The four chambers

The wall of the left ventricle is the thickest in the heart, because it has to push blood around the whole body.

Two that receive, two that pump

Open a heart from the front and you find four spaces. The two upper chambers are the atria (one is an atrium). They are thin-walled rooms that receive blood coming back through the veins. The two lower chambers are the ventricles. They have thick walls because their job is to push blood out of the heart with force. The difference in thickness comes from how much muscle each chamber has, and that matches how hard each one has to work.

Right and left are the body's right and left

In diagrams and in this model the heart faces you, so the right atrium and right ventricle appear on the left side of the screen. Doctors and textbooks always name the sides from the point of view of the person whose heart it is. It helps to imagine you are looking at a friend standing in front of you: their right hand is on your left.

The right atrium and right ventricle

The right atrium receives oxygen-poor blood from the body through two large veins. The superior vena cava brings blood from the head, neck and arms, and the inferior vena cava brings blood from the lower body. From the right atrium the blood passes down into the right ventricle. The right ventricle then pumps it through the pulmonary artery to the lungs. The lungs are close by and the vessels there offer little resistance, so the right ventricle wall is thinner than the left.

The left atrium and left ventricle

The left atrium receives oxygen-rich blood returning from the lungs through the pulmonary veins. It passes the blood down to the left ventricle. The left ventricle generates the high pressure needed to send blood through the aorta to the whole body, from the top of the head to the toes. That is the heaviest job in the heart, and the left ventricle has the thickest muscle to match. In the cut view you can compare the two ventricle walls directly.

The wall in the middle

Between the two sides runs the septum. The part between the ventricles is thick muscle that squeezes along with them. The part between the atria is thinner. Because the septum separates the two sides, the right side only ever handles oxygen-poor blood and the left side only ever handles oxygen-rich blood. The two atria contract at the same time, and a moment later the two ventricles contract at the same time, so the two pumps work in step.

A good way to remember the layout is to follow one drop of blood. It enters on the right at the top, drops into the right ventricle, leaves for the lungs, comes back on the left at the top, drops into the left ventricle and leaves for the body. Atrium first, ventricle second, on each side.

Reading the cut view

Turn on Cut open in the model and look at the flat surface where the heart has been sliced from top to bottom. The darker shapes are the spaces inside the chambers, and the lighter band around them is muscle. Around the left ventricle the band is wide; around the right ventricle it is much narrower, and the right ventricle curves around the front of the left one. The two atria sit above, each with a thin rim of wall. Between the two ventricles you can see the thick muscle of the septum, which squeezes together with the ventricles on every beat.

Where the chambers meet you will also find pale rings with flaps. These are the valves, the doors between the rooms. The next unit is about them, but notice already that each atrium opens into the ventricle below it and each ventricle opens into one large artery. No chamber has two exits.