Heart-rate

The rate at which the heart contracts to pump blood around the body. Most people have a heart-rate of between 60 and 100 beats per minute at rest. This rate tends to be faster in childhood and to slow slightly with age. Very fit people may have a resting rate below 60 beats per minute. The heart muscle responds automatically to any increase in the amount of blood returned to it from active muscles by increasing its output. During extreme exercise, heart-rate may increase to 200 contractions per minute and the output to almost 250 ml per beat. The heart-rate is also regulated by the autonomic nervous system. The parts of this system concerned with heart action are a nucleus of nerve cells, called the cardiac centre, in the brainstem, and 2 sets of nerves (the parasympathetic and sympathetic). At rest, the parasympathetic nerves – particularly the vagus nerve – act on the sinoatrial node to maintain a slow heart-rate. During or in anticipation of muscular activity, this inhibition lessens and the heart-rate speeds up. Sympathetic nerves release noradrenaline, which further increases the heart-rate and force of contraction. Sympathetic activity can be triggered by fear or anger, low blood pressure, or a reduction of oxygen in the blood. Release of adrenaline and noradrenaline by the adrenal glands also acts to increase heart-rate. The rate and rhythm of the heart can be measured by feeling the pulse or by listening with a stethoscope; a more accurate record is provided by an ECG. A resting heart-rate above 100 beats per minute is termed a tachycardia, and a rate below 60 beats per minute a bradycardia. (See also arrhythmia, cardiac.)