A pump that sets its own rhythm
Skeletal muscle waits for a nerve signal before it contracts. Cardiac muscle does not. The heart contains specialized cells that generate their own electrical impulses at regular intervals, so a heart removed from the body can keep beating for a short time if supplied with oxygen. The nervous system and hormones modify the rhythm, but they do not create it.
Understanding the heartbeat means following two linked events: an electrical signal that spreads through the heart, and the mechanical contraction it triggers.
The electrical pathway
The impulse follows a defined route.
- Sinoatrial (SA) node: A small cluster of cells in the wall of the right atrium acts as the natural pacemaker. Its cells slowly drift toward a threshold and then fire, setting the basic rate.
- Atria: The impulse spreads across both upper chambers, which contract and push blood down into the ventricles.
- Atrioventricular (AV) node: Located between the atria and ventricles, this junction holds the signal back briefly. The delay gives the ventricles time to fill before they contract.
- Bundle branches and Purkinje fibers: Fast-conducting fibers carry the impulse down the septum and up around the ventricles, so that the ventricles contract in a coordinated, wringing way from the bottom up.
Individual heart muscle cells are linked by tiny channels that let electrical current pass directly from cell to cell. That is what allows a wave of activation to sweep across the heart as a unit.
At the cell level, contraction is set off by a brief electrical event, an action potential, that opens channels allowing calcium to enter. The calcium triggers the muscle’s contractile machinery. A long plateau phase in the heart cell’s electrical signal keeps the muscle contracting long enough to expel blood and prevents the rapid repeated firing that could cause the heart to lock up.
What an ECG shows
An electrocardiogram (ECG or EKG) records the electrical activity from sensors on the skin. A normal tracing has a recognizable pattern of waves:
- The P wave reflects activation of the atria.
- The QRS complex, the sharpest spike, reflects activation of the ventricles.
- The T wave reflects the ventricles resetting for the next beat.
Doctors read the timing and shape of these waves for clues about rhythm and conduction. This article only describes the concepts. Interpreting an ECG takes training and is not something to attempt from a general description.
The mechanical cycle
Each beat has two main phases.
- Diastole is relaxation, when the chambers fill with blood. Most ventricular filling occurs passively, with a boost from atrial contraction at the end.
- Systole is contraction, when the ventricles squeeze. As pressure rises above the pressure in the atria, the valves between the atria and ventricles snap shut. When ventricular pressure exceeds that in the aorta and pulmonary artery, the outlet valves open and blood is ejected.
The typical “lub-dub” heart sounds come from the valves closing: the first sound with the closure of the valves between atria and ventricles, and the second with the closure of the outlet valves. Extra or unusual sounds, called murmurs, can be harmless or can point to a valve or flow problem, which is why clinicians listen.
The volume pumped per beat is the stroke volume. Multiplying it by the heart rate gives cardiac output, the amount of blood pumped per minute. Both can change, so the body can increase output either by beating faster or by pumping more per beat. Regular exercise training is associated with a larger stroke volume and, as a result, a lower resting rate.
Control by nerves and hormones
The autonomic nervous system adjusts the pacemaker. The sympathetic branch and the hormone adrenaline speed it up and strengthen contractions, as in the stress response. The parasympathetic branch, acting mainly through the vagus nerve, slows the rate. At rest, the slowing influence generally dominates, which is why the resting rate is lower than the SA node would fire if left alone. Breathing, posture, temperature, fitness and mood also influence the rate.
What your pulse reflects
Each time the left ventricle ejects blood, it sends a pressure wave along the arteries that can be felt at spots such as the wrist or neck. That is the pulse. Counting it gives the heart rate, because each pulse generally corresponds to a beat. Besides the rate, the pulse can reflect rhythm regularity and, roughly, the strength of the beat.
Resting heart rate in adults is commonly said to fall in the range of about 60 to 100 beats per minute, though healthy people, particularly very fit people, can fall outside it, and the rate changes through the day. Rate and blood pressure are separate measures, although they interact.
Wearable heart-rate sensors estimate pulse using light shone into the skin, a technology that belongs to the wider field of wearable health monitoring. Their accuracy varies with the device and the situation, such as vigorous movement. A natural variability from beat to beat also exists and is studied as a marker of how the nervous system is influencing the heart. Slow breathing practices such as deep breathing can influence this variability.
Rhythm problems, in general
Disturbances of the electrical system are called arrhythmias. Some are harmless and some are serious. Causes and implications are varied, and this article does not attempt diagnosis. Anyone with concerns about palpitations, fainting or chest discomfort should seek professional medical advice, and urgent symptoms need urgent care.