The Human Heart: The Most Important Muscle In Our Bodies

The heart is a 4-chambered fist-sized organ located in the centre of the chest just behind the breastbone (sternum). It is responsible for pumping the blood throughout the body. At rest, the heart will beat about 60-80 times per minute. During extreme exertion the heart can beat up to 200 beats per minute. At rest, each time the heart contracts it pumps about 70 ml of blood (in an adult). See below for more info on cardiac output.

The heart is a unique muscle, called cardiac, it is capable of initiating its own electrical activity, and working totally independent of the brain, if it needs to. No other muscle in the body can do this. In fact, under resting conditions, the brain usually slows down the heart rate. On its own, the heart would beat about 100 times per minute.

The entire cardiac contraction begins at a place called the sinoatrial node (SA). Here an initial electrical shock begins and travels through the cardiac muscles of the atriums. This, naturally, causes the atriums to contract and squeeze the blood to the ventricles. The current is absorbed by another node called the atrioventricular (AV). Here, it is stored for a fraction of a second, before being released and allowed to travel to the ventricles. First it travels down the septum (centre), then through the purkinje fibers to the outside of the ventricles. This causes the ventricles to contract squeezing blood out the aorta and out the pulmonary arteries. Then the cycle begins all over again.

An electrocardiogram is the process of monitoring the heart's electrical activity. Most heart problems can be detected through an ECG. It is a simple process and is done by attaching electrodes at certain spots on the chest. If anything abnormal is detected on the ECG then further tests can be performed.

Cardiac output (CO) is the volume of blood circulated with each heart beat. CO is determined by the heart rate (HR) and the stroke volume (SV). CO=HRXSV.

Example: 70 beats per minute X 0.07 L/beat = 5.0 L/minute.
As heart rate increases so will CO until a certain point. If heart rate is so high so that there is not enough time for the ventricles to fully fill then less blood will be pumped with each heart rate. Related to this, this is why exercising beyond a certain heart rate is counter productive. Even though heart rate is high CO is decreasing.