Anatomy of the Heart
The heart sits in the middle of the chest behind the sternum, and extends towards the left side. It is a strong muscular pump, about the size of its owner’s clenched fist. It has four chambers:
- two atria, which receive blood to pump to the ventricles
- two ventricles, one that pumps blood to the lungs (right), and another that pumps blood to the body (left)
In the average adult, the heart beats around 60 – 100 times per minute, sending about six litres of blood through well over 1,000 complete circuits of the body each day.
During a lifetime of 70 years, the heart will beat more than 2.5 billion times. The only rest it gets or needs is a split-second pause between beats.
The heart contains several valves that are designed to ensure that the blood flows in one direction. It normally beats rhythmically, pumping blood throughout the body through the arteries, veins and capillaries.
Oxygen-rich blood passes from the lungs to the left side of the heart. It is then pumped to all the different parts of the body where oxygen is delivered to the body cells.
Oxygen-depleted blood is returned to the right side of the heart and pumped to the lungs where it again becomes oxygenated. The blood then travels back to the left side of the heart, ready to make another journey around the body.
Just like any other muscle in the body, the heart also requires blood supply. It receives this by coronary circulation, a system of vessels which supplies oxygenated blood to the heart muscle cells and directs the deoxygenated blood back towards the right side of the heart.
The heart’s muscular contraction pumps blood through the lungs and around the body; and the coordinated and rhythmic contraction of all of the cardiac muscle cells for this purpose is controlled by the heart’s electrical system.
The electrical conduction system
All cardiac cells have an inherent pacemaker activity, that is, in isolation they would all contract at their own rate, in an unsynchronised way.
The electrical impulses that allow for a synchronised contraction of the heart originate in specialised pacemaker cells and are distributed to the cardiac muscle cells by an electrical conduction system.
The main pacemaker (the sinoatrial, or SA node) is situated in the wall of the right atrium. An electrical impulse spreads from the pacemaker through the walls of the atria, causing the muscle cells of the atria to contract and force blood into the ventricles.
The electrical impulse continues down the conduction pathway to the atrioventricular (AV) node where it is momentarily slowed to allow time for the atria to contract before it spreads through the ventricles, which contract. This contraction expels blood from the heart to the lungs (from the right ventricle) or throughout the rest of the body (from the left ventricle).