Q.Explain the process of Inspiration and Expiration with diagrams.

Inspiration = diaphragm and external intercostals contract → chest expands → pressure in lungs falls → air flows in. Expiration = these muscles relax → chest contracts → pressure in lungs rises → air flows out. (Accompany with labelled diagrams of the thorax.)

Breathing (pulmonary ventilation) is the mechanical process by which air is drawn into (inspiration) and driven out of (expiration) the lungs. It depends on changing the volume, and hence the pressure, of the thoracic (chest) cavity using two sets of muscles — the diaphragm (a dome-shaped muscular sheet below the lungs) and the intercostal muscles (between the ribs). Air always moves from higher to lower pressure.

Figure note: two diagrams should be drawn — one showing inspiration (diaphragm flattened, ribs raised, chest expanded, lungs inflated) and one showing expiration (diaphragm dome-shaped, ribs lowered, chest reduced, lungs deflated), each labelled with: ribs, sternum, external and internal intercostal muscles, lungs, diaphragm and the direction of air flow.

1. Inspiration (inhalation) — an active process:

• The diaphragm contracts and moves downward, becoming flattened. This increases the vertical (top-to-bottom) dimension of the thoracic cavity.

• The external intercostal muscles contract, pulling the ribs and sternum upward and outward. This increases the front-to-back and side-to-side dimensions of the chest.

• As a result, the volume of the thoracic cavity increases, and the lungs expand with it.

• Expansion lowers the pressure inside the lungs (intra-pulmonary pressure) to just below the atmospheric pressure.

• Because the outside air is now at higher pressure, air rushes in through the nostrils, pharynx, larynx, trachea and bronchi into the alveoli of the lungs — this is inspiration.

2. Expiration (exhalation) — normally a passive process:

• The diaphragm relaxes and moves upward, regaining its dome shape, decreasing the vertical dimension of the thorax.
• The external intercostal muscles relax (and during forced expiration the internal intercostals contract), so the ribs and sternum move downward and inward.
• Consequently, the volume of the thoracic cavity decreases and the elastic lungs recoil (shrink). …