Mechanism of Breathing (Inspiration & Expiration)

NCERT grounding

NCERT Class 11 Biology, Chapter 14, Section 14.2 (Mechanism of Breathing) places this subtopic immediately after the anatomy of the human respiratory system. The text defines breathing as a two-stage process — inspiration, during which atmospheric air is drawn into the lungs, and expiration, during which alveolar air is released out — and emphasises that air movement is driven entirely by a pressure gradient between the lungs and the atmosphere. It names the diaphragm and the two sets of intercostal muscles (external and internal) as the prime movers, and ends by noting that a healthy adult breathes about 12–16 times per minute. The NIOS lesson on pulmonary respiration adds two crucial pieces that NEET frequently exploits: that quiet inspiration is an active phase while quiet expiration is largely passive, and that forced breathing recruits the internal intercostals and abdominal muscles.

"Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure… Similarly, expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure."

— NCERT Biology XI, §14.2

How a breath happens

Lungs cannot pump themselves. They are elastic, passive bags suspended inside an air-tight thoracic chamber, and the only way to move air into or out of them is to change the volume of the chamber that surrounds them. Because the parietal pleura is anchored to the chest wall and the visceral pleura is glued (by a thin film of pleural fluid) to the lung surface, the lung is forced to follow whatever the thoracic cage does. Enlarge the thorax and the lung must enlarge with it; shrink the thorax and the lung must shrink. This is the central architectural fact behind every NEET question on breathing.

The link between thoracic volume and air flow is Boyle's law. At constant temperature the pressure exerted by a fixed amount of gas is inversely proportional to its volume — symbolically, P is proportional to 1/V. The lungs contain a fixed quantity of air at any instant. When the thorax expands and lung volume rises, the same quantity of gas now occupies a bigger space, so its pressure (the intra-pulmonary pressure) falls below the atmospheric pressure. Air then flows down its pressure gradient into the alveoli. When the thorax shrinks, the same gas is compressed into a smaller volume, intra-pulmonary pressure climbs above atmospheric pressure, and air is pushed out. NEET typically tests Boyle's law not by name but by asking what happens to intra-pulmonary pressure during one phase or the other.

Inspiration — the active phase

Inspiration is always active because at least two muscle groups must contract to enlarge the chest. The diaphragm is a dome-shaped sheet of skeletal muscle that separates the thoracic cavity from the abdomen. At rest, it bulges upward into the thoracic cavity. When its motor fibres fire, it contracts and flattens, dropping the dome downward and increasing thoracic volume along the antero-posterior (head-to-foot) axis. NCERT states this explicitly: contraction of the diaphragm increases the volume of the thoracic chamber in the antero-posterior axis. The diaphragm alone accounts for roughly two-thirds of the volume change in a normal breath.

Simultaneously, the external intercostal muscles, which run obliquely downward and forward between adjacent ribs, contract. Their geometry is such that contraction lifts the ribs and the sternum upward and outward — the so-called "pump-handle" and "bucket-handle" movements. This increases thoracic volume in the dorso-ventral (front-to-back) axis. Together, diaphragmatic descent and rib elevation enlarge the thoracic cage in two perpendicular directions at once. The lungs, glued to the chest wall by pleural cohesion, expand in step. Intra-pulmonary pressure falls about 1–3 mm Hg below atmospheric pressure during quiet inspiration, and roughly 500 mL of atmospheric air rushes in.

Expiration — passive at rest, active when forced

Quiet expiration costs the body almost no energy. It is the rebound phase: the diaphragm relaxes and springs back into its dome shape, the external intercostals relax and the ribs and sternum drop to their resting positions under gravity and elastic recoil, and the stretched lung tissue and rib-cage cartilages recoil inward. Thoracic volume falls, intra-pulmonary pressure climbs about 1–3 mm Hg above atmospheric pressure, and the same 500 mL of air leaves. Crucially, no expiratory muscle contracts during normal resting expiration — this is the single most testable distinction in this subtopic. NCERT's exact phrasing — "Relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions" — has been recycled almost verbatim into NEET stems.

Forced expiration is a different animal. When you cough, blow up a balloon, exhale during heavy exercise, or play a wind instrument, the body actively shrinks the thorax beyond its resting position. Two muscle groups now switch on. The internal intercostal muscles run perpendicular to the externals, and their contraction pulls the ribs downward and inward — the opposite direction to inspiration. The abdominal muscles (rectus abdominis, internal and external obliques, transversus abdominis) contract and push the abdominal viscera upward against the relaxed diaphragm, driving the diaphragm dome higher into the thorax than its resting position. Together they squeeze out the expiratory reserve volume — roughly 1,000–1,100 mL of additional air on top of the tidal 500 mL.

The pressure story — three pressures, one breath

Three pressures need to be kept separate. Atmospheric pressure is the constant reference at sea level, about 760 mm Hg. Intra-pulmonary (alveolar) pressure is the pressure inside the alveoli; it swings on either side of atmospheric pressure during each breath — negative (sub-atmospheric) during inspiration, positive (super-atmospheric) during expiration, and exactly equal to atmospheric pressure at the end of each phase when no air is flowing. Intra-pleural pressure is the pressure inside the thin film of pleural fluid between the visceral and parietal pleurae. It is always sub-atmospheric in a healthy individual — roughly −4 mm Hg at end-expiration and roughly −7 mm Hg at end-inspiration — because the lung's inward elastic recoil and the chest wall's outward spring continuously try to pull the two pleural surfaces apart, and the cohesive pleural fluid resists that pull.

The persistently negative pleural pressure is what keeps the lungs glued to the chest wall and prevents alveolar collapse. If a sharp object punctures the pleural seal — a clinical pneumothorax — air enters the pleural cavity, pleural pressure equalises with atmospheric pressure, and the lung on that side collapses inward under its own elastic recoil. NEET has not (yet) directly examined pleural pressure values, but understanding why pleural pressure stays subatmospheric is the cleanest explanation for why "any change in the volume of the thoracic cavity will be reflected in the lung cavity" — a phrase NCERT places verbatim in §14.1.1.

The numbers a NEET candidate must memorise

Three numbers anchor every numerical question on breathing. The respiratory rate of a healthy resting adult is 12–16 breaths per minute. The tidal volume — the volume of air inspired or expired during one normal breath — is approximately 500 mL. Multiplying the two yields the resting minute ventilation, which NCERT places at 6,000–8,000 mL per minute (i.e. 6–8 L/min). NEET 2020 Q.5 and NEET 2018 Q.171 both pivot on the candidate knowing these values without hesitation; mis-reading "tidal" as "inspiratory reserve" is the most common trap.

What controls the muscles — a brief bridge

The mechanical sequence above is rhythmic because a network of neurons in the medulla — the respiratory rhythm centre — fires phrenic-nerve impulses to the diaphragm and intercostal-nerve impulses to the external intercostals roughly 12–16 times per minute. A second cluster in the pons, the pneumotaxic centre, can shorten inspiration and thereby raise the breathing rate. A chemosensitive area next to the rhythm centre, plus peripheral receptors in the aortic arch and carotid artery, monitor blood CO₂ and H⁺ and feed back to the rhythm centre. Oxygen plays a minor role in this regulation. The mechanism of breathing on this page is the output of that control system — the actual muscle action. The control side is treated in its own subtopic.

One implication worth noting: the diaphragm and intercostals are skeletal muscles, so breathing is technically voluntary. You can override it briefly — hold your breath, hyperventilate, sing — but rising CO₂ very quickly forces the rhythm centre to reassert control. That is why "you cannot hold your breath until you die" is biologically accurate: the chemosensitive feedback wins.

Worked examples

Common confusion & NEET traps