Describe the differences in structure between an artery and a vein.

Blood is transported around the body by blood vessels. Arteries are responsible for carrying blood away from the heart, whilst veins are responsible for returning blood to the heart. Arteries and veins are both made up of the same 4 components: a tunica externa (the outside layer which is made of tough collagen fibres), a tunica media (the middle layer made of smooth muscle and elastic fibres), the endothelium (the thin inner lining) and the lumen (where the blood flows). It is the amount of each component that differs between arteries and veins.

Arteries have a thick tunica externa because the blood travels at a higher pressure in arteries than veins. They have a thick muscular layer, the tunica media, which allows for contraction - this helps propel the blood along the artery and contributes to the high blood pressure. And finally, they have an inner squamous epithelium, which is a very thin layer of cells. Veins, on the other hand, have a thinner tunica externa (the tough outside layer) because the blood pressure within the vein is less than the blood pressure in the artery. They have a thinner muscular layer which means there is less contraction and the blood travels at a lower pressure. Like arteries, veins have a thin layer of endothelium. Finally, veins have valves whereas arteries do not. Valves help prevent the backflow of blood in the wrong direction in the veins when they are returning blood to the heart.