Blood vessels

Blood vessels

Introduction to blood vessels
Blood vessels provide a channel of support necessary for the blood flow throughout our body . Blood vessels are the structural and functional units of the circulatory system. Blood vessels are the reason behind we all survive because without them it would have been impossible for our cells to recieve oxygen and to releases carbon dioxide. Thus, we all need healthy arteries and veins that play an important role in the normal circulation process . In addition to the gases, blood vessels carry different types of material through the blood plasma and some of them are nutrients, excretory material, enzymes,and hormones among others. The five types of blood vessels includes arteries, arterioles, capillaries, venules and veins. However the 2 fundamental types of blood vessels that are commonly in practice are the arteries and veins. The arteries carry the blood away from the heart, and the arterioles are the capillaries, where the exchange of water and chemicals between the blood and the tissues takes place .Similarly, the venules; and the veins are those that carry blood from the capillaries back towards the heart.

Arteries and veins
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. However, based on the gaseous composition of blood, the arteries are the blood vessels that carry arterial blood( oxygenated) and the veins carry deoxygenated blood with the only exception being the 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 branched-style. Arteries are so made that blood is pushed in a single direction with force . The amount of force is highest at the aortic output and it is least when the blood reaches the final destination( cell). 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 tiny venous capillaries.

Structure of Arteries
Arteries are highly elastic, thick and stronger than the veins .This is because, arteries have to carry the blood from the heart towards each cell of the body against the gravity. Arteries are so active that,each and every cell of our body receives oxygenated blood as a consequence of elasticity and the force created within the arterial lumens. A typical artery is surrounded by 3 different layers; 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.

Layers of artery-image (U.S National library)

Tunica Intima
This is the internal lumen formed by the combination of endothelial cells and the inner lamina. Tunica intima is the thinnest layer developed 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 and venules, the sub endothelial layer has only a single layer of cells, but the same can be much thicker as in the aorta. A thin basement membrane surrounds from below the tunica intima.

Tunica Media
This is the middle portion of the vessel wrapped in between the tunica external and the tunica intima. Tunica media is made up of a combination of smooth muscle cells and the elastic connective tissues arranged in a coiled fashion. 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. Tunica media is made up of circularly arranged elastic fiber, connective tissue, polysaccharide substances. Tunica media itself has 3 sublayers, the second and third layer are separated by another thick elastic band of cells called external elastic lamina.

Tunica Externa/adventitia
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 as the blood moves through the lumen. The tunica Externa is normally thicker in veins as it helps to prevent the collapse of veins during venous circulation.

Veins are relatively more fragile than the arteries as they carry blood towards the heart ( not against the gravity). Once the blood passes through the veins, it enters into the much thinner veins, called venules. Venules carry the blood with a high carbon dioxide content hence it is called venous blood or deoxygenated blood. The walls of veins are 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 and this is what makes them anatomically smoother and thinner than arteries. 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
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 because, they are directly immersed inside the fine tissues, muscles, organs, etc. See the picture below for more information.

Blood vessels (retouched)
Arterial and venous branches- source wiki images

Read more
Blood groups
Blood vessels in human beings
Anatomy of human heart

Check your understanding

Write 2 differences between the artery and vein.
Name the artery that carries impure blood
Explain the 3 layers of artery .
Define an artery and a vein.
What is the smallest arterial component called?