How Blood Flows Through the Heart

Blood in humans travels in two linked loops, which is why our system is called a double circulation:

• the pulmonary circulation runs from the heart to the lungs and back;
• the systemic circulation runs from the heart to the rest of the body and back.

The heart sits where the two loops cross, like the middle of a figure of eight. On one complete trip round the body, a drop of blood passes through the heart twice: once through the right side and once through the left.

Blood coming back from the body has given up much of its oxygen to the organs and is carrying carbon dioxide. It enters the heart through two large veins, the superior and inferior vena cava, and collects in the right atrium.

From there it passes through the tricuspid valve into the right ventricle. When the right ventricle contracts, the tricuspid valve closes so nothing leaks backwards, and the blood is pushed through the pulmonary valve into the pulmonary artery. This artery splits in two and carries the blood to both lungs.

In the lungs the blood flows through very thin capillaries wrapped around the air sacs. Carbon dioxide passes out of the blood to be breathed out, and oxygen passes in.

Now rich in oxygen, the blood returns through the pulmonary veins into the left atrium. It passes through the mitral valve into the left ventricle.

The left ventricle has the thickest muscle wall in the heart. When it contracts, the mitral valve shuts and the blood is driven through the aortic valve into the aorta. The aorta branches into smaller and smaller arteries that reach every organ. In the capillaries, oxygen is handed over to the body's cells, and the oxygen-poor blood flows back through the veins to the vena cava to start again.

The two sides of the heart work at the same time. The two atria contract together, then a moment later the two ventricles contract together. So a single beat pushes blood into the lungs and into the body at once.

Doctors call the squeezing phase systole and the relaxing, filling phase diastole. Most of the filling actually happens while the whole heart is relaxed: blood flows straight through the open valves into the ventricles. The atria's contraction only tops them up with the last 20 to 30 per cent. At a rate of 75 beats a minute, one full cycle of filling and squeezing takes about 0.8 seconds.

Each beat starts with an electrical signal from the sinoatrial (SA) node, a small patch of pacemaker cells in the wall of the right atrium. The signal spreads across both atria, which contract. It then reaches the atrioventricular (AV) node, which holds it back for a split second so the ventricles can finish filling. Finally it spreads down through the walls of the ventricles, which contract and pump blood out of the heart. Then the heart relaxes and the process starts again.

A fish has a single circulation: its two-chambered heart pumps blood through the gills and then straight on round the body, so the blood is not pumped again after it collects oxygen. That limits how much oxygen reaches the fish's organs.

In our double circulation, blood comes back to the heart after the lungs and the left ventricle pumps it out again, hard. Because the right and left sides are completely separated by the septum, the body receives only oxygen-rich blood, and the lungs receive only oxygen-poor blood.