Cardiac Cycle and ECG Explained: Heart Sounds, Pacemaker and Cardiac Output for NEET
Every heartbeat is a tiny, perfectly timed relay race: two chambers squeeze, two valves snap shut, and the sound you hear is just the doors closing. NEET loves this chapter because the whole story can be tested in a single graph or ECG strip. Learn the sequence once, and half a dozen questions become free marks.
Quick Summary
The heart beats in a fixed order: atrial systole, ventricular systole, joint diastole.
One cardiac cycle lasts about 0.8 seconds at 72 beats per minute.
Valves open and close because of pressure differences, not because the heart "decides" to.
In an ECG, P = atrial depolarisation, QRS = ventricular depolarisation, T = ventricular repolarisation.
Cardiac output = stroke volume × heart rate, roughly 5 litres per minute at rest.
What Is the Cardiac Cycle?
The cardiac cycle is the sequence of contraction and relaxation of the heart's chambers during one heartbeat. Contraction is called systole and relaxation is diastole.
At 72 beats per minute, one cycle takes about 0.8 seconds. Of this, atrial systole takes about 0.1 s, ventricular systole about 0.3 s, and the remaining 0.4 s is joint diastole when all chambers relax and the ventricles fill passively.
The Four Phases, Step by Step
Think of pressure as the driver and valves as one-way gates. Blood always moves from higher pressure to lower pressure, and the valves simply stop it from flowing backwards.
Ventricular filling: during joint diastole the AV valves (tricuspid and bicuspid) are open and blood flows from atria to ventricles. The semilunar valves are closed.
Atrial systole: the atria contract and push the last bit of blood into the ventricles.
Ventricular systole: pressure in the ventricles rises above that in the atria, so the AV valves shut (the "lub", the first heart sound). When ventricular pressure exceeds that in the aorta and pulmonary artery, the semilunar valves open and blood is ejected.
Ventricular diastole: the ventricles relax, pressure falls, and the semilunar valves close (the "dub", the second heart sound). The AV valves reopen and the cycle restarts.
Key Insight: The sounds are valves closing, not opening. "Lub" is the AV valves shutting at the start of ventricular systole; "dub" is the semilunar valves shutting at its end. Students who tie each sound to a pressure change, rather than memorising it as a fact, tend to answer valve-sequence questions in seconds, which is how problems like this are usually drilled at Sukriti NeetJee.
Why the Heart Beats on Its Own
The human heart is myogenic: the beat starts inside the heart muscle itself and does not need a nerve signal to begin. The starting point is the sinoatrial (SA) node, the pacemaker, located in the upper right atrium wall.
The signal then travels through a fixed path: SA node, then the atrioventricular (AV) node, then the AV bundle (bundle of His), its branches, and finally the Purkinje fibres in the ventricular walls. The brief delay at the AV node lets the atria finish emptying before the ventricles contract. The autonomic nervous system only speeds up or slows down the rhythm: sympathetic stimulation raises the rate, parasympathetic lowers it.
Reading the ECG Without Panic
An electrocardiogram (ECG) records the electrical activity of the heart, not its mechanical squeeze. A standard trace has three main parts, and each wave marks an electrical event, not a muscle contraction.
P wave: depolarisation of the atria, which leads to atrial contraction.
QRS complex: depolarisation of the ventricles, which leads to ventricular contraction. The atrial repolarisation is hidden inside it.
T wave: repolarisation of the ventricles, marking the end of ventricular systole.
To estimate heart rate from an ECG, count the number of QRS complexes in one minute. A very common NEET question asks you to find the rate when the gap between two successive R peaks is given: divide 60 by that gap in seconds.
Cardiac Output and Regulation
Stroke volume is the blood pumped by each ventricle per beat, about 70 mL. Cardiac output is stroke volume multiplied by heart rate, so 70 mL × 72 gives roughly 5000 mL (5 litres) per minute in a healthy adult.
Athletes often have a larger stroke volume and therefore a lower resting heart rate for the same output. Hormones such as adrenaline raise both rate and force of contraction.
Common Mistakes to Avoid
Mixing up the valves: the tricuspid sits on the right, the bicuspid (mitral) on the left.
Saying the T wave shows ventricular contraction. It shows repolarisation.
Forgetting that the right side carries deoxygenated blood to the lungs, and the left carries oxygenated blood to the body.
Treating "systole" as a property of the whole heart. Atria and ventricles each have their own timing.
Frequently Asked Questions
What is the duration of one cardiac cycle?
At a normal rate of 72 beats per minute, one cardiac cycle lasts about 0.8 seconds. The duration shortens as the heart rate rises, mainly because diastole becomes shorter.
What does the QRS complex represent in an ECG?
The QRS complex represents depolarisation of the ventricles, which triggers ventricular contraction. Atrial repolarisation also occurs at this time but is masked by the larger QRS signal.
Why is the heart called myogenic?
The heart is called myogenic because the impulse for contraction originates in the cardiac muscle itself, at the SA node. Nerves only modify the rate and strength of the beat.
Which heart sound is produced by the closing of the semilunar valves?
The second heart sound, "dub", is produced when the semilunar valves close at the end of ventricular systole. The first sound, "lub", comes from the closing of the AV valves.
How is cardiac output calculated?
Cardiac output equals stroke volume multiplied by heart rate. With a stroke volume of about 70 mL and 72 beats per minute, it comes to roughly 5 litres per minute.
Got a doubt about the cardiac cycle or any other NEET Biology chapter? Leave your prep questions in the comments, and share this post with a fellow aspirant who still mixes up the heart sounds.
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