The two circuits: lungs and body

On one full trip around the body, blood passes through the heart twice: once on its way to the lungs and once on its way back out.

Why two loops?

Blood picks up oxygen in the lungs and gives it away in the rest of the body. If the heart had only one pump, blood would leave the lungs and then have to push on through every organ with no fresh boost. Instead, the heart sends blood out twice. The right side pumps it through the lungs in a short loop called the pulmonary circuit. The left side then pumps it through the whole body in a long loop called the systemic circuit.

The pulmonary circuit

Oxygen-poor blood from the body enters the right atrium through the superior and inferior vena cava. The right atrium passes it to the right ventricle, which pumps it through the pulmonary valve into the pulmonary artery. The pulmonary artery splits in two, one branch for each lung. In the lungs the blood gives off carbon dioxide and takes up oxygen. The oxygen-rich blood then returns to the heart through the pulmonary veins and enters the left atrium.

The systemic circuit

From the left atrium the blood moves into the left ventricle. The left ventricle pumps it through the aortic valve into the aorta, the largest artery. The aorta arches over the top of the heart and sends branches to the head and arms before running down through the chest and belly. Its branches divide again and again until the blood reaches tiny capillaries in every tissue, where oxygen and nutrients leave the blood and waste enters it. Then the blood collects into veins, and the veins join until they form the two venae cavae that lead back to the right atrium. The trip begins again.

Arteries leave, veins return

Here is a point that trips up many learners. Arteries are defined by direction, not by oxygen: arteries carry blood away from the heart and veins bring blood to the heart. So the pulmonary artery carries oxygen-poor blood, and the pulmonary veins carry oxygen-rich blood. They are the exceptions to the rough rule that arteries are "red" and veins are "blue".

The colours in the model and in most diagrams are a code for oxygen level. Blood with less oxygen is darker, but it is still red; it is drawn blue only to tell the two kinds apart at a glance.

What changes in the lungs and in the body

The change in the blood happens in capillaries at both ends of the trip. In the lungs, carbon dioxide leaves the blood and oxygen enters it, so the blood in the pulmonary veins has a high oxygen content. In the rest of the body the opposite happens: cells use oxygen for their work and give off carbon dioxide, so the blood in the systemic veins has a reduced oxygen content when it returns to the right atrium. Gases cross the thin capillary walls by diffusion in both places.

A third, small loop for the heart itself

The heart muscle is part of the systemic circuit, but it has its own short route. The coronary arteries branch off the aorta just after the aortic valve, so the heart is one of the first organs to receive fresh blood. After the blood passes through the capillaries in the heart wall, coronary veins collect the oxygen-poor blood and return it to the right atrium, where it joins the rest of the blood heading to the lungs.

In the model, try switching on X-ray instead of Cut open. The outer walls fade and you can follow the blue stream from both venae cavae and the red stream from the four pulmonary veins at the same time. The two streams cross over each other above the heart, where the aorta arches over the pulmonary artery, but they never mix.