BiologyISC Class 11 · Body Fluids and Circulation

Human Heart: Structure, Cardiac Cycle, ECG, Cardiac Output and Double Circulation

Updated 17 October 2026 · Fact-checked

The human heart has four chambers with valves, is myogenic (the SA node is the pacemaker) and completes a cardiac cycle in about 0.8 seconds. Cardiac output = stroke volume × heart rate (about 70 mL × 72 per minute = 5 L). The ECG shows P, QRS and T waves. Circulation is double and closed.

Blue blood enters the right side and goes to the lungs; red blood returns to the left side and goes to the body. The left ventricle wall is thickest.

Illustration drawn for QuestionBuddy.

Understand Heart, Cardiac Cycle, ECG and Circulation

Structure. The heart is protected by the pericardium and has four chambers: two atria (upper, receiving blood) and two ventricles (lower, pumping blood). The right atrium receives deoxygenated blood through the superior and inferior vena cava and passes it through the tricuspid valve to the right ventricle, which sends it to the lungs by the pulmonary artery (semilunar valve). The left atrium receives oxygenated blood through the pulmonary veins and passes it through the bicuspid (mitral) valve to the left ventricle, which pumps it to the body through the aorta. The ventricular wall is thicker on the left.

Conducting system. The heart is myogenic: the sino-atrial node (SAN) in the right atrium is the pacemaker; the impulse spreads over the atria, reaches the atrio-ventricular node (AVN), then the bundle of His and the Purkinje fibres in the ventricles.

Cardiac cycle. One cycle of atrial and ventricular systole and diastole lasts about 0.8 seconds at 72 beats per minute: atrial systole about 0.1 s, ventricular systole about 0.3 s and joint diastole about 0.4 s. The first heart sound (lub) is the closure of the tricuspid and bicuspid valves; the second (dub) is the closure of the semilunar valves. *Stroke volume* is the blood pumped by each ventricle per beat (about 70 mL); *cardiac output* = stroke volume × heart rate (about 5000 mL per minute).

ECG. P wave: depolarisation of atria (atrial systole follows); QRS complex: depolarisation of ventricles (ventricular systole begins); T wave: repolarisation of ventricles (return to normal). The number of QRS complexes per minute gives the heart rate.

Circulation. Closed, double circulation: pulmonary (right ventricle → lungs → left atrium) and systemic (left ventricle → body → right atrium). The hepatic portal system carries blood from the intestine to the liver. Blood pressure is about 120/80 mm Hg (systolic/diastolic); pulse pressure = systolic − diastolic. Hypertension is a pressure persistently at or above 140/90 mm Hg. Coronary artery disease results from atherosclerosis (plaque of cholesterol); angina is chest pain from insufficient oxygen to the heart muscle; heart failure means the heart cannot pump enough blood.

Heart, Cardiac Cycle, ECG and Circulation: key facts

Valves: Tricuspid right atrium to ventricle
Bicuspid left atrium to ventricle
Semilunar at the pulmonary artery and aorta; One-way flow.
Cardiac output: $C O = s t r o k e v o l u m e \times h e a r t r a t e$; About 70 mL × 72 = 5040 mL min⁻¹.
ECG: P: atrial depolarisation
QRS: ventricular depolarisation
T: ventricular repolarisation; Heart rate = number of QRS per minute.
Blood pressure: $120/80 mm H g ∣ P u l se p r ess u r e = sy s t o l i c - d ia s t o l i c$; Hypertension: 140/90 or higher.

Tricuspid	right atrium to ventricle
Bicuspid	left atrium to ventricle
Semilunar	at the pulmonary artery and aorta

Heart, Cardiac Cycle, ECG and Circulation 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.

Output of a resting heart

Suppose a resting athlete has a heart rate of 60 beats per minute and a stroke volume of 90 mL.

Find his cardiac output and the duration of one cardiac cycle.

Try it first, then show the solutionSolution

Cardiac output = SV × HR = 90 × 60 = 5400 mL per minute.
Duration of one cycle = 60 s/60 = 1.0 s.
A normal person (72 beats, 70 mL) has 5040 mL per minute, so the athlete's heart pumps a similar (slightly greater) volume with fewer, stronger beats.

Answer: 5400 mL per minute; 1.0 s per cycle.

Memory hook: A bigger stroke volume lets the heart beat slower.

Tricks to remember Heart, Cardiac Cycle, ECG and Circulation

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

Lub is the AV valves, dub is the semilunar valves
The first sound is the closure of the tricuspid and bicuspid valves.
Use it when: Heart sounds.
P, QRS, T
Atria depolarise (P), ventricles depolarise (QRS), ventricles repolarise (T).
Use it when: ECG interpretation.

Heart, Cardiac Cycle, ECG and Circulation: solved examples

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

Example 1: Pacemaker · 1 mark · MCQBoard level

The pacemaker of the human heart is the:
(a) atrio-ventricular node
(b) sino-atrial node
(c) bundle of His
(d) Purkinje fibres

Try it first, then show the solutionSolution

The SA node in the right atrium initiates the heartbeat.

Answer: (b) sino-atrial node

Example 2: Heart valves · 3 marks · Short answerBoard level

Name the valves of the heart with their positions and the direction of blood flow they allow.

Try it first, then show the solutionSolution

Tricuspid valve between the right atrium and right ventricle. (1 mark)
Bicuspid (mitral) valve between the left atrium and left ventricle. (1 mark)
Semilunar valves at the openings of the pulmonary artery and the aorta; all valves permit one-way flow from atria to ventricles and from ventricles to arteries. (1 mark)

Answer: Tricuspid, bicuspid and semilunar valves ensure one-way flow.

Example 3: Cardiac output · 4 marks · Short answerHarder board level

The stroke volume is 70 mL and the heart rate 72 beats per minute. Find the cardiac output per minute and per day. If the heart rate falls to 60 and the output stays constant, find the stroke volume required.

Try it first, then show the solutionSolution

CO = 70 × 72 = 5040 mL per minute. (1 mark)
Per day: 5040 × 60 × 24 = 7 257 600 mL ≈ 7260 L. (1.5 marks)
New SV = 5040/60 = 84 mL. (1.5 marks)

Answer: 5040 mL min⁻¹; about 7260 L per day; 84 mL.

Example 4: Cardiac cycle and ECG · 4 marks · Short answerHarder board level

Describe the three events of the cardiac cycle with their durations, and explain what P, QRS and T waves represent on an ECG.

Try it first, then show the solutionSolution

Atrial systole (about 0.1 s), ventricular systole (about 0.3 s) and joint diastole (about 0.4 s). (1.5 marks)
P wave: depolarisation of the atria, leading to atrial systole. (0.75 mark)
QRS complex: depolarisation of the ventricles, the start of ventricular systole. (0.75 mark)
T wave: repolarisation of the ventricles, ending ventricular systole. (1 mark)

Answer: Atrial systole 0.1 s, ventricular systole 0.3 s, diastole 0.4 s; P, QRS, T.

Example 5: Double circulation · 5 marks · Long answerHOTS / competency

Trace the path of blood in the pulmonary and systemic circulations and explain why double circulation is advantageous. A blood pressure of 140/90 mm Hg is recorded: find the pulse pressure and comment.

Try it first, then show the solutionSolution

Pulmonary: right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium. (1.25 marks)
Systemic: left ventricle → aorta → body → vena cavae → right atrium. (1.25 marks)
Advantage: oxygenated and deoxygenated blood do not mix, and the blood regains pressure after the lungs before being sent to the body. (1 mark)
Pulse pressure = 140 − 90 = 50 mm Hg. (1 mark)
The pressure is at the hypertension limit (≥ 140/90 mm Hg) and needs medical attention. (0.5 mark)

Answer: Pulmonary and systemic loops; pulse pressure 50 mm Hg; hypertension.

Where students lose marks in Heart, Cardiac Cycle, ECG and Circulation

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

Mistake: Writing three chambers
Describing the human heart as three-chambered.
Do this: The human heart has four chambers.
Usually costs: 1 mark.
Mistake: Wrong valve names
Calling the left valve tricuspid.
Do this: Left: bicuspid (mitral); right: tricuspid.
Usually costs: 1 mark.
Mistake: Confusing P wave with T wave
Reversing atrial and ventricular events.
Do this: P: atrial depolarisation; T: ventricular repolarisation.
Usually costs: 1 to 2 marks.

Exam tips

Draw and label the heart in sectional view.
Tabulate the three parts of the cardiac cycle.
Show cardiac output calculations with units.
Link ECG waves to events.

Practice questions on Heart, Cardiac Cycle, ECG and Circulation

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 first heart sound "lub" is produced by:
(a) opening of AV valves
(b) closure of AV valves
(c) closure of semilunar valves
(d) contraction of atria
Hint
The ventricles begin to contract.
Answer
(b) closure of AV valves
Q2 · 2 marks · Short answerBoard level
Why is the human heart called myogenic?
Hint
Think about the origin of the beat.
Answer
Because the heartbeat originates in the cardiac muscle itself (the SA node), not from nerve impulses from outside.
Q3 · 3 marks · Short answerHarder board level
The ECG of a person shows 80 QRS complexes in one minute. Find the heart rate and the duration of one cardiac cycle.
Hint
One QRS per beat.
Answer
Heart rate = 80 beats per minute; one cycle = 60/80 = 0.75 s.
Q4 · 3 marks · Short answerHarder board level
Differentiate between arteries and veins on structure and function.
Hint
Wall thickness and valves.
Answer
Arteries have thick, elastic walls and carry blood away from the heart under high pressure (no valves). Veins have thinner walls, a wider lumen and valves, and carry blood to the heart under low pressure.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): The wall of the left ventricle is thicker than that of the right ventricle. Reason (R): The left ventricle has to pump blood to the whole body, a higher-pressure circuit. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Compare pulmonary and systemic pressure.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a doctor examines a patient with chest pain and an ECG.
(i) Name the disease from the narrowing of coronary arteries by plaque. (1 mark)
(ii) Name the pain caused by insufficient oxygen to the heart muscle. (1 mark)
(iii) Name the wave of ventricular repolarisation. (1 mark)
(iv) State one preventive measure. (1 mark)
Hint
Recall the disorders.
Answer
(i) Atherosclerosis (coronary artery disease). (ii) Angina pectoris. (iii) T wave. (iv) A diet low in saturated fat, regular exercise and no smoking.

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.Cardiac cycle duration?Show answer
About 0.8 s.
2.Normal blood pressure?Show answer
120/80 mm Hg.
3.QRS complex?Show answer
Ventricular depolarisation.
4.Valve at the aorta?Show answer
Semilunar.

Heart, Cardiac Cycle, ECG and Circulation: frequently asked questions

What is systole and diastole?

Systole is contraction of a chamber; diastole is its relaxation.

Why are veins fitted with valves?

To prevent backflow of blood as venous pressure is low.

What is the role of the Purkinje fibres?

They carry the impulse rapidly through the ventricular muscle, causing the ventricles to contract together.