[Cardiology] The heart

The heart is a sack of cardiac muscle cells which contract in order to squeeze blood through the circulatory system. The contraction of the muscle fibres transmits a force to the blood and increases the velocity of flow.

The organisation of the heart

The heart is organised into four chambers: the right atrium pushes blood to the right ventricle. The left atrium pushes blood to the left ventricle. The right side of the heart accepts blood from the venous system of the body and pushes this to the lungs. The blood passes close (perfusion) to the air sacs of the lungs (the alveoli), which fill with air (ventilation), and gas exchange occurs. The left side of the heart accepts blood from the venous system of the lungs and pushes it to the body through the biggest artery of the blood: the aorta.

The fibrous component

The heart is supported by a scaffold of structural tissues, including four valves, which prevent blood from flowing backwards. Each side of the heart has two valves.

Simplified representation of the blood flow of the heart

The electrical component

The heart coordinates contractions by generating electrical signals spontaneously in a patch of tissue called the sinoatrial node (SAN). These signals flow through special electrically conductive fibres (Purkinje fibers).

Simplified representation of the electrical pathways of the heart

The heart receives rich innervation (nerve supply) from the sympathetic and the parasympathetic nervous systems.

• Sympathetic innervation comes from the sympathetic chain. This increases the heart rate and stroke volume.
• Parasympathetic innervation comes from the vagus nerve (CN-X).  This decreases the heart rate and and stroke volume.

Schematic of the nervous supply to the heart and surrounding structures.
Drawn with guidance from the schematic by Dr. Jonathan Simmonds M.D.

Arterial supply of the heart

Freshly oxygenated blood from the lungs flows into the left atrium and ventricle. As the left ventricle contracts, this blood is ejected up the ascending aorta to reach the tissues throughout the body. Appropriately, the heart is the first organ to receive this blood.

Schematic of the arterial supply to the heart and surrounding structures.
Drawn with guidance from the schematic by Dr. Jonathan Simmonds M.D.