During inspiration the diaphragm and external intercostal muscles CONTRACT. This raises the volume of the thoracic cavity, which raises the volume of the lungs. When lung volume goes up, the intra-pulmonary pressure falls BELOW atmospheric pressure, so outside air is pushed into the lungs. Memory hook: "Contract, expand, pressure drops, air comes in."
During inspiration the diaphragm and external intercostal muscles contract. Thoracic volume rises, lung volume rises, intra-pulmonary pressure falls below atmospheric pressure, and air moves into the lungs.
Your doubts, answered
Does the diaphragm contract or relax during inspiration?
It CONTRACTS. The diaphragm is dome-shaped when relaxed. When it contracts it flattens and moves down. This makes the thoracic cavity bigger in the antero-posterior (top-to-bottom) axis. A common wrong idea is that the diaphragm relaxes to breathe in; that is expiration. NCERT: inspiration is initiated by contraction of the diaphragm.
Which muscles are active during inspiration?
Two sets: (1) the DIAPHRAGM and (2) the EXTERNAL intercostal muscles (between the ribs). The external intercostals contract and lift the ribs and sternum up and outward, increasing thoracic volume in the dorso-ventral (front-to-back) axis. The internal intercostals are NOT used in normal inspiration; they help in forced expiration.
Is intra-pulmonary pressure higher or lower than atmospheric during inspiration?
LOWER. Because thoracic volume rises, lung (pulmonary) volume rises too. By the volume-pressure rule, when volume goes up the pressure inside the lungs goes DOWN, below atmospheric pressure. This negative pressure (with respect to the atmosphere) is what forces outside air into the lungs. In expiration the reverse is true: pressure becomes slightly higher than atmospheric.
Why does air actually move into the lungs?
Air always moves from high pressure to low pressure. During inspiration the intra-pulmonary pressure becomes less than atmospheric pressure, so a pressure gradient is created. Air outside is now at higher pressure and moves in until pressures balance. You cannot change lung volume directly; you change thoracic volume, and the air-tight thoracic chamber makes the lung follow.
Does lung volume increase or decrease during inspiration?
It INCREASES. The chain is: muscles contract → thoracic volume increases → because the thorax is an air-tight chamber, pulmonary (lung) volume increases the same way → intra-pulmonary pressure decreases → air enters. If a question says 'pulmonary volume decreases during inspiration' it is describing expiration and is wrong.
⚠️ The NEET trap ✗ Inspiration happens when atmospheric pressure is less than intra-pulmonary pressure, and the diaphragm relaxes. ✓ Inspiration happens when intra-pulmonary pressure is LESS than atmospheric pressure, and the diaphragm CONTRACTS (with external intercostals). 🧠 NTA loves reversing the pressure inequality. Air moves IN only when the lung pressure is LOWER than outside. Contraction, not relaxation, starts inspiration.
Real NEET questions
NEET 2016/2018/2019/2020
Select the correct statement.
A · Expiration occurs due to external intercostal muscles
B · Intrapulmonary pressure is lower than the atmospheric pressure during inspiration. ✓
C · Inspiration occurs when atmospheric pressure is less than intrapulmonary pressure.
D · Expiration is initiated due to contraction of diaphragm.
Solution: Inspiration occurs only when intra-pulmonary pressure falls below atmospheric pressure, so air is drawn in (B is correct). Expiration is caused by RELAXATION of the diaphragm and intercostals, not by external intercostals contracting; and inspiration (not expiration) is started by diaphragm contraction. So the other three statements are false.
NEET 2020
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
A · (a), (b) and (d)
B · Only (d)
C · (a) and (b) ✓
D · (c) and (d)
Solution: Inspiration is started by contraction of the diaphragm and the external inter-costal muscles, so (a) and (b) are correct. These contractions INCREASE pulmonary volume and DECREASE intra-pulmonary pressure below atmospheric. So (c) 'volume decreases' and (d) 'pressure increases' both describe expiration and are wrong. Answer: (a) and (b).
Solved Breathing and Exchange of Gases NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Contraction of the diaphragm, which increases the volume of the thoracic chamber and starts drawing air in.
What is intra-pulmonary pressure?
It is the air pressure inside the lungs (alveoli). During inspiration it becomes lower than atmospheric pressure, and during expiration it becomes slightly higher.
Do external or internal intercostals help in inspiration?
EXTERNAL intercostals. They lift the ribs and sternum up and out. Internal intercostals are used mainly in forced expiration.
How much can we control our breathing muscles?
Normal inspiration uses the diaphragm and external intercostals. For a stronger breath in or out we can also use extra abdominal muscles. A healthy adult breathes about 12 to 16 times per minute.
Why is the thoracic chamber described as air-tight for NEET?
Because it is sealed, any change in thoracic volume is passed on directly to lung volume. This is why moving muscles (not the lungs directly) can change the pressure inside the lungs.
