It is necessary that lungs must be present in anatomically air-tight chamber because:
1. we cannot directly alter the pulmonary volume
2. our lungs are not made of any elastic tissue
3. the pleural cavity always has a positive pressure with respect to pulmonary pressure
4. the partial pressure of oxygen in atmosphere is less than that in the alveoli

During expiration, the diaphragm and external intercostal muscles are:

1. contracted

2. relaxed

3. flexed

4. both relaxed (diaphragm) and flexed (intercostal muscles)

When the diaphragm or external intercostal muscles contract, intrapulmonary pressure should

The volume of thoracic chamber in the antero-posterior axis can be increased by:
1. Contraction of diaphragm
2. Relaxation of diaphragm
3. Contraction of external intercostals
4. Contraction of internal intercostals