Blood Vessels

Left picture: Artery vs. Vein

Right picture: Capillary

Tunica Externa

This is the outmost layer comprised of collagen fibres to resist overstretching. 

Tunica Media

This is the middle layer and consists of elastic fibres and smooth muscle. It is thicker in the arteries than in the veins. In the arteries, the elastic fibres allowing stretching to accommodate the changes in pressure and flow rate as blood is pumped from the heart. These elastic fibres can recoil, pushing blood along the arteries and maintaining a high pressure. The smooth muscle can contract, which helps to maintain blood flow and a high pressure as the blood travels further away from the heart.

Endothelium

This is the innermost layer and is one cell thick. It has a smooth layer to decrease friction with a minimum resistance to blood flow. 

• Arteries carry blood away from the heart.
• They have a small diameter lumen to maintain a high blood pressure.
• They have thick muscular walls to withstand high blood pressure.
• They branch into arterioles, which in turn branch into capillaries. 

• The veins have larger diameter lumen and thinner muscular walls than the arteries as the pressure and flow rate they have to withstand is less.
• They carry blood back to the heart.
• Veins have semi lunar valves to ensure the flow of blood in one direction.

• The capillaries form a vast network that penetrates tissues and organs.
• The capillaries have walls made of endothelium: they are one cell thick and have pores between the cells to allow the diffusion of water and solutes.
• They have a small diameter and the rate of diffusion slows down. This creates time for the exchange of materials between the blood and the surrounding tissue fluid. 

Arteries, Veins 

• Arteries have narrow lumen, whilst the veins have wider lumen.
• Arteries carry blood away from the heart, whilst veins bring blood back to the heart.
• The arteries carry oxygenated blood (except for the pulmonary artery), whilst the veins carry deoxygenated blood (except for the pulmonary vein). 
• The arteries have thicker muscular walls, whilst the veins have thinner muscular walls.
• The veins contain valves, whilst the arteries do not.
• Blood is transported under high pressure and has a rapid flow rate in the arteries, whilst it is less in the veins.
• Pulse is often detectable in the arteries, whilst it is not in the veins.