BiologyICSE Class 8 · Circulatory System

Structure of the Human Heart, Valves, Pacemaker, Blood Vessels and Double Circulation (ICSE Class 8)

Updated 17 October 2026 · Fact-checked

The human heart has four chambers: two thin-walled atria that receive blood and two thick-walled ventricles that pump it out, separated into right and left halves by the septum. Valves keep blood moving one way and the pacemaker (SA node) in the right atrium starts each beat. Blood passes through the heart twice in each round: pulmonary circulation to the lungs and systemic circulation to the body.

Understand Structure of the Heart, Valves, Pacemaker and Double Circulation

Heart. A hollow muscular organ about the size of a fist, in the chest between the lungs, tilted slightly to the left, wrapped in a double membrane, the pericardium.

Chambers.

Two atria (upper, thin-walled) receive blood from veins.
Two ventricles (lower, thick-walled) pump blood into arteries. The left ventricle has the thickest wall because it pumps blood to the whole body.
The septum is a muscular wall dividing right from left, so oxygenated and deoxygenated blood do not mix. Right side: deoxygenated. Left side: oxygenated.

Valves (one-way flaps).

Valve	Position	Flaps
Tricuspid	right atrium and right ventricle	3
Bicuspid (mitral)	left atrium and left ventricle	2
Semilunar	exit of pulmonary trunk and aorta	3 pockets each

Thin cords hold the flaps so they do not turn inside out. The 'lubb' sound is closing of tricuspid and bicuspid valves; 'dupp' is closing of semilunar valves.

Pacemaker. The sinoatrial (SA) node, a patch of special muscle in the right atrium, produces the electrical signal that starts each beat and sets the rhythm. An artificial pacemaker can be fitted if it fails.

Main vessels.

Vessel	Path	Blood
Aorta	left ventricle → body	oxygenated
Pulmonary trunk	right ventricle → lungs	deoxygenated
Pulmonary veins	lungs → left atrium	oxygenated
Superior vena cava	head, neck, arms → right atrium	deoxygenated
Inferior vena cava	lower body → right atrium	deoxygenated
Coronary artery	base of aorta → heart muscle	oxygenated
Coronary vein	heart muscle → right atrium	deoxygenated

The heart muscle is fed by the coronary arteries; blocking one starves part of the muscle (heart attack).

Double circulation. Blood passes through the heart twice in one round.
1. Pulmonary: right atrium → right ventricle → pulmonary trunk → lungs → pulmonary veins → left atrium.
2. Systemic: left atrium → left ventricle → aorta → body → venae cavae → right atrium.
Advantage: blood is oxygenated before being sent at high pressure to the body, and the two kinds of blood never mix.

Flow chart. Body → vena cava → RA → tricuspid → RV → pulmonary trunk → lungs → pulmonary veins → LA → bicuspid → LV → aorta → body.

Draw-and-label checklist: schematic heart. Four chambers, septum, tricuspid and bicuspid valves, aorta, pulmonary trunk, venae cavae, pulmonary veins, SA node; the right side of the heart (deoxygenated) appears on the left of the page and the left side (oxygenated) on the right.

Structure of the Heart, Valves, Pacemaker and Double Circulation: key facts

Heart chambers: Atria receive, thin walls
Ventricles pump, thick walls
Septum separates right and left
Left ventricle thickest; Right deoxygenated, left oxygenated.
Valves and pacemaker: Tricuspid right side, 3 flaps
Bicuspid left side, 2 flaps
Semilunar aorta and pulmonary trunk
SA node right atrium; Lubb: AV valves close; dupp: semilunar close.
Double circulation: Pulmonary RV to lungs to LA
Systemic LV to body to RA
Coronary heart muscle supply; Pulmonary trunk is the artery with deoxygenated blood.

Atria	receive, thin walls
Ventricles	pump, thick walls
Septum	separates right and left
Left ventricle	thickest

Tricuspid	right side, 3 flaps
Bicuspid	left side, 2 flaps
Semilunar	aorta and pulmonary trunk
SA node	right atrium

Pulmonary	RV to lungs to LA
Systemic	LV to body to RA
Coronary	heart muscle supply

How to answer Structure of the Heart, Valves, Pacemaker and Double Circulation questions

Tasks include labelling a heart, explaining wall thickness, tracing the path of a drop of blood, naming vessels and the blood they carry, or giving the role of the septum, valves and pacemaker.

1Start a path at the right atrium (from the body) or the left atrium (from the lungs).
2Name every chamber, valve and vessel in order and the oxygen state of the blood.
3Remember arteries are defined by direction (away from heart), not by oxygen content.
4Explain wall thickness by the distance the blood must be pumped.
5In diagrams, the right side of the heart lies on the left of the page.

Structure of the Heart, Valves, Pacemaker and Double 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.

Heartbeats in a day

Suppose a sleeping child's heart beats 60 times a minute, and in a short run it rises to 120 times a minute.

How many beats occur in a night of 8 hours of sleep at the resting rate, and by what factor does the run change the rate?

Try it first, then show the solutionSolution

Beats in 8 hours = 60 × 60 × 8 = 28,800.
Factor of change = 120 ÷ 60 = 2.

Answer: 28,800 beats in the night; the run doubles the rate.

Memory hook: Small fist, huge workload.

A leaking valve

Suppose a doctor listening to a child's chest hears a hissing sound between the usual lubb and dupp, and says that a valve is not closing fully.

Why is a valve that fails to close a problem for the circulation?

Try it first, then show the solutionSolution

Valves make blood flow one way only.
If a valve leaks, some blood flows back when the ventricle contracts.
The heart then pumps less blood forward for the same effort.

Answer: Blood leaks backwards, so less is pumped on to the lungs or body.

Memory hook: Valves are one-way gates.

Tricks to remember Structure of the Heart, Valves, Pacemaker and Double Circulation

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

Tri on the right
Tricuspid is on the right; bicuspid (two) is on the left.
Use it when: Placing valves.
Atria Arrive, Ventricles Vanish
Atria receive blood; ventricles send it out.
Use it when: Functions of chambers.
Artery = Away
Arteries go away from the heart; the pulmonary artery carries deoxygenated blood.
Use it when: Vessel questions.

Structure of the Heart, Valves, Pacemaker and Double Circulation: solved examples

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

Example 1: Pacemaker site · 1 mark · MCQBoard level

The pacemaker (SA node) of the heart lies in the wall of the:
(a) left ventricle (b) right atrium (c) septum (d) aorta

Try it first, then show the solutionSolution

The SA node is in the right atrium.

Answer: (b).

Example 2: Role of septum · 2 marks · Short answerBoard level

State the function of the septum and name the blood on each side.

Try it first, then show the solutionSolution

The septum divides the heart into right and left halves so that oxygenated and deoxygenated blood do not mix (1 mark).
Right side: deoxygenated; left side: oxygenated (1 mark).

Answer: Prevents mixing; right deoxygenated, left oxygenated.

Example 3: Blood from the foot · 4 marks · Long answerHarder board level

Blood returns from the foot of a person. Trace it until it leaves the heart for the lungs, naming the vessels, chambers and valve, and state the oxygen state of the blood.

Try it first, then show the solutionSolution

Foot → inferior vena cava; the blood is deoxygenated (1 mark).
Inferior vena cava → right atrium (1 mark).
Right atrium → tricuspid valve → right ventricle (1 mark).
Right ventricle → semilunar valve → pulmonary trunk → lungs (1 mark).

Answer: Inferior vena cava, right atrium, tricuspid valve, right ventricle, pulmonary trunk to lungs; deoxygenated.

Example 4: A student's error · 3 marks · Short answerHarder board level

A student writes: the atria have the thickest walls because they hold blood from the whole body. Correct the statement and give the reason for the actual arrangement.

Try it first, then show the solutionSolution

Wrong: atria are thin-walled; the ventricles are thick-walled (1 mark).
Atria only receive blood and pass it a short distance to the ventricles (1 mark).
The left ventricle has the thickest wall because it must send blood to the whole body (1 mark).

Answer: Atria thin (receive); ventricles thick (pump); left thickest.

Example 5: Blocked supply · 4 marks · Case-basedHOTS / competency

A man has a blocked coronary artery. (a) Which vessel is blocked and what does it supply? (b) What is the likely result? (c) Why does the heart need its own blood supply when its chambers are always full of blood?

Try it first, then show the solutionSolution

(a) A coronary artery supplies oxygenated blood to the heart muscle (1 mark).
(b) Part of the muscle is starved of oxygen: a heart attack (1 mark).
(c) The wall is thick and blood in the chambers does not pass through it to the muscle cells (1 mark).
The muscle needs a constant supply of oxygen and food for its continuous work (1 mark).

Answer: Coronary artery feeds the heart muscle; blockage causes a heart attack; the muscle needs its own supply.

Where students lose marks in Structure of the Heart, Valves, Pacemaker and Double Circulation

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

Mistake: Swapping tricuspid and bicuspid
The left and right sides of a heart diagram are reversed.
Do this: Tri on the right; bi on the left.
Usually costs: 1 mark.
Mistake: All arteries carry oxygenated blood
Students define arteries by oxygen.
Do this: Arteries carry blood away from the heart; the pulmonary trunk is deoxygenated.
Usually costs: 1 mark.
Mistake: Path of blood skipping valves
Chambers and vessels are named, but valves are left out.
Do this: Write the valve between each atrium and ventricle.
Usually costs: 1 mark.
Mistake: Calling the atrium the pumping chamber
Atria and ventricles are mixed up.
Do this: Atria receive; ventricles pump.
Usually costs: 1 mark.

Exam tips

Include valves in the path of blood.
Left ventricle thickest: pumps to the whole body.
Pacemaker: SA node in the right atrium.
Label right side of the heart on the left of the page.

Practice questions on Structure of the Heart, Valves, Pacemaker and Double 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 · Very short answerBoard level
Name the artery that carries deoxygenated blood.
Hint
To the lungs.
Answer
The pulmonary trunk (pulmonary artery).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The pulmonary veins carry oxygenated blood.
Reason (R): They bring blood from the lungs to the left atrium.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Hint
Source of blood.
Answer
(c).
Q3 · 3 marks · Short answerHarder board level
What is meant by double circulation? State one advantage.
Hint
Twice through the heart.
Answer
Blood passes through the heart twice in one complete round: once in the pulmonary circulation (heart to lungs and back) and once in the systemic circulation (heart to body and back). The advantage is that oxygenated and deoxygenated blood never mix, and oxygenated blood is pumped to the body at high pressure.
Q4 · 4 marks · Long answerHarder board level
Describe the role of the pacemaker, the valves and the cords attached to them in a heartbeat.
Hint
Start, direction, support.
Answer
The pacemaker (SA node in the right atrium) produces the signal that starts each beat. The valves allow blood to flow one way only. Thin cords hold the flaps of the tricuspid and bicuspid valves so that they do not turn inside out when the ventricles contract.
Q5 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a heart is drawn with a hole in the septum between the two ventricles.
(a) What would happen to the two kinds of blood? (1)
(b) Why would the body get less oxygen? (1)
(c) Which chamber must work harder, and why? (1)
Hint
Mixing.
Answer
(a) Oxygenated and deoxygenated blood would mix. (b) The blood sent out by the aorta would carry less oxygen than normal. (c) The left ventricle, because it must push extra blood through the hole and through the body.

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.How many chambers does a human heart have?Show answer
Four.
2.Which valve has three flaps and lies on the right?Show answer
Tricuspid.
3.Which vessel carries blood from the left ventricle?Show answer
Aorta.
4.Which vessels feed the heart muscle?Show answer
Coronary arteries.
5.What causes the lubb-dupp sound?Show answer
Closing of the valves.

Structure of the Heart, Valves, Pacemaker and Double Circulation: frequently asked questions

Is the pulmonary vein an artery?

No; it is a vein because it brings blood to the heart, though it carries oxygenated blood.

Do I need the SA node?

Name it with the pacemaker: the SA node in the right atrium.

How should I draw the schematic heart?

Use simple boxes for chambers with arrows for the vessels, labelled clearly.

Where are blood groups covered?

In the next topic.