Zoology - Breathing and Exchange of Gases Question with Solution | TestHub

The muscles of respiration are attached to the rib cage. There are principal and accessory muscles who work to expand and contract the rib cage in order to inhale and exhale. Generally the process of inspiration is active and expiration is passive in nature. The contraction of internal intercostal muscles pushes the rib cage downwards and inwards. This decreases the intrapulmonary volume, increasing the intrapulmonary pressure. This leads to the removal of air from the lungs (expiration). Normal (effortless) expiration is brought about by relaxation of external intercostal muscles and diaphragm.