Mechanics of Respiration and Lung Volumes — NEET Biology

Correct answer: B — S2, S3 and S5

The core concept tested here is the mechanism of inspiration and expiration, including muscle actions, volume changes, and pressure gradients. S1: The contraction of diaphragm increases the volume of the thoracic chamber in the antero-posterior axis, initiating inspiration. This statement is correct. S2: Forceful expiration involves the contraction of additional muscles like the internal intercostals and abdominal muscles, which actively decrease thoracic volume beyond passive relaxation. It is not achieved solely by relaxation of the inspiratory muscles. This statement is NOT correct. S3: Inspiration occurs when the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, creating a negative pressure gradient. This statement is NOT correct. S4: The movement of air into and out of the lungs is indeed carried out by creating a pressure gradient between the lungs and the atmosphere. This statement is correct. S5: Normal expiration is a passive process resulting from the relaxation of the diaphragm and external intercostal muscles, which returns the thoracic volume to its normal position. Active contraction of internal intercostal muscles occurs during *forceful* expiration, not normal expiration. This statement is NOT correct. Therefore, statements S2, S3, and S5 are not correct. The final answer is B because S2, S3, and S5 are incorrect based on the NCERT description of breathing mechanics.