Normal Inspiration Process — NEET Biology

Correct answer: A — S3 and S4

The core concept tested here is the mechanism of breathing, involving the roles of muscles, volume changes, and pressure gradients during inspiration and expiration. S1: During normal expiration, the diaphragm and inter-costal muscles relax, returning to their normal positions. This reduces the thoracic volume and, consequently, the pulmonary volume. The reduction in volume increases the intra-pulmonary pressure to slightly above atmospheric pressure, causing air expulsion. Thus, S1 is correct. S2: Forced expiration is an active process involving the contraction of additional muscles in the abdomen (abdominal muscles) and internal inter-costal muscles, which helps to further decrease the thoracic volume and forcefully expel more air. Thus, S2 is correct. S3: Inspiration is initiated by the contraction of the diaphragm, which increases thoracic volume in the antero-posterior axis. The contraction of external inter-costal muscles lifts the ribs and sternum, increasing thoracic volume in the dorso-ventral axis. The statement incorrectly attributes the increase in antero-posterior axis volume solely to external inter-costal muscles. Thus, S3 is NOT correct. S4: The movement of air into and out of the lungs is carried out by creating a pressure gradient. Inspiration occurs when the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure (negative pressure). The statement incorrectly states that inspiration occurs when intra-pulmonary pressure is higher than atmospheric pressure. Thus, S4 is NOT correct. S5: Under normal conditions, expiration is indeed a passive process. It does not involve the active contraction of muscles but relies on the relaxation of the inspiratory muscles (diaphragm and external intercostals) and the elastic recoil of the lungs and rib cage. Thus, S5 is correct. Therefore, S3 and S4 are the statements that are NOT correct. The final answer is A