Arteries Carry Blood at High Pressure

1. Arteries carry blood away from the heart in pulses produced by ventricular contractions.
2. Their walls contain thick layers of smooth muscle that withstand high pressure and can alter the vessel diameter.
3. Their walls contain many elastic fibres that stretch when pressure rises and recoil between heartbeats, helping maintain blood flow.
4. A relatively small lumen helps the vessel maintain high pressure.
5. A tough outer layer prevents the wall from bursting.

Veins Return Blood at Low Pressure

1. Veins carry blood towards the heart after pressure has fallen through the capillary network.
2. Their walls are thinner and contain less muscle and elastic tissue because they do not experience arterial pressure.
3. A large lumen reduces resistance to flow at low pressure.
4. Valves open towards the heart and close if blood begins to move backwards.
5. Contractions of surrounding skeletal muscles squeeze veins, and the valves make this pressure move blood towards the heart.

Capillaries Allow Rapid Exchange

1. Capillaries form extensive networks close to body cells, giving a large total surface area for exchange.
2. Their walls consist of a single layer of flattened cells, so substances cross a very short diffusion distance.
3. Their lumen is narrow, which brings red blood cells close to the wall and slows their passage through the network.
4. Oxygen and glucose diffuse from the blood into tissues, while carbon dioxide diffuses from tissues into the blood.
5. Small gaps between capillary wall cells allow plasma and dissolved substances to move into tissue fluid, but blood cells remain inside.

Pulmonary Vessels Are Exceptions

1. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs.
2. The pulmonary vein carries oxygenated blood from the lungs to the left atrium.
3. Most systemic arteries carry oxygenated blood and most systemic veins carry deoxygenated blood, but direction remains the reliable definition.

Compare Vessels by Function

1. For an artery, link high pressure to a thick muscular and elastic wall, then explain that recoil maintains blood flow.
2. For a vein, link low pressure to a large lumen and explain that valves prevent backflow.
3. For a capillary, link exchange to a one-cell-thick wall, narrow lumen and large network.

Pressure and Exchange Explain Structure

1. Arteries need strength and elasticity because they receive blood directly from the ventricles.
2. Veins need help preventing reverse flow because their blood pressure is low.
3. Capillaries sacrifice thick walls for rapid exchange because pressure has already fallen before blood enters them.