Mechanism of the Heart Beat
The heartbeat is triggered by the sinoatrial node, a natural pacemaker of electrical impulses that makes the heart beat rhythmically on its own, in a repeating cycle of contraction (systole) and relaxation (diastole).
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The heart's natural pacemaker
Heart muscle is myogenic, meaning it can contract on its own rhythm without any signal from the nervous system. This rhythm is set by the sinoatrial node (SAN), a small patch of specialised muscle in the wall of the right atrium that acts as the heart's natural pacemaker.
The SAN generates an electrical impulse that spreads across both atria, making them contract together.
The impulse then reaches the atrioventricular node (AVN), where it is delayed slightly before travelling down the bundle of His and out along Purkinje fibres in the ventricle walls. This delay allows the atria to finish contracting and empty into the ventricles before the ventricles themselves start to contract from the bottom upward.
The cardiac cycle: systole and diastole
- Atrial systole: both atria contract together, pushing blood through the open atrioventricular valves into the ventricles below.
- Ventricular systole: the ventricles contract, the atrioventricular valves close to stop blood flowing back into the atria, and rising pressure forces open the semilunar valves, pushing blood into the pulmonary artery and aorta.
- Diastole: the ventricles and atria both relax, the semilunar valves close to stop blood flowing back into the ventricles, and all four chambers fill passively with blood ready for the next cycle.
- The closing of the two sets of valves produces the familiar 'lub-dub' sound of a heartbeat, heard through a stethoscope.
Controlling heart rate
The SAN sets a resting heart rate of roughly 60 to 100 beats per minute, but the actual rate can be adjusted to match the body's needs. Sympathetic nerves and the hormone adrenaline speed up the heart rate, for example during exercise or a stressful situation, while the parasympathetic (vagus) nerve slows it down during rest.
Each ventricular contraction sends a pressure wave along the arteries that can be felt as a pulse, so pulse rate is a simple way to measure heart rate.
How it is examined
Exam questions on this standard commonly ask you to explain why the heart is myogenic and continues beating even when removed from the body, to sequence the events of the cardiac cycle, or to interpret a graph of pressure or volume change in a heart chamber over time. You may also be asked why heart rate increases during exercise and how this is linked to a greater demand for oxygen by respiring muscles.
Worked exam-style question
Question. A graph shows the pressure inside the left ventricle over one complete cardiac cycle. The pressure rises sharply from a low value, reaches a peak, then falls sharply back to the low value before the cycle repeats.
(a) Name the phase of the cardiac cycle during which ventricular pressure is rising. (b) Explain, in terms of valves, why the ventricular pressure can rise so much higher than the pressure in the atrium above it.
(c) State what happens to the atrioventricular valve at the exact moment ventricular pressure exceeds atrial pressure. (d) Explain why an isolated heart, disconnected from all nerves, kept in a suitable nutrient solution, would continue to beat.
Model answer. (a) This is ventricular systole, the contraction phase of the ventricle. (b) Once the ventricle begins contracting, rising pressure closes the atrioventricular valve, sealing the ventricle off from the atrium, so continued contraction of the ventricle wall can raise the pressure inside the closed chamber without blood escaping back into the atrium.
(c) The atrioventricular valve closes at that moment, to prevent the backflow of blood into the atrium. (d) The heart is myogenic: its beat is initiated by the sinoatrial node, which generates its own electrical impulses without any signal from the nervous system, so as long as the heart muscle receives oxygen and nutrients from the solution, it will continue contracting and relaxing rhythmically on its own.
Practice question
Try this. During vigorous exercise, a person's heart rate rises from a resting value to a much higher value within two minutes. Explain, in terms of the sinoatrial node and the body's control systems, how this increase is brought about.
Exam tip
Key terms
Revise these linked terms to answer heartbeat questions precisely:
- Artery, the vessel in which a pressure wave from ventricular systole can be felt as a pulse.
- Double circulation, the two-loop circulatory system that the heart's two sides pump blood around.
- Hormone, the chemical messenger, adrenaline, that speeds up heart rate during stress or exercise.
- Neuron, the nerve cell type that carries sympathetic and parasympathetic signals to the heart.
Source:SRC-DSKP-EN
Frequently asked questions
Why does the heart keep beating even if all nerves connected to it are cut?
What is the difference between systole and diastole?
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