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The Structure of the Heart and the Cardiac Cycle

In this article, I intend to present an overview of the structure of its heart and how it functions in order to explain in later articles why a pacemaker may be needed and what the difference between various pacemakers are.

Structure of the Heart

Humans have a double circulatory system. This means that the heart is joined to two separate circuits. One pumps blood around the lungs, and the other pumps blood around the rest of the body.

The pulmonary artery transports deoxygenated blood to the lungs and the pulmonary vein returns the newly oxygenated blood back to the heart.

The vena cava brings deoxygenated blood from the rest of the body and the aorta takes the oxygenated blood to the rest of the body.

The heart itself is made up of four chambers. The two upper chambers are known as atria (singular: atrium) and the two lower chambers are called ventricles.

There are also a four valves within the heart which helps blood flow through the heart in the correct direction and prevents it from flowing backwards.

Cardiac Cycle

The cardiac cycle refers to the contraction and relaxation of the heart during one complete heartbeat.

Systole - Contraction of the heart muscle.

Diastole - Relaxation of the heart muscle.

The cardiac cycle has three distinct phases:

1) Atrial and Ventricular diastole - Chambers relax and deoxygenated blood enters the right atrium and oxygenated blood enters the left atrium.

2) Atrial systole - Atria contract and the remaining blood is pushed into the ventricles.

3) Ventricular systole - Ventricles contract and push blood out.

Cardiac Conduction System

The sino-atrial node (SAN) is found at the top of the right atrium. It is a group of specialised cells that emit electrical impulses which are carried through the walls of both atria. This causes atrial systole.

The impulse is then carried to the atrioventricular node (AVN) in the lower part of the right atrium. The signal, after a short delay, then diverges passing through the left and right Bundle of His (a collection of heart muscles specialised for electrical conduction) through the Purkinje fibres (conducting fibres in the left and right sides of the heart) as well as the endocardium (the innermost layer of tissue) in the apex of the heart, then finally to the ventricular epicardium (the outermost layer of tissue). This causes the contraction of the ventricles.

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