This MCQ module is based on: Mechanism Volumes
TOPIC 62 OF 90
Mechanism Volumes
🎓 Class 11
Biology
CBSE
Theory
Ch 14 – Breathing and Exchange of Gases
⏱ ~14 min
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Mechanism of Breathing, Respiratory Volumes and Capacities
Part 1 ended with a puzzle built into the anatomy: the lung has no muscle of its own, so it cannot inflate itself. This part shows how the body solves that — by changing the volume of the air-tight box the lung sits in, and letting pressure do the rest.
14.2 Mechanism of Breathing
Breathing involves two stages: inspiration, during which atmospheric air is drawn in, and expiration, by which the alveolar air is released out.
The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere.
The two conditions, stated exactly.
• Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure — i.e., there is a negative pressure in the lungs with respect to atmospheric pressure.
• Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.
• Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure — i.e., there is a negative pressure in the lungs with respect to atmospheric pressure.
• Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.
The diaphragm and a specialised set of muscles — external and internal intercostals between the ribs — help in the generation of such gradients.
Inspiration, step by step
- Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the antero-posterior axis.
- The contraction of the external intercostal muscles lifts up the ribs and the sternum, causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.
- The overall increase in the thoracic volume causes a similar increase in pulmonary volume.
- An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure, which forces the air from outside to move into the lungs — i.e., inspiration.
Two muscles, two axes. Notice that the two muscle groups enlarge the chest in different directions — the diaphragm along the antero-posterior axis, the external intercostals along the dorso-ventral axis. Together they expand the box in three dimensions, which is how a modest muscular effort produces a useful volume change. And notice the last step carefully: air is not pulled in. The body enlarges a sealed chamber, pressure inside falls, and the atmosphere pushes air in.
Expiration, step by step
- Relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions, and reduces the thoracic volume and thereby the pulmonary volume.
- This leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure, causing the expulsion of air from the lungs — i.e., expiration.
We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen. On an average, a healthy human breathes 12–16 times per minute.
The instrument. The volume of air involved in breathing movements can be estimated by using a spirometer, which helps in clinical assessment of pulmonary functions.
| Feature | Inspiration | Expiration |
|---|---|---|
| Muscle state | Diaphragm and external intercostals contract | Diaphragm and intercostals relax |
| Diaphragm shape | Flattens — increases volume along the antero-posterior axis | Returns to its dome shape |
| Ribs and sternum | Lifted up — increases volume along the dorso-ventral axis | Return to normal positions |
| Thoracic and pulmonary volume | Increases | Decreases |
| Intra-pulmonary pressure | Less than atmospheric (negative) | Slightly above atmospheric |
| Air movement | Into the lungs | Out of the lungs |
| Nature | Active — requires muscular contraction | Passive at rest — relaxation and recoil |
📐 Activity 14.2 — Build a working model of the chest
What to do. Take a transparent plastic bottle and cut off its base. Push a Y-shaped glass or plastic tube through a rubber stopper and fix two small balloons to the two arms of the Y; these are the lungs. Seal the stopper into the bottle's mouth so the bottle is air-tight. Now stretch a large rubber sheet across the open base and tie it firmly — this is the diaphragm. Pull the rubber sheet downward, then release it, and watch the balloons.
Predict: what happens to the balloons when you pull the sheet down, and when you release it? Then predict what happens if you punch a small hole in the side of the bottle and try again.
Observations. Pulling the rubber sheet down inflates both balloons; releasing it lets them deflate. The balloons were never squeezed or blown into — nothing touched them at all.
What the model shows. Exactly the real mechanism. Pulling the sheet down mimics contraction of the diaphragm, which increases the volume of the thoracic chamber. The overall increase in thoracic volume causes a similar increase in pulmonary volume, which decreases the intra-pulmonary pressure to less than the atmospheric pressure, and this forces the air from outside to move into the lungs. Releasing the sheet mimics relaxation, which reduces the volume and raises the pressure slightly above atmospheric, causing the expulsion of air.
What the model gets right that a diagram cannot. It makes vivid the point that we cannot directly alter the pulmonary volume — the balloons are inflated only through the sealed chamber around them.
The hole in the side. The balloons now fail to inflate, however hard you pull the sheet, because the chamber is no longer air-tight: air rushes in through the hole instead of down the tube, and no pressure difference develops across the balloon wall. This is precisely what happens in a real punctured chest, and it is why the chapter stresses that the thoracic chamber is anatomically an air-tight chamber.
One limitation to note honestly. The model represents only the diaphragm. The real chest also expands through the external intercostals lifting the ribs and sternum along the dorso-ventral axis, which a rigid bottle cannot imitate.
14.2.1 Respiratory Volumes and Capacities
By adding up a few respiratory volumes, one can derive various pulmonary capacities, which can be used in clinical diagnosis. Learn the four volumes first; every capacity is then just a sum of them.
The four volumes
Tidal Volume (TV) — volume of air inspired or expired during a normal respiration. It is approximately 500 mL, i.e., a healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute.
Inspiratory Reserve Volume (IRV) — the additional volume of air a person can inspire by a forcible inspiration. This averages 2500 mL to 3000 mL.
Expiratory Reserve Volume (ERV) — the additional volume of air a person can expire by a forcible expiration. This averages 1000 mL to 1100 mL.
Residual Volume (RV) — volume of air remaining in the lungs even after a forcible expiration. This averages 1100 mL to 1200 mL.
The five capacities
| Capacity | Definition | Composition |
|---|---|---|
| Inspiratory Capacity (IC) | Total volume of air a person can inspire after a normal expiration | TV + IRV |
| Expiratory Capacity (EC) | Total volume of air a person can expire after a normal inspiration | TV + ERV |
| Functional Residual Capacity (FRC) | Volume of air that will remain in the lungs after a normal expiration | ERV + RV |
| Vital Capacity (VC) | The maximum volume of air a person can breathe in after a forced expiration, or equivalently the maximum volume that can be breathed out after a forced inspiration | ERV + TV + IRV |
| Total Lung Capacity (TLC) | Total volume of air accommodated in the lungs at the end of a forced inspiration | RV + ERV + TV + IRV, i.e. vital capacity + residual volume |
A memory method that beats rote learning. Stack the four volumes in the order IRV → TV → ERV → RV, from the top of a full breath down to the air that can never leave. Then every capacity is just adjacent bands added together: two from the top gives IC, the middle two gives EC, the bottom two gives FRC, the top three gives VC, and all four gives TLC. The only fact you then need is which single band is unmeasurable — RV — and it follows at once that FRC and TLC cannot be found by spirometry alone.
🎯 Interactive: Volume or capacity? Look it up
Value / composition: Approximately 500 mL
Volume of air inspired or expired during a normal respiration. At 12 to 16 breaths per minute, a healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute.
🎯 Competency-Based Questions
Scenario: A spirometry test on a healthy adult gives TV = 500 mL, IRV = 2800 mL, ERV = 1100 mL. Residual volume, measured separately, is 1200 mL. A second person, a long-term smoker, is found to have a normal total lung capacity but a much reduced vital capacity and a raised residual volume.
Q1. Calculate IC, EC, FRC, VC and TLC for the healthy adult. L3 Apply
IC = TV + IRV = 500 + 2800 = 3300 mL.
EC = TV + ERV = 500 + 1100 = 1600 mL.
FRC = ERV + RV = 1100 + 1200 = 2300 mL.
VC = ERV + TV + IRV = 1100 + 500 + 2800 = 4400 mL.
TLC = RV + ERV + TV + IRV = 1200 + 4400 = 5600 mL, which is also VC + RV.
EC = TV + ERV = 500 + 1100 = 1600 mL.
FRC = ERV + RV = 1100 + 1200 = 2300 mL.
VC = ERV + TV + IRV = 1100 + 500 + 2800 = 4400 mL.
TLC = RV + ERV + TV + IRV = 1200 + 4400 = 5600 mL, which is also VC + RV.
Q2. In the smoker, TLC is normal but VC is reduced and RV raised. Explain why these three findings are consistent with each other. L4 Analyse
They are consistent because TLC = VC + RV. If residual volume rises and total lung capacity stays the same, vital capacity must fall by exactly the same amount — the lung is not smaller, but a larger share of it is air that can never be exhaled. Physiologically this is the picture of air trapping: the lungs hold plenty of air but cannot move it. Since vital capacity is the maximum volume of air a person can breathe in after a forced expiration, the useful, exchangeable volume has shrunk even though the container has not. This is why vital capacity, not total lung capacity, is the clinically meaningful measurement.
Q3. Fill in the blanks: Inspiration occurs when intra-pulmonary pressure is ______ atmospheric pressure. It is initiated by contraction of the ______, which increases thoracic volume in the ______ axis, while the ______ intercostals lift the ribs and sternum, increasing volume in the ______ axis. L1 Remember
less than; diaphragm; antero-posterior; external; dorso-ventral.
Q4. Why can a spirometer not measure residual volume, and which two capacities does this make unmeasurable by spirometry alone? L4 Analyse
Because a spirometer records only air that actually moves in or out of the lungs, and residual volume is by definition the volume of air remaining in the lungs even after a forcible expiration — it never leaves, so it cannot be recorded. Consequently the two capacities that include RV cannot be measured by spirometry alone: Functional Residual Capacity (ERV + RV) and Total Lung Capacity (RV + ERV + TV + IRV). All the others — IC, EC and VC — are made only of volumes that do move, so they are measurable. This is exactly why the chapter says one derives capacities by adding up a few respiratory volumes: RV has to be obtained by a separate method and then added in.
Q5. “Expiration is just inspiration in reverse.” Evaluate. L5 Evaluate
Superficially true, mechanistically misleading. The sequence does reverse: volume rises then falls, pressure falls then rises, air enters then leaves. But the two stages are not symmetrical in the way the statement implies.
(i) One is active, the other is not. Inspiration is initiated by the contraction of the diaphragm and of the external intercostal muscles — muscular work. Expiration at rest is produced by relaxation of the diaphragm and the intercostal muscles, which returns the diaphragm and sternum to their normal positions. Relaxation is not the opposite muscular act; it is the absence of one, with elastic recoil doing the work.
(ii) The pressure changes differ in size. Inspiration needs the intra-pulmonary pressure less than atmospheric; expiration needs it only slightly above atmospheric. The chapter's wording “slightly” is deliberate.
(iii) Different muscles serve the forced versions. We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen, and forced expiration additionally recruits the internal intercostals — muscles that play no part in quiet breathing at all.
(iv) The volumes are not mirror images. IRV is 2500–3000 mL but ERV only 1000–1100 mL, and RV can never be expelled at all — so the lung's capacity to take in far exceeds its capacity to push out.
The accurate statement: expiration reverses the direction of air flow, but it is a passive elastic return rather than an active mirror of inspiration.
(i) One is active, the other is not. Inspiration is initiated by the contraction of the diaphragm and of the external intercostal muscles — muscular work. Expiration at rest is produced by relaxation of the diaphragm and the intercostal muscles, which returns the diaphragm and sternum to their normal positions. Relaxation is not the opposite muscular act; it is the absence of one, with elastic recoil doing the work.
(ii) The pressure changes differ in size. Inspiration needs the intra-pulmonary pressure less than atmospheric; expiration needs it only slightly above atmospheric. The chapter's wording “slightly” is deliberate.
(iii) Different muscles serve the forced versions. We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen, and forced expiration additionally recruits the internal intercostals — muscles that play no part in quiet breathing at all.
(iv) The volumes are not mirror images. IRV is 2500–3000 mL but ERV only 1000–1100 mL, and RV can never be expelled at all — so the lung's capacity to take in far exceeds its capacity to push out.
The accurate statement: expiration reverses the direction of air flow, but it is a passive elastic return rather than an active mirror of inspiration.
🧠 Assertion–Reason Questions
For each pair choose: (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.
Assertion (A): Air moves into the lungs during inspiration.
Reason (R): An increase in pulmonary volume decreases the intra-pulmonary pressure to less than atmospheric pressure.
Answer: A. Both are true and the reason is the correct explanation. Air is not pulled in — the chamber is enlarged and the atmosphere pushes air down the pressure gradient.
Assertion (A): Total lung capacity is always greater than vital capacity.
Reason (R): Total lung capacity includes the residual volume, which is the air remaining in the lungs even after a forcible expiration.
Answer: A. Both are true and the reason explains the assertion exactly: TLC = VC + RV, and since RV can never be zero, TLC always exceeds VC.
Assertion (A): Contraction of the diaphragm increases the volume of the thoracic chamber in the dorso-ventral axis.
Reason (R): The diaphragm forms the lower side of the thoracic chamber and is dome-shaped.
Answer: D. The assertion is false — the diaphragm increases thoracic volume along the antero-posterior axis; it is the external intercostals that increase it in the dorso-ventral axis by lifting the ribs and sternum. The reason is a true statement about the diaphragm's position and shape.
Coming next. Part 3 takes up Section 14.3 — partial pressures of O₂ and CO₂ at every site of diffusion, Table 14.1, the three layers of the diffusion membrane, and why every factor in the body favours diffusion in exactly the directions it does.
Frequently Asked Questions - Mechanism of Breathing and Respiratory Volumes
How is inspiration brought about?
By creating a negative pressure in the lungs. Contraction of the diaphragm increases thoracic volume along the antero-posterior axis, and contraction of the external intercostals lifts the ribs and sternum, increasing it along the dorso-ventral axis. The increased thoracic volume increases pulmonary volume, which lowers intra-pulmonary pressure below atmospheric, so air is forced in from outside.
How does expiration occur?
Relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions, reducing thoracic and thereby pulmonary volume. This raises intra-pulmonary pressure to slightly above atmospheric pressure, causing the expulsion of air. At rest it needs no muscular work of its own.
How many times does a healthy human breathe in a minute?
On an average, 12 to 16 times per minute. With a tidal volume of about 500 mL, that means a healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute.
What is a spirometer used for?
It estimates the volume of air involved in breathing movements and so helps in clinical assessment of pulmonary functions. It cannot measure residual volume, since that air never leaves the lungs.
What are the four respiratory volumes and their values?
Tidal volume, about 500 mL, inspired or expired during normal respiration. Inspiratory reserve volume, 2500 to 3000 mL, which can be inspired by a forcible inspiration. Expiratory reserve volume, 1000 to 1100 mL, which can be expired forcibly. Residual volume, 1100 to 1200 mL, remaining even after a forcible expiration.
What is vital capacity and how does it differ from total lung capacity?
Vital capacity is the maximum volume of air a person can breathe in after a forced expiration, comprising ERV, TV and IRV. Total lung capacity is the total volume accommodated at the end of a forced inspiration, comprising RV, ERV, TV and IRV, that is vital capacity plus residual volume. TLC therefore always exceeds VC by the residual volume.
What is functional residual capacity?
The volume of air that will remain in the lungs after a normal expiration, made up of expiratory reserve volume plus residual volume. It is the volume the lungs sit at between ordinary breaths.
Why can we not inflate our lungs directly?
Because the lungs contain no muscle of their own. The thoracic chamber is anatomically air-tight, and any change in thoracic volume is reflected in the pulmonary cavity, so the lungs are inflated and deflated only by moving the walls of the chamber around them.
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