Mechanism of Inspiration and Expiration — NEET Biology
✅ Asked in NEET 2020Select the correct events that occur during inspiration. (i) Contraction of diaphragm (ii) Contraction of external inter-costal muscles (iii) Pulmonary volume decreases (iv) Intrapulmonary pressure increases
Select the correct events that occur during inspiration. (i) Contraction of diaphragm (ii) Contraction of external inter-costal muscles (iii) Pulmonary volume decreases (iv) Intrapulmonary pressure increases
Answer & NCERT explanation
Correct answer: D — (i) and (ii)
During inspiration, diaphragm contracts and moves down, external intercostal muscles contract lifting the ribs up and out. This increases thoracic volume, decreases intrapulmonary pressure below atmospheric pressure, causing air to rush in. Options (iii) and (iv) are incorrect as volume increases and pressure decreases during inspiration. Only (i) and (ii) are correct events.
Read more NCERT concept on the PYQ
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. Relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions and reduce 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/minute. The volume of air involved in breathing movements can be estimated by using a spirometer which helps in clinical assessment of pulmonary functions.
NTA tests your understanding of how diaphragm and inter-costal muscle contraction causes inspiration by increasing thoracic volume and decreasing intra-pulmonary pressure. Students commonly confuse the pressure gradient direction—remember that a decrease in lung pressure (below atmospheric) pulls air IN, while an increase in lung pressure (above atmospheric) pushes air OUT. The key trap is forgetting that inspiration is an active process requiring muscle contraction, while expiration involves muscle relaxation. To score correctly, always link muscle movement → volume change → pressure change → air movement in that sequence.
What does NCERT say about overall increase thoracic volume?
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