Left alone, your heart would beat about 100 times a minute
The heart's built-in pacemaker, the sinoatrial node, would fire at roughly 100 beats per minute with no input from the brain. Resting rates are lower because the vagus nerve rides the brake constantly. To speed up, the body mostly just lifts its foot off; only beyond about 100 does it need to press the accelerator.
Heart rate is simply how many times the heart contracts each minute. The American Heart Association puts a normal resting adult between 60 and 100 beats per minute. Above 100 at rest counts as tachycardia and below 60 as bradycardia, though 40 to 50 during sleep is common and normal, and a highly trained athlete may rest at 37 or 38.
Control is shared between the heart and the brainstem. The sinoatrial node sets the rhythm, while paired centres in the medulla oblongata adjust the pace. One sends sympathetic signals through the accelerans nerve, releasing norepinephrine, which opens channels for sodium and calcium and hurries each beat along. The other works through the vagus nerve, releasing acetylcholine, which opens potassium channels and stretches the pause before the next beat. At rest both are gently active, and the vagal brake dominates.
A web of sensors feeds these centres. Stretch receptors in the aorta, carotid arteries and elsewhere report blood pressure: when pressure rises, the brain eases the accelerator and presses the brake. Chemical sensors track carbon dioxide, acidity and oxygen, while receptors in muscles and joints flag exercise. Emotions reach the heart through the limbic system, which is why stress lifts the rate. In one small study of eight student actors, those offstage reacted to an unexpected event immediately, while those onstage showed a rising heart rate over the following five minutes.
Chemistry nudges it too. Adrenal hormones mimic sympathetic signals, and thyroid hormones raise the rate over longer periods. Caffeine speeds the pacemaker directly, whereas nicotine works by stimulating sympathetic nerves. Medicines exploit the same machinery: some blood pressure drugs block the receptor norepinephrine binds to, and calcium channel blockers slow the heart by restricting calcium's entry.
Source: Heart rate
