The human body has a vast circulatory system that allows blood to travel from our hearts, at the center of the system, to the outer extremeties of our bodies and back again. Within the circulatory system, blood travels along two main types of blood vessels: veins and arteries. Even though they serve the same essential function -- carrying blood around the body -- there are a number of differences between veins and arteries.
The major difference between veins and arteries is the direction in which blood travels through them. Arteries carry blood away from the heart, while veins carry blood towards it. This seemingly small functional difference is actually very significant as it is because of this that there are major structural differences between veins and arteries.
Veins and arteries are both composed of the same three types of tissue, which is arranged in layers: the tunica media, the tunica intima, and the tunica adventitia. Many of the functional differences between veins and arteries stem from differences in the thickness of each of these layers depending on whether they make up a vein or an artery. The thinnest layer in both veins and arteries is the innermost layer, the tunica intima. The tunica intima is made up of a single layer of cells, which are in direct contact with the blood that flows through the vessel.
In arteries the thickest layer is the tunica media, the middle layer of the vessel tissue. The thickness of this layer varies depending on how close an artery lies to the heart. For example, the aorta, which branches directly off the heart itself, has the thickest tunica media of any artery. This is due to the fact that blood is pumped out of the heart through the left ventricle with great force, which the aorta must be able to contain. The aorta is able to handle the high pressure of blood flowing out of the heart because the tunica media is composed of smooth muscle and elastic tissue, which allow it to expand and contract in response to differences in blood pressure that occur between heart beats.
Further away from the heart, the tunica media in the arteries gets thinner, but it is still very thick relative to the tunica media of the veins as it provides structural stability and allows the arteries to maintain enough pressure for blood to move to the outermost parts of the body i.e. the head, fingers, and toes.
The thickest layer in veins is the tunica adventitia, which is also known as the tunica externa. This is the outermost layer of the blood vessel, which provides similar structural stability in veins to the tunica media in arteries. While blood is moved through arteries by the activity of the tunica media, it travels trhough veins using a different mechanism called 'skeletal muscle pump.' In skeletal muscle pump, blood is moved passively through the veins by the contraction of skeletal muscle throughout the body, which forces blood to move upwards towards the heart instead of pooling in the body's lower extremeties (the hands and feet).
This difference in the mechanism of blood flow also leads to one of the other major differences between veins and arteries: the presence of valves in veins. In large veins, the tunica intima contains valves. These valves ensure that blood travelling towards the heart via skeletal muscle pump cannot flow backwards and start to pool in the lower extremeties. Failure of the valves to close properly is the main cause of varicose veins, which occurs when the veins of the lower leg become enlarged due to the blood that collects in them instead of being forced back up towards the heart.