Blood Vessels

There are three main types of blood vessels:

1. Arteries
2. Veins
3. Capillaries

Transverse Section

Arteries vs Veins

Arteries	Veins
Blood is under high pressure and flows in pulses	Blood is under low pressure and flows smoothly
Thick, tough, elastic walls	Thin walls
Small lumen	Large lumen
Does not contain valves	Contains valves
Usually carries oxygenated blood away from the heart (except for the pulmonary artery)	Usually carries deoxygenated blood towards the heart (except for the pulmonary vein)

• Valves in veins prevent backflow of blood, ensuring it flows in the correct direction towards the heart.
• As blood in veins is under low pressure, valves are needed to prevent the blood flowing backwards.
• Skeletal muscles surrounding veins contract and squeeze the veins, helping to push blood back to the heart.
• The muscle in arteries and veins is involuntary smooth muscle.
• Arteries carry blood to smaller vessels called arterioles.

Capillaries

• Capillaries are tiny, highly branched blood vessels that allow for the exchange of materials between blood and body cells.
• They connect arterioles to venules.
• Their walls are one cell thick, allowing materials to pass in and out efficiently.

Adaptations of Capillaries:

1. Numerous: Large surface area for exchange of materials.
2. Thin-walled: Only one cell thick for efficient exchange.