BiologyISC Class 11 · Breathing and Exchange of Gases

Mechanism of Breathing, Lung Volumes and Capacities

Updated 17 October 2026 · Fact-checked

Breathing is brought about by the diaphragm and the external intercostal muscles changing the volume of the thoracic cavity: on inspiration the volume rises and the pressure falls below atmospheric, so air enters; on expiration the volume falls and air leaves. Lung volumes (TV, IRV, ERV, RV) and capacities (IC, EC, FRC, VC, TLC) are measured by spirometry.

Understand Mechanism of Breathing and Respiratory Volumes

Respiratory system. Air passes through the nostrils, nasal chamber, pharynx, larynx (the sound box, with the epiglottis covering the glottis during swallowing), trachea (C-shaped cartilage rings), primary bronchi, secondary and tertiary bronchi, bronchioles and the terminal alveoli, the sites of gas exchange. The two lungs are covered by a double pleura with pleural fluid between the layers, and are located in the thoracic chamber bounded by the vertebral column, ribs, sternum and diaphragm.

Mechanism. *Inspiration:* the diaphragm contracts and flattens, and the external intercostal muscles contract, lifting the ribs and sternum outward; the volume of the thorax increases in both dorso-ventral and antero-posterior axes; the lungs expand and the intra-pulmonary pressure falls below atmospheric pressure, so air flows in. *Expiration:* the diaphragm and external intercostals relax, returning to their original positions; the thoracic volume decreases, the intra-pulmonary pressure rises above atmospheric, and air flows out. Additional muscles of the abdomen assist in forced expiration.

Volumes (approximate, adult human). Tidal volume (TV): about 500 mL at each normal breath. Inspiratory reserve volume (IRV): 2500 to 3000 mL. Expiratory reserve volume (ERV): 1000 to 1100 mL. Residual volume (RV): 1100 to 1200 mL, which remains after forced expiration.
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 a forced inspiration, about 3500 to 4500 mL); total lung capacity TLC = VC + RV (about 5000 to 6000 mL).

Ventilation. Minute volume = TV × breaths per minute. Dead space (about 150 mL, the air in the conducting passages that does not take part in gas exchange) is subtracted for alveolar ventilation = (TV − dead space) × rate. A healthy adult breathes about 12 to 16 times a minute.

Mechanism of Breathing and Respiratory Volumes: key facts

Capacities: $I C = T V + I R V ∣ E C = T V + E R V ∣ F R C = E R V + R V$; Sums of volumes.
Vital capacity: $V C = E R V + T V + I R V ∣ T L C = V C + R V$; RV cannot be expelled.
Minute volume: TV × breaths per minute; Alveolar ventilation uses TV − dead space.
Muscles: Inspiration: diaphragm + external intercostals contract | Expiration: relax; Pressure changes drive the air.

Mechanism of Breathing and Respiratory 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 readings of a student

Suppose a student's spirometer test gives TV = 500 mL, IRV = 3000 mL, ERV = 1100 mL and RV = 1200 mL, and she breathes 12 times a minute with a dead space of 150 mL.

Find the VC, TLC, FRC, minute volume and alveolar ventilation.

Try it first, then show the solutionSolution

VC = ERV + TV + IRV = 1100 + 500 + 3000 = 4600 mL; TLC = 4600 + 1200 = 5800 mL.
FRC = ERV + RV = 1100 + 1200 = 2300 mL.
Minute volume = 500 × 12 = 6000 mL; alveolar ventilation = (500 − 150) × 12 = 4200 mL per minute.

Answer: VC 4600 mL; TLC 5800 mL; FRC 2300 mL; 6000 mL min⁻¹ and 4200 mL min⁻¹.

Memory hook: Dead space air does not reach the alveoli.

Tricks to remember Mechanism of Breathing and Respiratory Volumes

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

Diaphragm down, air in
The diaphragm flattens in inspiration.
Use it when: Mechanism questions.
VC excludes the residual volume
Vital capacity can be breathed out; RV remains.
Use it when: Distinguishing VC and TLC.

Mechanism of Breathing and Respiratory Volumes: solved examples

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

Example 1: Muscles of inspiration · 1 mark · MCQBoard level

During normal inspiration, the diaphragm:
(a) relaxes and becomes dome-shaped
(b) contracts and flattens
(c) relaxes and flattens
(d) contracts and becomes more curved

Try it first, then show the solutionSolution

Contraction flattens the diaphragm, increasing thoracic volume.

Answer: (b) contracts and flattens

Example 2: Inspiration and expiration · 3 marks · Short answerBoard level

Explain how inspiration and expiration are brought about in man.

Try it first, then show the solutionSolution

Inspiration: the diaphragm contracts and flattens, the external intercostals lift the ribs and sternum, increasing the thoracic volume. (1 mark)
The lung pressure falls below atmospheric pressure and air rushes in. (1 mark)
Expiration: the muscles relax, the thoracic volume decreases, the pressure in the lungs rises above atmospheric and air is forced out. (1 mark)

Answer: Volume and pressure changes: diaphragm and intercostal muscles.

Example 3: Volumes and capacities · 4 marks · Short answerHarder board level

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

Try it first, then show the solutionSolution

IC = TV + IRV = 3000 mL; EC = TV + ERV = 1500 mL. (1.5 marks)
FRC = ERV + RV = 2200 mL. (1 mark)
VC = ERV + TV + IRV = 4000 mL. (0.75 mark)
TLC = VC + RV = 5200 mL. (0.75 mark)

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

Example 4: Minute volume and dead space · 4 marks · Short answerHarder board level

A person breathes 14 times per minute with a tidal volume of 450 mL. Find the minute volume. If the anatomical dead space is 150 mL, find the alveolar ventilation and the percentage of each breath that reaches the alveoli.

Try it first, then show the solutionSolution

Minute volume = 450 × 14 = 6300 mL. (1 mark)
Alveolar ventilation = (450 − 150) × 14 = 4200 mL per minute. (1.5 marks)
Percentage reaching alveoli = 300/450 × 100 = 66.7%. (1.5 marks)

Answer: 6300 mL min⁻¹; 4200 mL min⁻¹; 66.7%.

Example 5: Lung volumes after exercise and disease · 5 marks · Long answerHOTS / competency

A student breathes more deeply during exercise: TV rises to 1500 mL and the rate to 20 per minute. (i) Find the minute volume. (ii) Find the alveolar ventilation with 150 mL dead space. (iii) Which reserve volumes are used? (iv) A person with emphysema has a normal TLC but a reduced VC; explain what has changed.

Try it first, then show the solutionSolution

(i) 1500 × 20 = 30 000 mL min⁻¹. (1 mark)
(ii) (1500 − 150) × 20 = 27 000 mL min⁻¹. (1.5 marks)
(iii) The inspiratory and expiratory reserve volumes. (1 mark)
(iv) In emphysema the alveolar walls are damaged, the lungs lose elasticity and air is trapped, so the residual volume rises and VC (TLC − RV) falls. (1.5 marks)

Answer: 30 000 mL; 27 000 mL; IRV and ERV; higher RV and reduced VC in emphysema.

Where students lose marks in Mechanism of Breathing and Respiratory Volumes

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

Mistake: Including RV in the vital capacity
Adding RV to ERV, TV and IRV.
Do this: VC = ERV + TV + IRV; RV is added only for TLC.
Usually costs: 1 to 2 marks.
Mistake: Saying air enters because the pressure in the lungs rises
Reversing the pressure change.
Do this: Pressure falls below atmospheric in inspiration.
Usually costs: 2 marks.
Mistake: Ignoring dead space
Using TV × rate for alveolar ventilation.
Do this: Use (TV − dead space) × rate.
Usually costs: 1 mark.

Exam tips

Draw a labelled diagram of the respiratory system.
Write the pressure and volume changes clearly in inspiration and expiration.
Tabulate volumes and capacities.
Give the unit mL or L.

Practice questions on Mechanism of Breathing and Respiratory 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
Which lung capacity is the sum of the expiratory reserve volume and the residual volume?
(a) inspiratory capacity
(b) vital capacity
(c) functional residual capacity
(d) total lung capacity
Hint
It is the air in the lungs at the end of a normal expiration.
Answer
(c) functional residual capacity
Q2 · 2 marks · Short answerBoard level
What is the function of the epiglottis?
Hint
Think about swallowing.
Answer
It is a thin cartilaginous flap that covers the glottis during swallowing, preventing food from entering the larynx.
Q3 · 3 marks · Short answerHarder board level
If TV = 500 mL and the breathing rate is 15 per minute, find the volume of air breathed per minute and the volume in an hour.
Hint
Multiply.
Answer
500 × 15 = 7500 mL min⁻¹; per hour 7500 × 60 = 450 000 mL = 450 L.
Q4 · 3 marks · Short answerHarder board level
Explain the term vital capacity and give its value for an average person with the volumes involved.
Hint
Add three volumes.
Answer
The maximum volume of air a person can breathe out after a forced inspiration: ERV + TV + IRV; about 3500 to 4500 mL.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): The residual volume cannot be removed from the lungs by forced expiration. Reason (R): The lungs remain partly inflated because the thoracic cavity is airtight. 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
Think about the lungs and the pleura.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a spirometer test records TV = 500 mL, ERV = 1100 mL, IRV = 3000 mL, RV = 1200 mL.
(i) Find the VC. (1 mark)
(ii) Find the TLC. (1 mark)
(iii) Find the IC. (1 mark)
(iv) State the instrument used. (1 mark)
Hint
Use the capacity formulas.
Answer
(i) 4600 mL. (ii) 5800 mL. (iii) 3500 mL. (iv) A spirometer.

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.Value of the dead space?Show answer
About 150 mL.
3.Muscles of inspiration?Show answer
Diaphragm and external intercostals.
4.Instrument for lung volumes?Show answer
Spirometer.

Mechanism of Breathing and Respiratory Volumes: frequently asked questions

Why are the C-shaped rings present in the trachea?

They keep the trachea open while the oesophagus behind expands during swallowing.

What is the pleura?

A double membrane around each lung; the fluid between the layers reduces friction.

What is the difference between breathing and respiration?

Breathing is the physical movement of air in and out of the lungs; respiration includes gas exchange and cellular oxidation.