries 

The capillaries are microscopic thin-walled vessels that act as a bridge between the arteries and veins. The thin walls of the capillaries allow oxygen and nutrients to pass from the blood into the tissue, they also allow waste products from the tissue to pass into the blood. The capillary walls are made up of a thin cell layer called endothelium that’s surrounded by a thin layer of the basement membrane. There are three types of capillaries in the body and they differ in how much gases, water, sugar (glucose) hormones, molecules and new blood cells pass-through 

• Continuous capillaries are the most common and contain small gaps between the endothelial cell that allows gases, water, sugar (glucose), and some hormones to pass through. The exception is in the brain. These capillaries are part of the blood-brain barrier and have no gaps between the endothelial cells, which protects the brain by only allowing essential nutrients to pass through. The basement membrane is also thicker.  
• Fenestrated Capillaries are more permeable than the continuous capillaries and contain small pores and small gaps between the endothelial cells, which allow larger molecules to pass through. They are found in places that require a lot of exchange between the blood and tissues, like the small intestine to absorb nutrients from food and the kidneys to filter out waste products from the blood.
• Sinusoid capillaries are the most permeable and rarest capillary. The large gaps in the capillary wall and the pores and the small gaps in the endothelial cells allow for large molecules and cells to pass through. The surrounding basement membrane, in addition, has openings in many places. This type of capillary is found in certain tissues such as the liver, spleen and bone marrow. In the bone marrow, where new blood cells are produced, whole blood cells in this area enter the bloodstream and circulate in the cardiovascular system.

The different capillaries are distributed thought the body in a varying degree that corresponds to the metabolic activity of body tissue. Skeletal muscles, liver and kidney, have extensive capillary networks because they are metabolically active and depend on an abundant supply of oxygen and nutrients. Other types of tissues don’t require such an abundant supply of oxygen and nutrients, such as connective tissue. The epidermis, the lens and the cornea of the eye are almost void of a capillary network. 

Overall within the capillaries, 5 per cent of the total volume of blood is at any given time in the systemic capillaries, while 10 per cent is in the lungs. 

- Veins

The role of veins is to transport blood to the heart. Blood flows from the capillaries into venules that are tiny veins that lead into progressively larger and larger veins until they reach the heart. The veins in the pulmonary circuit transport the oxygenated blood from the lungs to the heart's left atrium. In the systemic circuit, the veins transport the deoxygenated blood to the right atrium of the heart. The blood has a reduced amount of oxygen because the oxygen has been used for metabolic activities in the tissue cell. 

The walls of veins have the same three layers of arteries see - Layers of the blood vessels (above). Even though the three layers are present in veins, there is less smooth muscle and connective tissue. As there is less pressure in the veins, the walls can be thinner, and as they are thinner and less rigid, veins can hold more blood. Within the veins, there is almost 70 per cent of the total blood volume at any given time. So that the blood moves in the direction of the heart under low pressure, there are venous valves in the medium and large veins, which prevents the backflow of blood.