Circulatory system

The circulatory system (Fig. 1) includes the heart, blood vessels (arteries and veins) and the blood. There are two circuits of blood circulation: the pulmonary and the systemic.

– The pulmonary circuit carries the oxygen-poor blood from the heart to the lungs for gas exchange and returns oxygen-rich blood to the heart.

– The systemic circuit carries oxygen-rich blood from the heart to the rest of the body and returns oxygen-poor blood to the heart.

Fig. 1: The circulatory system, the heart and main arteries (red) and veins (blue) are illustrated. Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.

How the heart works

Below is a narrated animation of blood flow through the heart. Click here to license this video and/or other cardiovascular related videos on Alila Medical Media website.

The heart has two sides: right and left; each side has two chambers : an atrium and a ventricle (Fig. 2). The right side of the heart is in charge of the pulmonary circuit, it receives oxygen-poor blood from the body and pumps it into the lungs for gas exchange. The left side of the heart is in charge of the systemic circuit, it receives oxygen-rich blood from the lungs and pumps it through the aorta to the rest of the body.

The heart has four valves : two atrioventricular valves (AV valves) and two semilunar valves. Their function is to ensure that the blood flows only in one direction throughout the heart, i.e. they prevent blood backflow.

– The right AV valve, also called tricuspid valve, is located between the right atrium and right ventricle.

– The left AV valve, also called mitral valve, is located between the left atrium and left ventricle.

– The semilunar pulmonary valve is located at the base of the pulmonary trunk – the large artery that takes blood to the lungs.

– The semilunar aortic valve is located at the base of the aorta – the large artery that takes oxygen-rich blood to the rest of the body.

Fig. 2: The pathway of blood flow through the heart. Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.

If you have watched the video, there is no need to read all the text below!

Oxygen-poor blood from your body returns to the right atrium of the heart. Blood from upper body returns through superior vena cava, blood from lower body returns through inferior vena cava. As the right atrium is filled with blood, it contracts, the tricuspid valve opens and blood is pumped into the right ventricle of your heart. When the right ventricle is full, the tricuspid valve closes to prevent blood from flowing back into the atrium. The right ventricle contracts, pulmonary valve opens and blood is pumped into the pulmonary artery and to your lungs. Pulmonary valve closes to prevent blood from flowing back into the ventricle.

Oxygen-rich blood from the lungs returns to the left atrium of the heart. As the left atrium is filled with blood, it contracts, the mitral valve opens and blood is pumped into the left ventricle of your heart. This occurs at the same time as the right atrium pumps blood into the right ventricle on the other side of the heart. As the left ventricle is full, the mitral valve closes, the aortic valve opens, the left ventricle contracts and oxygen-rich blood is pumped into the aorta to reach all parts of your body. This happens at the same time as the right ventricle pumps blood into the pulmonary artery on the other side of the heart. The aortic valve quickly closes to prevent blood from flowing back to the heart. Meanwhile, the atria have filled with blood and the cycle repeats itself.