How does the heart work?

Cardiac anatomy

Structure

The heart is a vital organ divided into four distinct chambers, each with a specific role: the right and left atria and the right and left ventricles.

The left atrium receives oxygen-rich blood from the lungs via the pulmonary veins. This blood is then sent to the left ventricle, which expels oxygenated blood to the rest of the body through the aorta, providing blood to the organs and tissues.

After circulating through the body, the oxygen-depleted blood returns to the right atrium via the superior and inferior venae cavae. The right atrium ejects it towards the right ventricle, which sends it to the lungs via the pulmonary artery, where it is reoxygenated.

To ensure that the blood flows correctly from one chamber to another without flowing back, cardiac valves create a unidirectional flow by opening and closing at the right time.

Here are the main heart valves:

• The tricuspid valve is located between the right atrium and the right ventricle,
• The mitral valve sits between the left atrium and the left ventricle,
• The pulmonary valve between the right ventricle and the pulmonary artery allows oxygen-depleted blood to be sent to the lungs,
• The aortic valve between the left ventricle and the aorta ensures that oxygenated blood flows to the rest of the body.

Cells

The heart is made up of several different types of cell, each with a very specific role to play in ensuring that it functions properly.

Cardiomyocytes are the muscle cells of the heart responsible for its contraction. They are linked together by intercellular junctions, which allow the synchronized contraction of the heart muscle, essential for an efficient heartbeat.

Purkinje cells located in the ventricular walls are responsible for conducting the electrical impulse. They transmit electrical impulses from the conduction nodes and bundles to the cardiomyocytes, ensuring coordinated contraction of the ventricles.

Fibroblasts are the main cells of cardiac connective tissue. They play a crucial role in repairing and healing the heart, particularly after an injury or stress to the heart muscle. 

Endothelial cells line the inside of blood vessels and heart chambers. They facilitate the exchange of nutrients and waste products between the blood and heart tissue, and are involved in regulating the vessel diameter, which influences blood pressure. They play a part in inflammatory responses and blood coagulation too.

The heart also has nerve cells that regulate the heartbeat and the force of contraction in response to the body’s needs.

Cardiac cycle

The heart works in two main phases to fulfil its role as a pump: diastole and systole. 

• Diastole is the relaxation phase of the heart muscle. During this phase, the right and left atria fill with blood then the tricuspid and mitral valves open to allow blood to flow to the right and left ventricles.

• Systole is the contraction phase during which the ventricles contract to expel blood from the heart. The tricuspid and mitral valves close to prevent blood from flowing back into the atria during contraction, while the pulmonary and aortic valves open to allow blood to flow to the rest of the body and the lungs.

Blood circulation

The circulatory system is made up of two circuits: pulmonary circulation and systemic circulation. 

• Pulmonary circulation transports blood between the heart and the lungs. Its main role is to oxygenate the blood, replacing carbon dioxide with oxygen.

• Systemic circulation is the circuit that transports oxygenated blood from the heart to all the organs and tissues of the body. It then returns the oxygen-poor blood from the organs and tissues to the heart, where it is sent back to the lungs.

Electrical activity

To contract regularly, the heart uses an electrical conduction system that regulates the heartbeat.

The sinus node (or sinoatrial node) is located in the right atrium and generates the initial electrical impulse. This impulse triggers the contraction of the atria, allowing blood to flow into the ventricles.

The atrioventricular (AV) node lies between the atria and the ventricles. It receives the electrical impulse from the sinus node and transmits it to the ventricles, triggering their filling.

The bundle of His and Purkinje cells conduct the electrical impulse through the ventricles, causing them to contract in a coordinated manner to expel blood to the lungs and the rest of the body.