Q.________ muscles contract when the external intercostal muscles contract.
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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 …
The diaphragm contracts at the same time as the external intercostal muscles during …
The diaphragm contracts at the same time as the external intercostal muscles during inspiration.
Inspiration is driven jointly by two muscle groups acting together: the external intercostal muscles contract to pull the ribs and sternum up and outward, while the diaphragm simultaneously contracts and flattens downward, enlarging the thoracic cavity from both th …
- Assuming only the intercostal muscles are involved in inspiration and overlookin …
- 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 i …
- 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.
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- 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.
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- 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 rel …
- 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. …
- 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. …
- 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. …
- 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 tho …
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