BiologyCBSE Class 11 · Breathing and Exchange of Gases

Mechanism of Breathing, Respiratory Volumes and Capacities

Updated 17 October 2026 · Fact-checked

Breathing involves inspiration, when the diaphragm and external intercostal muscles contract to enlarge the thoracic cavity and lower lung pressure, and expiration, when they relax. Tidal volume is about 500 mL, vital capacity = IRV + TV + ERV, and total lung capacity = VC + RV. Pulmonary ventilation = tidal volume x breathing rate.

Notice that VC = IRV + TV + ERV, and TLC = VC + RV; residual volume always stays in the lungs.

Illustration drawn for QuestionBuddy.

Understand Mechanism of Breathing and Lung Volumes

Mechanism. Air moves because of a pressure gradient: inspiration when intrapulmonary pressure is less than atmospheric, expiration when it is greater.

Inspiration: the diaphragm contracts and flattens (increasing the volume of the thoracic chamber in the antero-posterior axis) and the external intercostal muscles contract, lifting the ribs and sternum (increasing volume in the dorso-ventral axis). Lung volume rises, intrapulmonary pressure falls below atmospheric pressure, and air rushes in.
Expiration: the diaphragm and external intercostals relax to their normal positions, reducing thoracic and lung volume; pressure rises above atmospheric, and air moves out. Forceful expiration also uses internal intercostals and abdominal muscles. The lungs are held in the thoracic cavity by the pleural fluid and the double pleura.
The thoracic cavity is a closed chamber; the lungs have no muscle of their own. Respiratory rate in a healthy human adult about 12 to 16 times per minute.

Respiratory volumes (a spirometer is used). Typical adult values:

Tidal volume (TV): air inspired or expired in a normal breath, about 500 mL.
Inspiratory reserve volume (IRV): extra air inspired by forceful inspiration, 2500 to 3000 mL.
Expiratory reserve volume (ERV): extra air expired by forceful expiration, 1000 to 1100 mL.
Residual volume (RV): air remaining after forced expiration, 1100 to 1200 mL.

Capacities.

Inspiratory capacity (IC) = TV + IRV.
Expiratory capacity (EC) = TV + ERV.
Functional residual capacity (FRC) = ERV + RV.
Vital capacity (VC) = ERV + TV + IRV (maximum air that can be breathed out after maximum inspiration), about 3500 to 4500 mL.
Total lung capacity (TLC) = VC + RV, about 5000 to 6000 mL.

Ventilation. Pulmonary ventilation = tidal volume x breaths per minute. A healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute. Dead space (air in conducting passages that does not take part in gas exchange) is about 150 mL. Alveolar ventilation = (TV - dead space) x breathing rate.

Draw and label: a spirogram marking TV, IRV, ERV, RV, VC and TLC.

Mechanism of Breathing and Lung Volumes: key facts

Capacities: $I C = T V + I R V ∣ E C = T V + E R V ∣ F R C = E R V + R V$; Capacities
VC and TLC: $V C = E R V + T V + I R V ∣ T L C = V C + R V$; Lung capacities
Pulmonary ventilation: TV x breathing rate; mL per minute
Alveolar ventilation: (TV - dead space) x breathing rate; Dead space about 150 mL

How to answer Mechanism of Breathing and Lung Volumes questions

Questions give volumes and ask for capacities or ventilation.

1List the volumes given with units.
2Choose the correct sum for the capacity.
3Subtract dead space for alveolar ventilation.
4Convert the final answer to mL per minute or litres per minute.
5Check that TLC is greater than VC and VC greater than TV.

Mechanism of Breathing and Lung Volumes 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.

Spirometer test

Suppose a student blows into a spirometer: normal breath 500 mL, maximal further inhalation 2500 mL, maximal exhalation after normal breath 1000 mL, remaining 1200 mL.

Find VC and TLC, and the ventilation at 12 breaths per minute.

Try it first, then show the solutionSolution

VC = 1000 + 500 + 2500 = 4000 mL.
TLC = 4000 + 1200 = 5200 mL.
Ventilation = 500 x 12 = 6000 mL per minute.

Answer: VC 4000 mL; TLC 5200 mL; 6000 mL per min.

Memory hook: Add the reserves; add the residue.

Tricks to remember Mechanism of Breathing and Lung Volumes

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

Diaphragm down, air in
Contract diaphragm, volume up, pressure down.
Use it when: Inspiration
VC = no residue
Vital capacity excludes residual volume.
Use it when: VC
ERV + RV = FRC
Functional residual capacity.
Use it when: FRC

Mechanism of Breathing and Lung Volumes: solved examples

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

Example 1: Inspiration · 3 marks · Short answerBoard level

Explain how inspiration takes place, with the muscles and pressure change.

Try it first, then show the solutionSolution

Diaphragm contracts and flattens; external intercostals lift the ribs and sternum (1 mark).
Thoracic and lung volume increase (1 mark).
Intrapulmonary pressure falls below atmospheric, so air enters (1 mark).

Answer: Muscle contraction, volume up, pressure down.

Example 2: Capacities · 2 marks · Very short answerBoard level

Define vital capacity and total lung capacity in terms of volumes.

Try it first, then show the solutionSolution

VC = ERV + TV + IRV (1 mark).
TLC = VC + RV (1 mark).

Answer: VC = ERV + TV + IRV; TLC = VC + RV.

Example 3: Numerical · 5 marks · Long answerHarder board level

A person has TV = 500 mL, IRV = 2500 mL, ERV = 1000 mL, RV = 1200 mL. Find IC, EC, FRC, VC and TLC.

Try it first, then show the solutionSolution

IC = TV + IRV = 500 + 2500 = 3000 mL (1 mark).
EC = TV + ERV = 500 + 1000 = 1500 mL (1 mark).
FRC = ERV + RV = 1000 + 1200 = 2200 mL (1 mark).
VC = ERV + TV + IRV = 1000 + 500 + 2500 = 4000 mL (1 mark).
TLC = VC + RV = 4000 + 1200 = 5200 mL (1 mark).

Answer: IC 3000; EC 1500; FRC 2200; VC 4000; TLC 5200 mL.

Example 4: Ventilation · 3 marks · Short answerHarder board level

Find pulmonary and alveolar ventilation for TV = 500 mL, dead space 150 mL, and 12 breaths per minute.

Try it first, then show the solutionSolution

Pulmonary ventilation = 500 x 12 = 6000 mL per min (1 mark).
Alveolar ventilation = (500 - 150) x 12 = 350 x 12 (1 mark).
= 4200 mL per min (1 mark).

Answer: 6000 and 4200 mL per min.

Example 5: Shallow breathing · 5 marks · Long answerHOTS / competency

Two people each breathe 6000 mL per minute: A takes 12 breaths of 500 mL, B takes 30 breaths of 200 mL. Compare their alveolar ventilation (dead space 150 mL) and explain which breathing pattern is more effective for gas exchange.

Try it first, then show the solutionSolution

A pulmonary ventilation = 12 x 500 = 6000 mL; B = 30 x 200 = 6000 mL (1 mark).
A alveolar = (500 - 150) x 12 = 4200 mL (1 mark).
B alveolar = (200 - 150) x 30 = 1500 mL (1 mark).
A delivers nearly three times more air to alveoli (4200 / 1500 = 2.8) (1 mark).
Dead space is a fixed loss per breath, so deep slow breaths are more effective (1 mark).

Answer: A: 4200 mL; B: 1500 mL; deeper is more effective.

Where students lose marks in Mechanism of Breathing and Lung Volumes

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

Mistake: VC includes RV
Capacities confused.
Wrong working
VC = TLC.
Right working
VC = TLC - RV.
Do this: No residue.
Usually costs: 1 mark.
Mistake: Alveolar ventilation without dead space
Dead space ignored.
Wrong working
Alveolar ventilation = TV x rate.
Right working
(TV - dead space) x rate.
Do this: Subtract 150.
Usually costs: 1 mark.
Mistake: Inspiration passive
Muscles forgotten.
Wrong working
Inspiration is passive.
Right working
Inspiration is active; normal expiration is passive.
Do this: Active in.
Usually costs: 1 mark.

Exam tips

Write each capacity as a sum with numbers.
Include units in every line.
Draw the spirogram.
State dead space value used.

Practice questions on Mechanism of Breathing and Lung Volumes

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 volume of air remaining in the lungs after forceful expiration is
(a) TV (b) ERV (c) RV (d) VC
Hint
Cannot be expelled.
Answer
(c).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Alveolar ventilation is less than pulmonary ventilation.
Reason (R): Some inspired air fills the dead space and does not reach alveoli.
(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
Dead space.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
A spirometer reading gives IRV = 3100 mL, TV = 500 mL, ERV = 900 mL and RV = 1300 mL. Find VC and TLC.
Hint
Sum.
Answer
VC = 900 + 500 + 3100 = 4500 mL. TLC = 4500 + 1300 = 5800 mL.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a person has TLC = 5800 mL, VC = 4600 mL and a breathing rate of 15 per minute with TV = 500 mL.
(a) Find RV. (1)
(b) Find pulmonary ventilation. (1)
(c) Find alveolar ventilation with dead space 150 mL. (2)
Hint
TLC - VC.
Answer
(a) RV = 5800 - 4600 = 1200 mL. (b) 500 x 15 = 7500 mL per min. (c) (500 - 150) x 15 = 5250 mL per min.
Q5 · 3 marks · Short answerHOTS / competency
Suppose the diaphragm of a patient is paralysed but the intercostals work. Predict the effect on inspiration and on tidal volume, and explain with the mechanism.
Hint
Volume change.
Answer
Inspiration is weaker because the diaphragm contributes the antero-posterior increase in thoracic volume. The external intercostals still raise the ribs, so some air enters, but tidal volume and ventilation fall.

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.Tidal volume?Show answer
About 500 mL.
2.Which muscle flattens in inspiration?Show answer
Diaphragm.
3.Dead space?Show answer
About 150 mL.
4.What is FRC?Show answer
ERV + RV.
5.Normal rate?Show answer
12 to 16 per minute.
6.Instrument?Show answer
Spirometer.

Mechanism of Breathing and Lung Volumes: frequently asked questions

How does air enter the lungs?

Pressure falls below atmospheric after chest volume rises.

What is vital capacity?

Maximum air breathed out after maximum inspiration.

What is residual volume?

Air left after forceful expiration.

What is dead space?

Air in conducting passages not used for exchange.

What is pulmonary ventilation?

Tidal volume times breaths per minute.