In human, which one among the following, with reference to breathing, is correct ?

During inhalation, the diaphragm and external intercostal muscles contract, causing the thoracic cavity to expand [4]. This expansion increases the volume of the pleural cavity, which, according to Boyle's Law, leads to a decrease in pressure [3]. Consequently, the intrapleural pressure, which is already negative relative to atmospheric pressure to keep the lungs open, becomes even more negative (sub-atmospheric) during the inspiratory phase [5]. This drop in pressure creates a gradient that allows air to be sucked into the lungs [4]. Option 1 is incorrect because the diaphragm contracts and flattens during inhalation [1]. Option 2 is incorrect because the thoracic cavity volume decreases during exhalation as the ribs move down and the diaphragm relaxes upwards [1]. Option 4 is incorrect because normal (quiet) exhalation is a passive process driven by elastic recoil, whereas inhalation is the active phase [5].

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 9: Life Processes in Animals > Activity 9.2: Let us make a model > p. 130
2. [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC9237333/
3. [3] https://training.seer.cancer.gov/anatomy/respiratory/mechanics.html
4. [5] https://www.sciencedirect.com/topics/medicine-and-dentistry/mechanics-of-breathing
5. [2] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Do You Know? > p. 90