Q.Explain the mechanism of inspiration in humans, with reference to the diaphragm, the external intercostal muscles, and the resulting change in pulmonary pressure.
Concept understanding — Mechanism Of Breathing
Mechanism Of Breathing
The Basic Idea: Suction and Squeeze
Your lungs cannot pull air in by themselves — they have no muscles of their own and behave like passive elastic bags. Think of a syringe with no needle: pulling the plunger back increases the volume inside the barrel, the pressure inside drops below the pressure outside, and air rushes in to equalise it. Pushing the plunger in does the reverse. Your chest cavity works the same way — the diaphragm and the muscles between the ribs act as the plunger, and the lungs are the balloon pulled along with them.
Volume Change Drives Pressure Change
Breathing follows a simple physical rule: for a fixed amount of gas at a constant temperature, pressure and volume are inversely related — increase the volume of the thoracic cavity and the pressure inside falls; decrease it and the pressure rises. Air always flows from higher pressure to lower pressure.
Inspiration (Inhalation)
Inspiration is an active process. Two sets of muscles contract together:
- The diaphragm, a dome-shaped sheet of muscle separating the chest from the abdomen, flattens and moves downward.
- The external intercostal muscles, between the ribs, pull the rib cage upward and outward.
Together these increase the volume of the thoracic cavity in every direction.
The lungs are held against the inner chest wall by a thin layer of pleural fluid, so as the thoracic cavity expands, the lungs are pulled open and expand passively along with it.
As the cavity expands, pressure inside the alveoli drops to roughly 1 mmHg below atmospheric pressure, and air flows in until the pressures equalise.
Expiration (Exhalation)
Quiet, normal expiration is passive — the diaphragm and external intercostals simply relax. The diaphragm domes back upward, the ribs fall back under gravity and elastic recoil, thoracic volume decreases, alveolar pressure rises to about 1 mmHg above atmospheric, and air flows out.
During forced expiration — blowing out a candle, coughing, or heavy exercise — the internal intercostal muscles and abdominal muscles contract actively to push the diaphragm up harder and pull the ribs down faster.
A Common Misconception
Many students imagine the lungs actively expand to suck air in. It is the opposite: the expanding chest wall stretches the lungs open. If the chest wall is punctured (a pneumothorax), the lung collapses, because the negative pressure that was holding it open is lost — the lung itself has no power to inflate on its own.
Do not confuse the pressure in the space between lung and chest wall (always negative during normal breathing) with the pressure inside the alveoli, which oscillates between slightly negative during inspiration and slightly positive during expiration. The first keeps the lungs stuck to the chest wall; the second is what actually drives airflow.
One-line summary: breathing is a pressure-driven flow caused by volume changes of the thoracic cavity, produced by the diaphragm and intercostal muscles — the lungs themselves are passive passengers.
Students searching for "Mechanism Of Breathing class 11 biology" or "Mechanism Of Breathing: definition and examples" will find this topic sits squarely inside the Breathing and Exchange of Gases unit of the NCERT/CBSE Class 11 Biology syllabus. It is also a recurring favourite in "Mechanism Of Breathing important questions" lists compiled for NEET and state-level medical/CET aspirants, since Breathing and Exchange of Gases carries real weightage in competitive Biology papers.
[!TLDR]
Inspiration: diaphragm + external intercostals contract, enlarging the thorax and lowering pulmonary pressure below atmospheric, drawing air in.
[!ANSWER]
Diaphragm contracts and flattens; external intercostal muscles contract, pulling ribs up and out; thoracic volume increases; pulmonary pressure falls below atmospheric; air flows in.
Inspiration begins when the diaphragm contracts, pulling its dome downward and increasing the vertical dimension of the thoracic cavity, while at nearly the same moment the external intercostal muscles contract, pulling the ribs upward and outward and increasing the front-to-back and side-to-side dimensions as well. Because the lungs are elastic and mechanically coupled to the thoracic wall via the pleural membranes, this enlargement of the thoracic cavity is transmitted to the lungs, increasing pulmonary volume. By the inverse pressure-volume relationship of an enclosed gas, this increase in pulmonary volume lowers pulmonary pressure to slightly below atmospheric pressure, and air flows in from the higher-pressure atmosphere down this gradient until the pressures equalise, completing inspiration.
[!ANSWER]
Diaphragm contracts and flattens; external intercostal muscles contract, pulling ribs up and out; thoracic volume increases; pulmonary pressure falls below atmospheric; air flows in.
Trace the causal chain: muscle contraction to enlarged thoracic volume to lowered pulmonary pressure (inverse relationship) to inflow of air down the resulting pressure gradient.
Describing air as being 'sucked in' by the lungs actively, rather than correctly explaining that it flows passively down a pressure gradient created by the muscles enlarging the thoracic cavity.
- CBSE 2026Set ANNUAL1 markQ.On an average, a healthy human breathes _______ times per minute.
›Reveal solutionSolution
The average resting breathing rate in a healthy human is roughly 12–16 breaths per minute.
Breathing rate (the number of complete breathing cycles — one inspiration plus one expiration — per minute) is one of the basic respiratory volumes/parameters discussed alongside tidal volume, vital capacity, and residual volume. For a healthy adult at rest, this rate averages about 12 to 16 times per minute. It rises during exercise, fever, high altitude, or emotional stress because the body's oxygen demand and CO₂ output increase, and it is regulated by the respiratory rhythm centre in the medulla, which responds mainly to blood CO₂/H⁺ levels sensed by chemoreceptors.
✓Final answerA healthy human breathes approximately 12–16 times per minute.
- CBSE 2026Set ANNUAL1 markMCQQ.The normal breathing rate in a healthy adult is(a) 10-12 breaths/min(b) 12-16 breaths/min(c) 20-25 breaths/min(d) 25-30 breaths/min
›Reveal solutionSolution
The normal breathing rate in a healthy adult is 12-16 breaths per minute, so the answer is (B).
The respiratory rate is the number of breaths (one inspiration + one expiration) taken per minute. In a healthy adult at rest, this is normally about 12 to 16 breaths per minute.
The rate rises during exercise, fever or excitement (mainly because of increased CO2 in the blood) and is higher in infants. Under normal resting conditions, however, the standard value is 12-16 breaths per minute.
✓Final answer(B) 12-16 breaths/min.
- CBSE 2026Set ANNUAL1 markMCQQ.During inspiration, the diaphragm(a) Relaxes(b) Contracts and moves downwards(c) Moves upwards(d) Becomes dome-shaped
›Reveal solutionSolution
During inspiration the diaphragm contracts and moves downwards, so the answer is (B).
Inspiration is an active process. The diaphragm (dome-shaped at rest) contracts and flattens, moving downwards, while the external intercostal muscles lift the ribs and sternum. Together these increase the volume of the thoracic cavity and lungs, lowering the intrapulmonary pressure below atmospheric pressure so air rushes in.
During expiration the diaphragm relaxes and becomes dome-shaped again (moving upwards), reducing thoracic volume. So during inspiration the diaphragm contracts and moves downwards.
✓Final answer(B) Contracts and moves downwards.
- CBSE 2025Set ANNUAL1 markQ.Choose True or False: The diaphragm and the inter-costal muscles are relaxed on inspiration.
›Reveal solutionSolution
The statement is False — contraction, not relaxation, of the diaphragm and inter-costal muscles drives inspiration.
Breathing in humans is driven by changes in the volume of the thoracic cavity. During inspiration, the diaphragm contracts and flattens (moving downward), and the external inter-costal muscles contract, lifting the ribs and sternum outward and upward. Together these actions increase the volume of the thoracic chamber, decreasing the pulmonary pressure below atmospheric pressure so air rushes into the lungs. During expiration, the diaphragm and inter-costal muscles relax, the thoracic volume decreases, and air is pushed out — exactly the opposite of what the statement claims for inspiration.
✓Final answerFalse. The diaphragm and inter-costal muscles contract (not relax) during inspiration; they relax during expiration.
- CBSE 2024Set zoology-sz1 markMCQQ.The process of inspiration during breathing involves:(a) Contraction of diaphragm and external inter-coastal muscles(b) Relaxation of diaphragm and external inter-coastal muscles(c) Contraction of diaphragm and relaxation of external inter-coastal muscles(d) Relaxation of diaphragm and contraction of external inter-coastal muscles
›Reveal solutionSolution
Inspiration happens when the diaphragm and the external intercostal muscles both CONTRACT, enlarging the thoracic cavity and drawing air in — option (a).
Step 1 — Muscles involved in breathing.
Breathing (pulmonary ventilation) is achieved by rhythmic contraction and relaxation of the diaphragm (a dome-shaped muscle separating thorax from abdomen) and the intercostal muscles between the ribs.
Step 2 — What happens during inspiration.
- The diaphragm contracts, so its dome flattens and moves downward, increasing the vertical dimension of the thoracic cavity.
- The external intercostal muscles contract, pulling the ribs and sternum upward and outward, increasing the antero-posterior and lateral dimensions of the thoracic cavity.
- Together this enlarges the thoracic volume. By Boyle's law, an increase in volume causes a decrease in the intra-pulmonary pressure below atmospheric pressure, so air flows from the atmosphere (high pressure) into the lungs (low pressure) — this is inspiration.
Step 3 — Why the other options are wrong.
Relaxation of either muscle (as in options b, c, d) would decrease or fail to increase thoracic volume, which is what happens during (passive) expiration, not inspiration.
✓Final answerThe correct option is (a) Contraction of diaphragm and external inter-costal muscles.
- CBSE 2024Set ANNUAL1 markMCQQ.During inspiration, the diaphragm______(a) expands(b) shows no change(c) contracts and flatten(d) relaxes to become dome shaped
›Reveal solutionSolution
Inspiration begins when the diaphragm's muscle fibres contract, flattening its dome shape and enlarging the thoracic cavity so air is drawn in.
Breathing (pulmonary ventilation) works on pressure differences created by changing the volume of the thoracic chamber. The diaphragm is a dome-shaped muscular partition between the thoracic and abdominal cavities.
- During inspiration, the diaphragm contracts, which pulls it downward and flattens its dome shape; simultaneously the external intercostal muscles contract, lifting the ribs and sternum outward. Both actions increase the volume of the thoracic cavity, decreasing intra-pulmonary pressure below atmospheric pressure, so air flows into the lungs.
- During expiration, the diaphragm and external intercostal muscles relax, allowing the diaphragm to return to its dome shape and the thoracic volume to decrease, raising intra-pulmonary pressure above atmospheric and pushing air out.
So the option describing inspiration correctly is that the diaphragm contracts and flattens (not relaxes and domes, which describes expiration).
✓Final answer(c) contracts and flatten
- CBSE 2023Set ANNUAL1 markQ.Fill in the blank: Air passes in atmosphere from lungs by the process of ____.
›Reveal solutionSolution
Expiration (exhalation) is the breathing movement that pushes air out of the lungs into the atmosphere, occurring when the thoracic (chest) volume decreases.
During expiration:
- The diaphragm relaxes and domes upward, and the external intercostal muscles relax, allowing the ribcage to move down and inward.
- This decreases the volume of the thoracic cavity and lungs, which increases the pulmonary (intra-pulmonary) pressure above atmospheric pressure.
- Because gases flow from a region of higher pressure to lower pressure, air is pushed out of the lungs and alveoli, through the bronchioles, bronchi, trachea, and finally out through the nose/mouth into the atmosphere.
This is the reverse of inspiration, where diaphragm/intercostal muscle contraction increases thoracic volume and draws air in.
✓Final answerAir passes from the lungs into the atmosphere by the process of expiration (exhalation).
- CBSE 2019Set ANNUAL1 markMCQQ.Match the following - Diaphragm:(a) Expiration(b) Regeneration(c) Solanaceae(d) Fluorine(e) Renal tubule
›Reveal solutionSolution
The diaphragm's contraction and relaxation, together with the intercostal muscles, drive breathing: relaxation of the diaphragm assists expiration.
During inspiration, the diaphragm contracts and flattens, increasing the volume of the thoracic cavity and drawing air in. During expiration, the diaphragm relaxes and returns to its dome (upward-curved) shape, decreasing thoracic volume and helping to push air out, aided by the elastic recoil of the lungs and relaxation of the external intercostal muscles.
✓Final answer(a) Expiration.
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