Q.Explain the mechanism of breathing with neat labelled sketches.

Breathing has two stages driven by pressure differences created between the lungs and the atmosphere. During inspiration, the diaphragm contracts and flattens, increasing thoracic volume front-to-back, while the external intercostal muscles contract to lift the ribs and sternum, increasing thoracic volume side-to-side; because the thoracic chamber is air-tight, this raises pulmonary volume and lowers intra-pulmonary pressure below atmospheric pressure, so air is drawn in. During expiration, the diaphragm and intercostal muscles simply relax, the diaphragm domes back upward and the ribs/sternum fall back, reducing thoracic and pulmonary volume, which raises intra-pulmonary pressure above atmospheric pressure and pushes air out. A labelled sketch would show the diaphragm in its lower, flattened position with the ribs raised during in …

Breathing is the mechanical process of moving air into and out of the lungs, and it depends on creating a pressure difference between the lungs and the atmosphere. This pressure difference is produced by the diaphragm together with the external and internal intercostal muscles.

Inspiration. The diaphragm contracts, flattening from its resting dome shape and increasing the volume of the thoracic chamber along the front-to-back (antero-posterior) axis. At the same time, the external intercostal muscles contract, lifting the ribs and the sternum outward and upward, which increases thoracic volume along the dorso-ventral axis as well. Because the thoracic chamber is air-tight, this overall rise in thoracic volume is passed on directly to the lungs, and the larger pulmonary volume lowers the intra-pulmonary pressure below atmospheric pressure — so air is drawn in from outside. …

Method: Explaining a two-phase active/passive cyclic mechanism

This method applies to any question asking you to explain a repeating cycle made up of one active (muscle-driven) phase and one passive (muscle-relaxation) phase, such as the breathing cycle.

Steps

Step 1: Identify which phase is active and which is passive

First establish which half of the cycle involves muscles actively contracting, and which half is simply those same muscles relaxing and letting the system return to its resting state — this distinction structures the whole explanation.

Step 2: For the active phase, describe the muscle actions and the volume/pressure change they cause

Name the specific muscles that contract, describe the physical change each produces (e.g., a muscle flattening or lifting a structure), and connect that physical change to a change in chamber volume — then apply the pressure-volume rule (Boyle's Law) to say what happens to pressure as a result.

Step 3: For the passive phase, describe the reverse

Explain that the same muscles simply relax, the structures they had moved return to their resting positions, chamber volume falls back, and pressure rises again — driving the flow in the opposite direction.

Step 4: Represent both phases as a labelled sketch …