Structure of blood vessels – Blood vessels

Structure of blood vessels: Understanding what blood vessels are is key to better understanding the cardiovascular system and how blood is transported through our body.

Structure of blood vessels

Blood vessels are tubular structures through which blood circulates, present throughout the body. These vessels form a large system of tubes that ensure that the blood pumped by the heart travels towards the body and then returns to the heart.

→ Structure of blood vessels

Structure of blood vessels: Blood vessels are made up of the tunica intima, media and adventitia. Tunics are nothing more than layers in which different fabrics are associated. Below, see some of the main characteristics of each of the tunics that form the blood vessels.

  • Tunica intima: This layer, which is the innermost layer, is formed by endothelial cells that are supported by a layer of loose connective tissue, which may have some cells of non-striated muscle tissue. It is also possible to observe the presence of an internal elastic lamina, separating the tunica intima from the tunica media. This lamina is seen in arteries and is not usually seen in veins.
  • Tunic media: This tunic, which is the median layer, is made up mainly of non-striated muscle tissue cells. Among these cells, there is a variable amount of extracellular matrix, which has, among other components, elastic and reticular fibers.
  • Adventitial tunic: This tunic, which is the outermost layer, is mainly formed by collagen and elastic fibers.

→ Types of blood vessels

The three main types of blood vessel are: arteries, veins and capillaries. Let’s learn more about each of them:

Arteries are vessels that transport blood from the heart to the tissues and organs of the body. Contrary to what many people think, arteries do not guarantee the exclusive transport of arterial blood (blood rich in oxygen gas and poor in carbon dioxide). The pulmonary arteries, for example, are an exception and guarantee the transport of blood rich in carbon dioxide.


Arteries carry blood under high pressure. This happens because they start from the heart, pumping blood drives it in the form of a rapid flow. Due to this characteristic, it is essential that the walls of the arteries are strong, elastic and resistant. The walls of these vessels, therefore, are quite thick. Inside the organs, the arteries branch into smaller-caliber vessels, which are called arterioles.

Structure of blood vessels

  • What are capillaries?

Capillaries are blood vessels that have only one layer of endothelial cells and a very small diameter. Its thin walls ensure the exchange of substances between the blood and the interstitial fluid present around the cells.

  • What are veins?

Veins are blood vessels that ensure that blood from organs and tissues is transported back to the heart. Veins are the result of the convergence of capillaries. Capillaries converge into venules, which converge into veins. Veins, unlike what many people think, do not just transport blood rich in carbon dioxide, popularly called venous blood. The pulmonary veins, for example, are an exception to this rule and ensure the transport of oxygen-rich blood.

The blood flowing in the veins is under lower pressure than that seen in the arteries. To ensure a unidirectional flow of blood, the veins have valves that prevent the backflow of blood. In addition, to ensure adequate blood flow, skeletal muscles play an important role. The contraction of skeletal muscles compresses the veins, ensuring flow. That’s why sitting or standing for a long time causes swelling in the lower limbs.

Like arteries, veins have all three tunics. However, unlike arteries, the tunica media of veins has a smaller amount of muscle tissue and is therefore less thick.

→ Summary

Blood vessels
arteries veins capillaries
Vessels that carry blood from the heart to the body’s tissues and organs.


Blood transported under high pressure


They have the tunica intima, tunica media and tunica adventitia.

Vessels that carry blood from tissues and organs to the heart.


Blood transported under low pressure.


Have valves that prevent backflow of blood


They have the tunica intima, tunica media and tunica adventitia.

Small caliber blood vessels that guarantee the exchange of substances by diffusion.


They have a thin and thin wall.


They lack the tunica media and adventitia

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