The Blood Vessels – Arteries, Capillaries and Veins

Blood Circulatory System in Humans

The circulatory system of humans has 3 basic components.

In this article, we will discuss the blood vessels which carry blood towards and away from the heart to all parts of the body.

The Blood vessels–Arteries, Capillaries, and Veins


Blood vessels are the channels or avenues through which blood is distributed to body tissues. The vessels make up two closed systems of tubes that begin and end at the heart. One system, the pulmonary vessels, that transport blood from the right ventricle to the lungs and back to the left atrium.

The other system, the systemic vessels, brings blood from the left ventricle to the tissues in all parts of the body and then returns the blood to the right atrium. Based on their structure and function, blood vessels are classified as either arteries, blood vessels, or veins.

Also Read: Difference Between Arteries, Veins and Capillaries


An artery is an elastic vessel that carries blood away from the heart. This is the opposite in the function of veins, which carry blood to the heart. Arteries are components of the cardiovascular system. This system distributes nutrients to and removes the waste products from the cells of the body.

Types of Arteries

There are two primary types of arteries: pulmonary arteries and systemic arteries.

Pulmonary arteries

Pulmonary arteries bring blood from the heart to the lungs where the blood carries oxygen. The oxygen-rich blood is then gone back to the heart through the pulmonary veins.

Systemic arteries

Systemic arteries provide blood to the remainder of the body. The aorta is the main systemic artery and the biggest artery of the body. It stems from the heart and branches off into smaller arteries that supply blood to the head area (brachiocephalic artery), the heart itself (coronary arteries), and the lower regions of the body.


The largest and most important artery in the circulatory system is the aorta. Without the aorta, the body’s tissues would not get the oxygen and nutrients that they need. The aorta is connected to the heart by means of the aortic valve. It’s formed of the following parts:

Ascending aorta

The ascending aorta distributes oxygen and nutrients to the heart by means of the coronary arteries.

Aortic arch:

This has three significant branches– the brachiocephalic trunk, the left typical carotid artery, and the left subclavian artery. It sends blood to the upper body, consisting of the head, neck, and arms.

Descending aorta

The descending aorta sends blood to your torso, abdomen, and lower body. It’s referred to as the thoracic aorta above the diaphragm, but after passing the diaphragm, it becomes the abdominal aorta.

Structure of Arteries

The artery wall includes 3 layers:

Tunica Adventitia (Externa)

The strong external covering of arteries and veins. It is composed of connective tissue along with collagen and elastic fibres. These fibres permit the arteries and veins to extend to prevent overgrowth due to the pressure that is exerted on the walls by blood circulation.


Tunica Media

The middle layer of the walls of arteries and veins. It is made up of smooth muscle and flexible fibers. This layer is thicker in arteries than in veins.

Tunica Intima

The inner layer of arteries and veins. In arteries, this layer is composed of a flexible membrane lining and smooth endothelium (a special kind of epithelial tissue) that is covered by flexible tissues.

The artery wall expands and contracts due to pressure applied by blood as it is pumped by the heart through the arteries. Arterial growth and contraction or pulse coincides with the heart as it beats. The heartbeat is created by heart conduction to force blood out of the heart and to the rest of the body.

The smallest arteries are called arterioles and they play an important role in microcirculation.


A vein is a flexible blood vessel that carries blood from numerous areas of the body to the heart. Veins are components of the cardiovascular system, which circulates blood to offer nutrients to the cells of the body. Unlike the high-pressure arterial system, the venous system is a low-pressure system that relies on contraction to return blood to the heart.

Types of Veins

Veins can be classified into four main types: pulmonary, systemic, superficial, and deep veins.

Pulmonary veins:

These bring oxygenated blood from the lungs to the left atrium of the heart.

Systemic veins:

These collect and return deoxygenated blood from the rest of the body to the best right of the heart.

Superficial veins:

These are located near the surface of the skin.

Deep veins:

These lie deep within muscle tissue and are usually located near a corresponding artery with the exact same name (for example coronary arteries and veins).

Structure of Veins


As discussed in the arteries structure.

Superior Vena Cava and Inferior Vena Cava

The two largest veins in the body are the superior vena cava, which carries blood from the upper body straight to the right atrium of the heart, and the inferior vena cava, which carries blood from the lower body directly to the right atrium. Like arteries, veins form a complex, branching system of larger and smaller vessels.

The smallest veins are called venules. They receive blood from blood vessels and transfer it to bigger veins. Each venule gets blood from numerous capillaries.



Capillaries are the smallest blood vessels in the body, connecting the tiniest arteries to the smallest veins. These vessels are often referred to as the “microcirculation.”


Just two layers of cell thickness, the function of capillaries is to play the central function in the circulation, providing oxygen in the blood to the tissues, and getting carbon dioxide to be eliminated. They are likewise the place where nutrients are provided to feed all of the cells of the body.

Structure of capillaries

Capillaries are extremely thin, approximately 5 micrometres in size, and are made up of only 2 layers of cells– an inner layer of endothelial cells and an outer layer of epithelial cells. They are so little that red blood cells require to flow through the file.

Although the blood appears confined within the capillary walls, the latter are permeable with the result, that water and dissolved materials pass in and out exchanging oxygen, CO2, dissolved food and excretory products with the tissues around the capillary. The capillary network is so dense that no living cell is far from a supply of oxygen and food. In the liver, every cell remains in direct contact with a capillary.

The diameter of a blood vessel can be changed by nervous stimulation, which tends to close them, and by chemicals, such as histamine, which dilate them. The modification in size is brought about by a modification in the shape of the cells, constituting their walls. The pre-capillary sphincters likewise manage the quantity of blood flowing in blood vessels. Therefore, the quantity of blood flowing in a particular tissue is managed.

The blood vessels are the sites where the materials are exchanged between the blood and body tissues.

This exchange occurs in three methods.

  • (i) Active transport and diffusion through the cells lining the capillary wall into the interstitial or extracellular fluid, and after that to the body cells, and vice versa.
  • (ii) Through the intercellular spaces of the endothelial lining of the wall of the capillary to and from the extracellular fluid.
  • (iii) Materials from the cavity of blood vessels are likewise taken up by endocytosis, and after that passed to the other side by exocytosis. The same holds true for some things entering from the intercellular areas (extracellular fluid) into the blood.

Thus, the exchange of products takes place between blood and tissues via the extracellular or interstitial fluid. Capillaries join to form venules, which join together to form veins.

Exchange of gases and nutrients in Capillaries

The pressure within blood vessels causes a constant leakage of fluid from the blood plasma into the areas that surround the capillaries and tissues. This fluid, called interstitial fluid consists primarily of water, in which the dissolved nutrients, hormones, gases, wastes, and small proteins from the blood exist.

Large proteins red cell and platelets can not cross the intercellular spaces of the capillary wall, so they stay within blood vessels. But some leukocyte can squeeze out through the intercellular spaces of the capillary wall. Interstitial fluid is the medium through which the exchange of products between the blood and adjacent cells happens.