Double Circulation

Explained

Two circuits, one heart, and the reason it works

In a double circulation, blood passes through the heart twice for every complete circuit of the body. Humans have one, and the word double refers to those two passes, not to having two hearts.

The two loops

The pulmonary circuit runs between the heart and the lungs. The right side of the heart pumps deoxygenated blood to the lungs, where it loses carbon dioxide and gains oxygen, and the now oxygenated blood returns to the left side of the heart.

The systemic circuit runs between the heart and the rest of the body. The left side pumps oxygenated blood out to all the organs, where the tissues take the oxygen they need, and the deoxygenated blood returns to the right side.

Follow one red blood cell round and the order is right side, lungs, left side, body, back to the right side. It has been through the heart twice.

Why two circuits are better than one

This is the question the topic exists for, and the answer is about pressure.

Blood loses a great deal of pressure as it squeezes through the narrow capillaries of the lungs. In a single circulation, as in a fish, that low pressure blood would have to travel straight on to the rest of the body, so it would move slowly.

In a double circulation the blood comes back to the heart in between and is pumped again. The pressure is restored before the blood sets off round the body, so it travels faster, delivers oxygen and glucose to the tissues more quickly, and removes waste more quickly.

Faster delivery supports a higher rate of respiration, which is what a mammal needs to maintain a constant body temperature and a high level of activity.

The two sides are not identical

The left ventricle has a much thicker muscular wall than the right. Both pump the same volume of blood, but the right only has to push it as far as the lungs, which are next door, while the left has to push it to the head, the fingers and the toes.

A thicker wall contracts more powerfully and generates a higher pressure. If a question asks why the walls differ, the distance is the reason, and the pressure is the mechanism.

The lungs also need the lower pressure. Their capillaries are delicate, and blood at systemic pressure would damage them and force fluid into the air spaces.

Naming the vessels

Four vessels connect to the heart, and their names follow a simple rule. Arteries carry blood away from the heart, veins carry it towards the heart, and the pulmonary ones go to or from the lungs.

So the vena cava brings deoxygenated blood from the body into the right atrium. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. The pulmonary vein brings oxygenated blood from the lungs into the left atrium. The aorta carries oxygenated blood from the left ventricle to the body.

The pulmonary vessels are the exceptions people remember wrongly, because the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. They are not really exceptions at all. Artery and vein describe direction of travel, and they always have.

One vessel that is neither

The hepatic portal vein carries nutrient rich blood from the gut to the liver. It is unusual because it runs from one organ's capillaries to another's rather than back to the heart, and it exists so the liver can process absorbed food before it reaches the rest of the body.