In 1859 French scientist √Čtienne-Jules Marey noticed that when at rest a person's pulse rate is inversely proportional to blood pressure. Or in other words, the slower your heart beats, the higher your blood pressure is. This has come to be known as Marey's law.

It is important to note that the dependent variable in this equation is the blood pressure. Assuming that everything else is normal, and you are not exercising, unhealthy, or stressed, your blood pressure will determine the rate at which your heart beats.

This effect comes about because the body responds to lowered blood pressure, in the short term, by increasing the heart rate. Conversely, when blood pressure is too high, the heart will slow.

This 'law' does not apply in all conditions; for example, during exercise heart rate will increase (to supply oxygen to the working muscles), but diastolic blood pressure will remain approximately the same (because blood vessels in the working muscles dilate to accept more blood flow). A more technical counterexample is the Bainbridge reflex, when an increase in right atrial pressure results in an increased heart rate.

If you have hypertension this law will apply to you, but your 'default' blood pressure will be set at a higher level than a healthy person's. In other words, the law will hold, but your heart will not be beating at a lower rate than your healthier friends' just because your resting blood pressure is higher. The law is consistent within the individual, not between individuals.