The wall of blood vessels is made up of three layers. Tunica Media is the middle layer of the wall of blood vessels. It is made up of smooth muscle and elastic fibres. The other two layers are tunica media (middle) and tunica externa (external).
Blood Vessels
Blood vessels are the closed tubes through which blood flows. In a closed circulatory system, the blood flows through a closed pathway of blood vessels. The closed circulatory system is present in all animals from annelids to chordates.
Blood vessels transport nutrients, hormones, oxygen and other substances to various tissues and also take away carbon dioxide and waste products from the tissue.
Types of Blood Vessels
Blood vessels include a network of arteries, veins, capillaries, arterioles and venules. Blood vessels are mainly two types, arteries and veins.
- Arteries – They carry oxygenated blood from the heart to various tissues, except for the pulmonary artery. The pulmonary artery carries deoxygenated blood from the heart to the lungs.
- Veins – They carry deoxygenated blood from various tissues and organs back to the heart, except for the pulmonary vein. The pulmonary vein carries oxygenated blood from the lungs to the heart.
The blood flows with high pressure in arteries, and because of this the walls of arteries are thicker compared to those of veins. Veins have valves present in them, and the blood flows with lower pressure. In arteries, the tunica media is thicker compared to veins.
Structure of Blood Vessels
The walls of blood vessels are made up of three layers. These are:
- Tunica Intima – the innermost layer, made up of squamous endothelium.
- Tunica Media – the middle layer, made up of smooth muscle and elastic fibres.
- Tunica Externa or adventitia – the outermost layer, made up of connective tissues.
Tunica Media
It is the middle layer of the wall of blood vessels. The tunica media is present between the tunica externa and the tunica interna. It is the thickest layer of the three layers of blood vessels. The tunica media of arteries is thicker than veins.
The structure of tunica media slightly differs in different types of arteries and veins. The important characteristics of the tunica media are as follows:
- The tunica media is made up of smooth muscle and elastic fibres.
- The smaller arteries have fine bundles of smooth muscle fibres. One to six layers of lamellae may be present depending on the size of the vessel.
- In the larger arteries, elastic fibres and collagen form lamellae that alternate with the bundles of smooth muscle fibres. The elastic fibres connect these lamellae to the elastic layer of the inner lining.
- The aorta and other bigger arteries have a thick lining of elastic tissue. White connective tissues are also present.
- The middle layer of the veins is much thinner than those of arteries.
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