Part one

Part two

The heart

The heart is a muscular organ about the size of your fist, located in the center of the chest, between the two lungs.


It is a fantastic organ, almost untiring. It makes the blood circulate throughout the body.

The 4 cardiac chambers

The interior of the heart is composed of four hollow that we call “chambers”. The upper cavities are called the atria. They are reservoirs of blood waiting to fill the ventricles.

The ventricles are the lower chambers of the heart. Their muscles are stronger then the atriums’. It is the ventricles that pump the blood throughout the body.

A right and a left heart

A division between these cavities - called septum - divides the heart into two sections - a right heart (in blue) and a left heart (in red), each one having one atrium and one ventricle.

The heart has a “skeleton”

The heart is built on a fibrous skeleton, which means a rigid structure on which it is assembled. This skeleton is made of 4 rings.

The atriums and the ventricles assemble themselves on the two larger rings that are called atrioventricular rings.

The two smaller rings attach the pulmonary artery to the right ventricle and the aorta to the left ventricle.

The 4 heart valves

The heart possesses 4 valves that ensure the blood circulates in the right direction … without any backflow.

Two tricuspid valves

Two of these cardiac valves are the aortic and pulmonary valves. They are found at the base of the large vessels thus at the exit of the ventricles.

These valves are tricuspid which means they have three cusps attached to the ring. They separate the aorta from the left ventricle and the pulmonary artery from the right ventricle respectively.

The cusps are extremely flexible pockets that fold up against their ring as the ventricle ejects the blood.

After ejection, the column of blood tries to come back in the ventricle because of the lower pressure and the suction caused by the ventricle’s relaxation. This backflow causes the cusps to open and stop the blood from coming back into the ventricle by temporary sealing the opening. 

Two atrio-ventricular valves

The two other cardiac valves are the atrioventricular valves that separate the atrium from the ventricle.

These valves are built on the atrioventricular rings and possess leaflets, chordae tendini (a sort of rope) and muscular pillars in the ventricles.

On the right side, there is the tricuspid valve. On the left side there is the mitral valve that possesses two leaflets.

When the ventricles contracts, the pressure generated by the muscle expels the blood, which closes the leaflets.

The leaflets are held by the chordae tendini (ropes) that stop them from being sucked into the atriums.

Thus, the only way out for the blood is through the pulmonary valve on the right and the aortic valve on the left side.

The envelope of the heart

If you were asked to rub your hands together constantly, the friction would eventually produce irritation, inflammation and possibly blisters. 

As the heart is in constant motion, it is surrounded by a protective envelope with a small quantity of liquid inside. This liquid works as a lubricant, thus allowing the heart to move freely. This pocket is called the pericardium.

An electrical system exclusively for the heart

The heart possesses a fascinating electrical system that is unique. It allows the synchronised contraction of the different chambers.

The leader of this electrical system is found at the top of the heart in the right atrium. It is the natural stimulator or pacemaker of the heart. It is the one who initiates every beat and determines the heart rate.

The transmission between the atriums and ventricles is only possible in one site of the heart. It is another group of cells called the atrioventricular node. It has the same task as a customs officer but for the electrical signals. It lets the electrical signals travel from the atriums to the ventricles. It thus determines the maximal heart rate in normal conditions.

When the electrical signals have passed through the atrioventricular node, the current is transmitted to the right and left ventricles by two branches. These two electrical highways are called: rapid conduction pathways.

This organized electrical transmission in the ventricles ensures the heart contracts from bottom to top in order to send the blood towards the pulmonary and aortic valves.

The large vessels attached to the heart

Large blood vessels, arteries and veins, are connected to the heart. They are called the inferior and superior vena cava, the pulmonary artery, the 4 pulmonary veins and the aorta.

The arteries that nourish the heart

The blood networks that nourish the heart consist of a right and a left coronary artery.

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