During inspiration

Options

A. Diaphragm and external intercostal muscles relax
B. Diaphragm and internal intercostal muscles relax
C. Diaphragm and external intercostal muscles contract
D. Diaphragm and internal intercostal muscles contract

Answer

C

Explanation

Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the anterior-posterior axis.
During inspiration diaphragm contract and become tiat.
The contraction of external inter-coastal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis. The overall increase in the thoracic volume causes a similar increase in pulmonary volume.
An increase in pulmonary volume decreases the intra pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs, i.e. inspiration.