BiologyCBSE Class 11 · Body Fluids and Circulation

Cardiac Cycle, Cardiac Output and the ECG

Updated 17 October 2026 · Fact-checked

The cardiac cycle is the sequence of events in one heartbeat; it lasts about 0.8 seconds at 72 beats per minute. Atrial systole 0.1 s, ventricular systole 0.3 s and joint diastole 0.4 s. Cardiac output = stroke volume x heart rate, about 5 litres per minute at rest. The ECG shows P (atrial depolarisation), QRS (ventricular depolarisation) and T (repolarisation) waves.

Match the waves to the phases: P comes before atrial systole, QRS before ventricular systole (lub), T near its end (dub), and 0.1 + 0.3 + 0.4 = 0.8 s.

Illustration drawn for QuestionBuddy.

Understand Cardiac Cycle, Cardiac Output and ECG

The heart. Four chambers (two atria, two ventricles) with tricuspid valve (right) and bicuspid (mitral) valve (left) between atria and ventricles, and semilunar valves at the exits of the aorta and pulmonary artery. Double circulation: pulmonary and systemic. The heart is myogenic: the sino-atrial node (SAN), the pacemaker in the right atrium, initiates the impulse (70 to 75 per minute); the impulse spreads to the atrio-ventricular node (AVN), then the AV bundle (bundle of His) and Purkinje fibres over the ventricles.

Cardiac cycle (heart rate ~72 beats per minute, so each cycle ≈ 0.8 s):
1. Joint diastole (about 0.4 s): all chambers relaxed; blood flows from veins into atria and through open AV valves into ventricles; semilunar valves closed.
2. Atrial systole (about 0.1 s): atria contract and push the remaining blood into the ventricles.
3. Ventricular systole (about 0.3 s): ventricles contract, AV valves close, pressure rises, semilunar valves open and blood is pumped out.
4. Ventricular diastole: ventricles relax; semilunar valves close; the cycle repeats (ventricular diastole ≈ 0.5 s, including the joint diastole).
Check: 0.1 + 0.3 + 0.4 = 0.8 s.

Heart sounds. First sound, 'lub': closure of the tricuspid and bicuspid valves at the start of ventricular systole. Second sound, 'dub': closure of the semilunar valves at the end of ventricular systole. Detected with a stethoscope.

Stroke volume and cardiac output. Stroke volume (SV): blood pumped by each ventricle per beat, about 70 mL. Cardiac output (CO) = stroke volume x heart rate = 70 mL x 72 = 5040 mL ≈ 5 L per minute in a healthy person. Athletes have a higher stroke volume and lower resting heart rate. Heart rate can be changed by neural signals (sympathetic increases, parasympathetic decreases) and adrenal medullary hormones (adrenaline, noradrenaline).

Electrocardiograph (ECG). A graphic record of the electrical activity of the heart. Three electrodes (two on wrists, one on the left ankle) in the standard leads. Waves:

P wave: depolarisation of atria (leading to atrial contraction).
QRS complex: depolarisation of ventricles (initiating ventricular contraction); the start of the Q wave marks ventricular systole.
T wave: repolarisation of ventricles (return to normal state); atrial repolarisation is hidden in the QRS.
The end of the T wave marks the end of systole.
Heart rate = number of QRS complexes per minute; a deviation indicates abnormality.

Disorders. High blood pressure (hypertension; 120/80 normal; 140/90 or above is high), coronary artery disease (atherosclerosis), angina, heart failure (heart not pumping effectively; congestive heart failure), cardiac arrest.

Draw and label: a normal ECG with P, Q, R, S, T marked.

Cardiac Cycle, Cardiac Output and ECG: key facts

Cycle times: Atrial systole 0.1 s
ventricular systole 0.3 s
joint diastole 0.4 s
total 0.8 s; At 72 per minute
Cardiac output: $C O = S V x H R$; About 5040 mL per min
Heart rate: $H R = 60/ cy c l e t im e (s)$; Beats per minute
ECG: P: atrial depolarisation
QRS: ventricular depolarisation
T: ventricular repolarisation; Waves

How to answer Cardiac Cycle, Cardiac Output and ECG questions

Questions give stroke volume, heart rate or ECG details.

1Convert time to rate with 60 divided by cycle duration.
2Multiply stroke volume by heart rate for cardiac output, then convert to litres.
3For ECG, count QRS complexes in a known time and scale to a minute.
4Identify the wave from the event: P atria, QRS ventricles, T repolarisation.
5Link heart sounds to valve closures.

Cardiac Cycle, Cardiac Output and ECG in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Pulse at rest

Suppose a student counts 18 pulse beats in 15 seconds at rest and assumes a stroke volume of 70 mL.

Find the heart rate and the cardiac output.

Try it first, then show the solutionSolution

Scale 15 s to 60 s: factor 4, so rate = 18 x 4 = 72 beats per minute.
CO = 70 x 72 = 5040 mL per minute.
That is about 5 litres per minute.

Answer: Heart rate 72 per minute; CO 5040 mL per minute.

Memory hook: Scale the count to a minute.

Tricks to remember Cardiac Cycle, Cardiac Output and ECG

Short memory aids. Say each one aloud once, then use it on a question the same day.

Lub = valves, Dub = semilunar
Lub: AV valves close; dub: semilunar valves close.
Use it when: Heart sounds
P-Q-T
P atria, QRS ventricles, T recovery.
Use it when: ECG
CO = SV x HR
Output per minute.
Use it when: Cardiac output

Cardiac Cycle, Cardiac Output and ECG: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Heart sounds · 2 marks · Very short answerBoard level

Name the two heart sounds and the valves whose closure produces them.

Try it first, then show the solutionSolution

First sound lub: closure of tricuspid and bicuspid valves (1 mark).
Second sound dub: closure of semilunar valves (1 mark).

Answer: Lub: AV valves; dub: semilunar valves.

Example 2: Cardiac output · 2 marks · Very short answerBoard level

A resting person pumps 75 mL per beat from each ventricle at 68 beats per minute. Calculate the cardiac output in mL and litres per minute.

Try it first, then show the solutionSolution

CO = SV x HR = 75 x 68 (1 mark).
= 5100 mL per minute = 5.1 litres (1 mark).

Answer: 5100 mL per min.

Example 3: Heart rate from cycle time · 3 marks · Short answerHarder board level

During a jog each cardiac cycle shortens to 0.6 s while the stroke volume rises to 80 mL. Find the heart rate and the cardiac output, and compare with a resting output of 5040 mL per minute.

Try it first, then show the solutionSolution

HR = 60 / 0.6 = 100 per minute (1 mark).
CO = 80 x 100 = 8000 mL per min = 8 litres (1 mark).
8000 / 5040 is about 1.6 times the resting output (1 mark).

Answer: 100 per min; 8 L per min; about 1.6 times rest.

Example 4: Reading an ECG · 4 marks · Short answerHarder board level

An ECG strip of 15 seconds shows 20 QRS complexes. Find the heart rate and explain what P, QRS and T represent.

Try it first, then show the solutionSolution

Scale: 60 / 15 = 4; 20 x 4 = 80 beats per minute (1 mark).
P: atrial depolarisation (1 mark).
QRS: ventricular depolarisation (1 mark).
T: ventricular repolarisation (1 mark).

Answer: 80 per min; P atria, QRS ventricles, T recovery.

Example 5: Athlete's heart · 4 marks · Short answerHOTS / competency

Suppose an athlete has a resting heart rate of 50 per minute and a stroke volume of 100 mL, while a non-athlete has 72 and 70 mL. Compare their cardiac outputs and explain the advantage in exercise.

Try it first, then show the solutionSolution

Athlete CO = 100 x 50 = 5000 mL per min (1 mark).
Non-athlete CO = 70 x 72 = 5040 mL per min (1 mark).
Similar resting output, but the athlete's heart beats less often (1 mark).
Higher stroke volume allows a larger output during exercise without a very high rate, giving a bigger reserve (1 mark).

Answer: About equal at rest (5000 vs 5040 mL); athlete has a bigger reserve.

Where students lose marks in Cardiac Cycle, Cardiac Output and ECG

The slips examiners see most often, and what to write instead.

Mistake: Lub from semilunar
Valve closure mixed.
Wrong working
Lub is closing of semilunar valves.
Right working
Lub is closure of AV valves.
Do this: Lub = AV.
Usually costs: 1 mark.
Mistake: CO in litres without conversion
Units.
Wrong working
CO = 5040 L.
Right working
CO = 5040 mL = 5.04 L.
Do this: Convert.
Usually costs: 1 mark.
Mistake: P wave ventricles
Waves mixed.
Wrong working
P is ventricular.
Right working
P is atrial depolarisation.
Do this: P atria.
Usually costs: 1 mark.

Exam tips

Show CO as SV x HR with units.
Write the three durations and add to 0.8 s.
Define each ECG wave.
Convert ECG strip time to a minute.

Practice questions on Cardiac Cycle, Cardiac Output and ECG

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The QRS complex of an ECG represents
(a) atrial depolarisation (b) ventricular depolarisation (c) ventricular repolarisation (d) atrial repolarisation
Hint
Ventricles contract.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The heart is called myogenic.
Reason (R): The impulse for contraction arises in the heart muscle itself, at the SAN.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Pacemaker.
Answer
(a).
Q3 · 2 marks · Very short answerHarder board level
A heart beats 75 times per minute. Find the duration of one cardiac cycle and, if the joint diastole is 0.4 s, the combined atrial and ventricular systole time.
Hint
60/75.
Answer
Cycle = 60/75 = 0.8 s. Systole total = 0.8 - 0.4 = 0.4 s (atrial 0.1 s + ventricular 0.3 s).
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose the stroke volume is 55 mL and the heart rate is 84 per minute.
(a) Find cardiac output in mL per min. (1)
(b) Convert to litres. (1)
(c) How much less is it than 5040 mL per min? (2)
Hint
CO = SV x HR = 55 x 84; then divide by 1000 for litres.
Answer
(a) 55 x 84 = 4620 mL per min. (b) 4.62 L per min. (c) 5040 - 4620 = 420 mL per min less.
Q5 · 3 marks · Short answerHOTS / competency
Suppose the SAN fails and the AVN takes over as pacemaker at a lower rate. Predict the effect on heart rate and explain why the heart still beats.
Hint
Backup pacemaker.
Answer
The heart rate falls because the AVN fires more slowly than the SAN. The heart still beats because the impulse is generated in the heart muscle (myogenic) and the AVN can act as a backup pacemaker.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Cycle duration?Show answer
About 0.8 s.
2.Pacemaker?Show answer
SAN.
3.Normal BP?Show answer
120/80 mm Hg.
4.What is Q-R-S?Show answer
Ventricular depolarisation.
5.Stroke volume?Show answer
About 70 mL.
6.Which valve closes at lub?Show answer
Tricuspid and bicuspid.

Cardiac Cycle, Cardiac Output and ECG: frequently asked questions

What is the cardiac cycle?

The events of one heartbeat.

What is cardiac output?

Blood pumped by the heart per minute.

What does an ECG show?

Electrical activity of the heart.

Why is the heart myogenic?

Its impulse arises in the muscle.

What is the SAN?

The natural pacemaker.