Breathing Mechanics: Pressure Gradients — NEET Biology
📚 Practice ConceptBreathing 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. 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. Similarly, 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 generation of such gradients. Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero-posterior axis. The contraction of external inter-costal muscles lifts up the ribs and the
Which of the following statements accurately describe the mechanism of breathing? S1: Contraction of the diaphragm initiates inspiration by increasing the volume of the thoracic chamber in the antero-posterior axis. S2: The external inter-costal muscles contract to lift the ribs and sternum, thereby increasing thoracic volume along the dorso-ventral axis. S3: An overall increase in thoracic volume directly leads to an increase in intra-pulmonary pressure, forcing air inwards. S4: Normal expiration is a passive process primarily involving the relaxation of the diaphragm and inter-costal muscles, reducing thoracic volume. S5: During forceful expiration, the active contraction of abdominal muscles further increases thoracic volume for greater air expulsion.
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Correct answer: C — S1, S2 and S4 are correct
The core concept tested is the role of different muscles and their effects on thoracic and pulmonary volume, and pressure changes during inspiration and expiration. S1: Correct. The contraction of the diaphragm increases the volume of the thoracic chamber in the antero-posterior axis, initiating inspiration. S2: Correct. The contraction of external inter-costal muscles lifts the ribs and sternum, causing an increase in the volume of the thoracic chamber in the dorso-ventral axis. S3: Incorrect. An overall increase in thoracic volume causes a similar increase in pulmonary volume, which then *decreases* the intra-pulmonary pressure to less than the atmospheric pressure, forcing air inwards (inspiration). S4: Correct. Normal expiration is a passive process due to the relaxation of the diaphragm and inter-costal muscles, which returns the diaphragm and sternum to their normal positions and reduces thoracic volume. S5: Incorrect. During forceful expiration, the active contraction of abdominal muscles helps *decrease* thoracic volume by pushing the diaphragm up, leading to expulsion of more air, not increasing thoracic volume. Based on the analysis, statements S1, S2, and S4 are correct.
What does NCERT say about Breathing involves two stages?
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Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.