Transport

Both have three layers around a hollow lumen: the tunica intima (smooth endothelium), the tunica media (smooth muscle and elastic fibres) and the tunica externa (connective tissue with collagen).

In a micrograph, compare the wall with the lumen. An artery has a thick wall and a narrow lumen and tends to look round. A vein has a thin wall and a wide lumen and often looks squashed.

Watch out: size alone does not tell you which is which. Use wall thickness relative to lumen diameter.

Each heartbeat sends a surge of blood into the arteries at high pressure, reaching about 120 mm Hg in the aorta. The wall has two jobs:

• Withstand the pressure: a thick wall of muscle, with collagen, prevents the artery from rupturing.
• Maintain the pressure: elastic fibres stretch as the surge arrives, storing energy, and then recoil between beats, pushing the blood on. Contracting smooth muscle also narrows the lumen.

Further from the heart, arteries have less elastic tissue and more smooth muscle. Arterioles use this muscle to control how much blood reaches each capillary bed.

By the time blood reaches a vein it has lost nearly all its pressure, at about 5 mm Hg, and has no pulse. Backflow is a real risk, so veins rely on two features:

• Valves: blood moving to the heart pushes the flaps aside. If it flows backwards, it fills the pockets and the flaps close.
• A thin, flexible wall: contracting skeletal muscles can squeeze the vein, pushing blood along it.

The wide lumen also reduces friction. This is why walking helps blood to return from the legs.

Coronary arteries branch from the aorta and supply heart muscle. Occlusion means that a vessel is blocked or closed.

1. Fatty deposits, or atheroma, form under the endothelium.
2. The lumen narrows and blood pressure rises, which damages the wall further.
3. Fibrous repair tissue makes the wall less elastic.
4. The lining breaks down, forming a plaque.
5. A ruptured plaque triggers clotting, and the clot is a thrombus. If it breaks free it is an embolus.

Consequences: a narrowed artery can cause chest pain, or angina, during exercise. A full block deprives muscle downstream of oxygen, so it dies. If enough muscle is lost, the heart cannot pump effectively. This is a heart attack, or myocardial infarction.

Evaluating data: studies across populations link coronary heart disease with factors such as saturated fat, smoking and high blood pressure. When you evaluate this evidence, look at sample size, spread of data, whether other variables were controlled, and how the data were collected. A strong correlation does not prove a cause.

Watch out: plaque forms within the artery wall, beneath the endothelium. It does not simply stick on top.