Question

When the external intercostal muscles are relaxed, the lungs are at
A) Residual volume
B) Expiration phase
C) Total lung capacity
D) None of the above

Answer 1

Respiration occurs in two phases namely inspiration and expiration. During inspiration, the thoracic cage enlarges and the lungs expand so that air enters the lungs easily. During expiration, the thoracic cage and lungs decrease in size and attain the original position so that air leaves the lungs easily.

Complete answer:
During normal quiet breathing, inspiration is the active process and expiration is the passive process. Respiratory muscles are of two types:
1. Inspiratory muscles: Muscles involved in inspiratory movements are known as inspiratory muscles. Inspiratory muscles are the diaphragm and external intercostal muscles.
2. Expiratory muscles: Muscles involved in expiratory movements. Expiratory muscles are the internal intercostal muscles.
External intercostal muscles are found in between the ribs. They are used during inspiration. So, when it relaxes, lungs are at the expiration phase.

Thus the correct answer is option ‘B’.

Note: Contraction of external intercostal muscles causes elevation of these ribs and upward and forward movement of the sternum. The movement of the diaphragm increases the vertical diameter of the thoracic cage.