Blood vessels provide a structural framework necessary for the blood flow throughout the circulatory system. Healthy blood vessels, whether they are arteries or veins, has a significant role in the normal circulation process of an individual. Blood vessels help in carrying the different material by combining them with the blood plasma. Blood transports nutrients, excretory material, enzymes, hormones, gases and many other substances. Depending upon the direction in which they carry the blood, they are classified as arteries and veins. Arteries carry blood away from the heart whereas veins carry the blood towards the heart. Based on the gaseous composition of blood, the definition of blood vessels differ. According to this, arteries are the blood vessels that carry arterial blood and the veins carry deoxygenated blood with the exception of pulmonary arteries (where deoxygenated blood is carried) and the pulmonary veins ( as they carry the oxygenated blood). Blood vessels across the body are spread in a branching fashion. Aorta and the pulmonary vessels are the largest vessels, from these vessels, all the major arteries branch out to form arterioles and arterioles further redivide to form the arterial capillaries. Similarly, major veins get split into venosus, venosus further branch out to form venules and finally the venous capillaries.
Structure of Arteries
Arteries are highly elastic, thick and stronger than the veins, this is because they have to carry the blood against the gravity to each and every cell of our body. A typical artery has 3 layers; they are, external layers (Tunica externa), middle layers ( Tunica media) and internal layers (Tunica Intima)as seen in the image below. The major central arteries that come out of the heart are coronary arteries, abdominal aortic vessels, and carotid arteries.
This is the internal lumen formed by the combination of endothelial cells and the inner lamina. Tunica intima is the thinnest layer formed into a single continuous channel. It is formed of endothelial cells supported by a sub-endothelial layer. Endothelial layers act as connective tissues that hold the vessels together in place. In the case of smaller arterioles or venules, the subendothelial layer has only a single layer of cells, but the same can be much thicker in larger vessels like the aorta. There is also a thin basement membrane that surrounds from below the tunica intima.
This is the middle portion of the vessel enfolding around the tunica intima. Tunica media has a combination of smooth muscle cells and the elastic connective tissues coiled together. It is much thicker in arteries than in the veins as the veins contain fewer elastic fibres than the arteries. The coiled elastic tissues play an important role in maintaining the blood pressure required for the circulation in our body.
The outermost layer is the tunica Externa or tunica adventitia. Tunica externa is formed by connective tissues surrounded by an external elastic lamina. Lamina helps to coordinate the vessels with surrounding supportive tissues when the blood travels through the vessels. The tunica Externa is usually thicker in veins as it helps to prevent the collapse of veins during venous circulation.
Veins are relatively thinner and less strong than the arteries and they carry blood towards the heart. Once the blood passes through the veins, it enters into the much thinner veins, called venules. Venules carry the blood that has high carbon dioxide content hence it is called venous blood or deoxygenated blood. The walls of veins also made up of 3 layers as seen in the arteries. However, veins have a much lesser proportion of smooth muscles and connective tissues when compared to the arteries. This is what makes them anatomically smoother and thinner. As the inner walls of the veins are thin and less rigid than arteries, veins can hold more blood than the arteries Veins of medium to larger size have venous valves, similar to the semilunar valves associated with the heart. The valves help to keep the blood flowing in a forward direction by instantly closing themselves after every contraction, so this will ensure that blood will not leak back each time when the heart contracts. As the elasticity and pushing capacity of the veins is much lower, the semilunar valves help in creating a certain pressure that avoids the backflow of blood in response to the pull of gravity.
Blood capillaries can be venosus and arterioles. The capillaries are the finest branches of the systemic circulation that carry blood deep inside the tissues and cells,. This is becasue, they are directly immersed inside the fine tissues, muscles, organs, etc. See the picture below for more information.