Heart Rate

The Heart Rate

Heart rate
The heart rate is the number of contractions of the heart in one minute. It is measured in beats per minute (bpm). When resting, the adult human heart beats at about 70 bpm (males) and 75 bpm (females), but this rate varies between people.
The body can increase the heart rate in response to a wide variety of conditions in order to increase the cardiac output (the amount of blood ejected by the heart per unit time). Exercise causes a normal person's heart rate to increase above the resting heart rate. As the physical activity becomes more vigorous, the heart rate increases more. With very vigorous exercise, a maximum heart rate can be reached.
The pulse is the most straightforward way of measuring the heart rate, but it can be deceptive when some strokes do not lead to much cardiac output. In these cases (as happens in some arrhythmias), the heart rate can be (much) higher than the pulse.

Control of heart rate
The heart contains cardiac pacemakers that spontaneously cause the heart to beat. These can be controlled by the autonomic nervous system and circulating adrenaline. The heart beats more quickly than average in an obese person, and less quickly than average in athletes.
Systole of the heart is induced as the sarcolemma of the myocardial cells of the sinoatrial node slowly depolarises beyond the threshold. At this point, the voltage-gated calcium channels open and allow calcium ions to pass through, into the sarcoplasm of the muscle cell. Some calcium ions bind to the receptors on the sarcoplasmic reticulum causing their intrinsic calcium channels to open and an influx of calcium ions into the sarcoplasm results. The calcium ions bind to the troponin, causing a conformation change, breaking the bond between tropomyosin, to which the troponin is attached, and the myosin binding sites. This allows the myosin heads to bind to the myosin binding sites on the actin filament and contraction results as the myosin heads draw the...