Biology · Breathing and Exchange of Gases · NEET
It CONTRACTS during inspiration. A contracted diaphragm flattens and moves down, which increases the volume of the thoracic chamber along the front-to-back (antero-posterior) axis. This is the main muscle that starts every normal breath. When it relaxes, it returns to its dome shape and pushes air out (expiration). Students often reverse this because 'relax' sounds like 'let air in' — remember instead that you must make space (contract) before air can enter.
They are two sets of muscles between the ribs with opposite jobs. The EXTERNAL intercostals contract during INSPIRATION — they lift the ribs and sternum up and outward, increasing thoracic volume in the dorso-ventral (back-to-front) axis. The INTERNAL intercostals contract during FORCED EXPIRATION — they pull the ribs down and inward to push extra air out. In quiet normal breathing the internal intercostals are not needed; expiration then happens just by relaxation.
Normal (quiet) breathing uses only two: the diaphragm and the external intercostal muscles for inspiration; normal expiration is passive (just relaxation, no muscle effort). For a deep or forced breath, NCERT says we use 'additional muscles in the abdomen' plus the internal intercostals to make inspiration and expiration stronger. So the extra abdominal and internal intercostal muscles are the answer whenever a question mentions 'forceful' breathing.
Normal expiration is PASSIVE. When the diaphragm and external intercostal muscles simply RELAX, the diaphragm and sternum return to their resting positions, thoracic and pulmonary volume decrease, intra-pulmonary pressure rises slightly above atmospheric pressure, and air is expelled — no muscle contraction is needed. Only FORCED expiration is active, using the internal intercostals and abdominal muscles. This active-vs-passive point is a common NEET trap.
The lungs have no muscle of their own, so they cannot expand or shrink by themselves. They sit in an air-tight thoracic chamber, and NCERT states that any change in the volume of the thoracic cavity is reflected in the lung cavity. So the diaphragm and intercostal muscles change the THORACIC volume, and the lungs passively follow. This is why breathing is controlled by these skeletal muscles and not by the lungs directly.
Select the correct statement.
Select the correct events that occur during inspiration. (a) Contraction of diaphragm (b) Contraction of external inter-costal muscles (c) Pulmonary volume decreases (d) Intrapulmonary pressure increases
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It contracts and flattens, moving downward. This increases the thoracic volume in the antero-posterior (front-to-back) axis, which lowers pressure inside the lungs and draws air in. It is the first muscle to act in a normal breath.
The internal intercostal muscles and the abdominal muscles. NCERT says we can increase the strength of expiration with 'additional muscles in the abdomen'. Normal quiet expiration, however, needs no muscle contraction and is passive.
No. External intercostals contract only during INSPIRATION to lift the ribs. During normal expiration they RELAX. A NEET option pairing external intercostal contraction with expiration is a classic wrong answer.
Contraction enlarges the thoracic chamber, and because the chamber is air-tight, the lungs expand with it. Bigger lungs mean lower intra-pulmonary pressure than the atmosphere, so outside air moves in to equalise the pressure.
Inspiration is always active (needs diaphragm and external intercostal contraction). Normal expiration is passive (just relaxation). Only forced expiration becomes active, using internal intercostal and abdominal muscles.
