Question

During inspiration, the diaphragm
A. Relaxes to become dome-shaped
B. Contracts and flattens
C. Expands
D. Shows no change

Answer 1

Hint: The action of the inspiratory muscles results in an increase in the volume of the thoracic cavity. As the lungs are held against the inner thoracic wall by the pleural seal, they also undergo an increase in volume.

Complete Answer:
- The lungs are situated in the thoracic chamber which is anatomically an air-tight chamber. The thoracic chamber consists of the vertebral column dorsally, the sternum the lateral side, the ribs laterally and the dome-shaped diaphragm at the bottom.

- The process of breathing involves inspiration through which atmospheric air is drawn in and expiration in which the alveolar air is pushed out. Air flow into and out of the lungs occurs by generating a pressure gradient with the lungs and the atmosphere. The diaphragm and a complex group of muscles external and internal intercostals between the ribs, help to generate such gradients.

- Inspiration begins by contraction of the diaphragm that raises the volume of the thoracic chamber in the antero-posterior axis. The ribs and the sternum rises as the external intercostal muscles contract leading to a rise in the volume of the thoracic chamber in the dorso-ventral axis. A similar rise in pulmonary volume is caused by the overall increase in the thoracic volume. The intrapulmonary pressure becomes less than the atmospheric pressure which causes air to move into the lungs.

- The relaxation of the diaphragm and the intercostal muscles causes the diaphragm and the sternum to return to their normal positions and reduces the volume of the thoracic and thus the pulmonary volume. This leads to an increase in intrapulmonary pressure that is slightly above the atmospheric pressure causing air to be expelled from the lungs.

Thus, the correct answer is option B, i.e., contracts and flattens.

Note: Inspiration may occur if the pressure inside the lungs (intrapulmonary pressure) is lower than the ambient pressure, i.e. there is a negative pressure in the lungs compared to the atmospheric pressure. Similarly, expiration occurs when the intrapulmonary pressure is more than the ambient pressure.