In the lungs the blood drops its load of carbon dioxide and receives a fresh supply of oxygen,  then  passes  the  blood  to  the  left   side  of the  heart   (which  also  has two chambers) and from there it is pumped to all parts of our body.  The walls of the heart consist of powerful muscle fibers that have the power to contract or beat rhythmically. This constant rhythmic beating keeps the circulation going.

A Ceaseless Worker

	It beats over 100,000 times a day.
	It pumps the blood through more than 60,000 miles of tiny blood vessels.  These tiny capillaries are only a tenth of an inch long, but if they could be placed together, they would stretch two and a half times around the earth at the equator.
	During our life time of say 75 years, our  heart must have beaten more than 3000 million times and pumped over 200 million litres of blood.

An Electronics Marvel

The motions of the heart are maintained by a specially designed structure  known as the Sinoatrial Node or SA node. This little structure is capable of initiating a regular rhythm, and transmitting these electrical impulses to all the muscle fibers of the heart wall.   This SA node or the Pace Maker is located near the top of the heart. It is the Radio Station of our heart flashing out its signals in regular electric waves. These are the P Waves that are  seen on the electro-cardiogram.

Lower down near the main valves of the heart, there is another small structure, the Atrioventricular  or the AV node– that picks up the signals transmitted by the SA node and transmit them through the special conduction fibers or filaments to all the muscle cells of the pumping chambers of the heart. These electrical impulses are the QRS complex.  Almost instantly after the QRS complex, heart muscle contracts, emptying the pumping left lower chamber (left ventricle) into the aorta and out to  all the arteries. This is the Pulse you can feel at your wrist.

The Oxygen Suppliers

For maintaining all these activities, the heart must feed itself. It must also be constantly supplied with oxygen.  The hearts own blood supply is maintained by two very important vessels known as coronary arteries.  They are among the most important vessels in the body. If one of these should fail, the story of life ends.