Q.Explain the process of inspiration under normal conditions.

Inspiration is the drawing of outside air into the lungs, and under normal conditions it works by enlarging the thoracic chamber so that air is pulled in along a pressure gradient.

• The diaphragm contracts, which increases the volume of the thoracic chamber along its front-to-back axis.
• The external intercostal muscles contract, lifting the ribs and the sternum and increasing the thoracic volume along the back-to-front axis.
• Because the thoracic chamber is air-tight, this rise in thoracic volume raises the volume of the lungs in the same way.
• The larger lung volume lowers the pressure inside the lungs below the atmospheric pressure. …

Inspiration is the first stage of breathing — the process by which fresh atmospheric air is pulled into the lungs. The whole thing rests on a simple principle: air always flows from a region of higher pressure to one of lower pressure. So to bring air in, the body has to make the pressure inside the lungs fall below the pressure of the outside air.

Here lies an important constraint. We cannot squeeze or stretch the lungs directly. Instead, the lungs sit inside an air-tight box, the thoracic chamber, so any change we make to the volume of that box is passed straight on to the lungs. Enlarging the chest therefore enlarges the lungs.

The enlargement is produced by two sets of muscles working together:

• The diaphragm, the dome-shaped sheet of muscle below the lungs, contracts and flattens. This increases the volume of the thoracic chamber along its front-to-back (antero-posterior) axis.
• The external intercostal muscles, lying between the ribs, contract and lift the ribs and the sternum upward and outward. This increases the thoracic volume along its back-to-front (dorso-ventral) axis.

Method: Explaining a Phase of the Breathing Cycle Using the Pressure-Volume Rule

Use this structure for any question asking you to explain inspiration or expiration mechanistically.

Steps

Step 1: State the governing rule first

Pressure inside a sealed chamber falls when its volume rises, and gas always flows from higher to lower pressure. Every later step of the explanation should trace back to this one rule.

Step 2: Name the muscles and their exact effect

Identify the muscles active in the phase being explained (the diaphragm and external intercostals for inspiration) and state precisely which way each one changes the thoracic volume. …