Biology · Breathing and Exchange of Gases · NEET
It INCREASES. During inspiration the diaphragm contracts and moves down, and the external intercostal muscles pull the ribs up and out. Both actions make the thoracic cavity bigger, so pulmonary (lung) volume increases. A common NEET trap says 'pulmonary volume decreases' during inspiration — that is wrong; volume decreasing belongs to expiration.
Because when lung volume increases, the intrapulmonary (inside-lung) pressure falls BELOW the atmospheric pressure outside. Air always flows from high pressure to low pressure, so outside air rushes in until the pressures become equal. No pump sucks the air — the pressure difference does the work.
Normal (quiet) expiration is PASSIVE. The diaphragm and external intercostal muscles simply relax and return to their resting position, so chest volume drops, intrapulmonary pressure rises above atmospheric pressure, and air leaves. No muscle contraction is needed for calm breathing. Forced expiration (like blowing hard) is active and uses extra muscles such as the internal intercostals.
They are INVERSELY related (Boyle's law idea): when the chest volume goes up, the pressure inside the lungs goes down, and vice versa. Inspiration = volume up, pressure down. Expiration = volume down, pressure up. If you remember this one line you can answer most breathing-mechanism questions.
The change is small but decisive. During inspiration intrapulmonary pressure falls slightly below atmospheric pressure (a small negative difference) so air enters; during expiration it rises slightly above atmospheric pressure so air leaves. NCERT stresses the DIRECTION of the difference, not exact numbers — always compare lung pressure to the atmosphere.
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 Choose the correct answer from the options given below:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Pressure differences between the lungs and the atmosphere. Muscles change chest volume, volume changes intrapulmonary pressure, and air flows from high pressure to low pressure until the two are equal.
The diaphragm (contracts and flattens downward) and the external intercostal muscles (lift the ribs up and out). Together they enlarge the thoracic cavity.
Because quiet expiration is passive: the inspiratory muscles just relax and elastic recoil of the lungs and chest reduces the volume, raising pressure and pushing air out. No active contraction is needed.
It is the pressure of the air inside the lungs (alveoli). When it is below atmospheric pressure, air enters (inspiration); when above, air leaves (expiration).
