The circulatory system is comprised of a pump (the heart) which is responsible for pumping blood through the vessels thereby bringing O2 and nutrients to the cells. The heart has 4 parts:

1. Muscle cells (myocardial cells): during a heart attack, the coronary arteries are blocked and these cells do not get the oxygen they need and may ultimately die.
2. An electrical system: in arrhythmias, this system is deregulated and the heart beats at an irregular rate.
3. Valves: diseases such as rheumatic heart disease and aortic stenosis typically alter the functioning of these valves and do not let the blood travel properly.
4. An envelope (pericardium): pericardial tamponade (accumulation of fluid) is the most common problem at this level

The bottom line is that any or all of the aforementioned parts can malfunction leading to a decreased or absent cardiac output. That is what is being checked when you check the pulse. You are looking to feel a change in pressure in the carotid artery caused by increased flow of blood caused by the heart contracting and ejecting blood during systole. So again, is there a pulse? Is the pulse sufficient to sustain life? A normal pulse is usually in the vicinity of 70 beats per minute as reflected by the following equation:

Where CO is cardiac output, SV stroke volume and HR heart rate. Normal values render the equation as follows:

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