The circulatory system

Biology AS · Transport in mammals. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Mammals have a closed double circulation. Closed means the blood stays inside vessels. Double means the blood passes through the heart twice in one full circuit: once for the pulmonary circulation to the lungs and once for the systemic circulation to the rest of the body.

Blood leaves the heart in arteries, then flows through arterioles, capillaries and venules and returns in veins. The wall of each type of vessel suits the pressure of the blood inside it.

Exchange happens at the capillaries. High pressure at the arterial end forces water and small solutes out of the plasma to form tissue fluid, which bathes the cells and carries substances between them and the blood.

Rules to remember

Pulmonary artery: deoxygenated blood from the heart to the lungs. Pulmonary vein: oxygenated blood from the lungs to the heart.
Aorta: oxygenated blood from the left ventricle to the body. Vena cava: deoxygenated blood from the body to the right atrium.
Elastic arteries (such as the aorta): much elastic tissue, which stretches and recoils to smooth the flow. Muscular arteries: more smooth muscle, to control flow to organs.
Veins: thin wall, wide lumen and valves, for blood at low pressure.
Capillaries: wall one endothelial cell thick, for a short diffusion distance.
Tissue fluid is plasma without most of the proteins, which are too large to leave the capillary.
Water in blood: a good solvent for transport, and a high specific heat capacity for carrying heat.

Common mistake

Saying that all arteries carry oxygenated blood. Arteries carry blood away from the heart, so the pulmonary artery carries deoxygenated blood.

Worked example

A photomicrograph shows two vessels side by side in transverse section. Vessel X has a thick wall and a small round lumen. Vessel Y has a thin wall and a large lumen with an irregular shape. Identify X and Y.

A thick wall with much muscle and elastic tissue resists high pressure and keeps its round shape, so X is an artery.
A thin wall and a wide lumen suit blood at low pressure, and a thin wall flattens easily, so Y is a vein.
Neither is a capillary, because a capillary wall is only one cell thick.

Answer: X is an artery and Y is a vein.

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