Blood Vessels

Explained

Three blood vessels, three different pressures

Arteries, veins and capillaries look different because the blood inside them behaves differently. Arteries carry blood at high pressure away from the heart, veins return it at low pressure, and capillaries do the actual exchanging. Every structural feature follows from one of those three facts.

Arteries

Blood leaving the heart surges at high pressure, so an artery has thick walls containing muscle and elastic fibres. The elastic fibres stretch as blood is forced in and recoil afterwards, which smooths the flow between heartbeats. The lumen is relatively narrow, helping to maintain the pressure.

Away from the heart is the direction, not "carrying oxygenated blood". The pulmonary artery carries deoxygenated blood to the lungs, and questions use it to check that you have learned the definition rather than the usual case.

Veins

By the time blood reaches a vein the pressure has fallen, so thick muscular walls would serve no purpose. Veins have thinner walls and a wide lumen, which reduces resistance to flow.

Low pressure creates a problem: blood could flow backwards. Veins contain valves that close if that starts to happen, and they are squeezed by the skeletal muscles around them, which pushes blood along.

Capillaries

Capillaries are where substances actually move between blood and tissues, so everything about them favours diffusion. Their walls are one cell thick, giving a very short diffusion distance. They form dense networks, giving a very large surface area. They are narrow enough that red blood cells pass through in single file, which slows the flow and allows more time for exchange.

Coronary heart disease

The coronary arteries supply the heart muscle itself with blood. Fatty material builds up inside them and narrows the lumen, so less blood reaches the muscle and it receives less oxygen.

The consequences follow from that shortage. Less oxygen means less aerobic respiration, so the muscle has less energy and pain may follow during exertion. If a vessel becomes completely blocked, that region of muscle is starved of oxygen altogether.

Risk factors include a diet high in saturated fat, smoking, high blood pressure and lack of exercise. Naming a risk factor and saying how it contributes is worth more than listing several without explanation.

Answering a comparison question

Take one feature at a time and give all three vessels: wall thickness, then lumen width, then valves, then what each carries. A table in your head with three columns is more reliable under pressure than three separate descriptions.