Question

Choose the correct answer from the alternatives given.
Which of the following sequences of events best describes inspiration?

• A. Contraction of diaphragm and external intercostals $\to$ increase in size of thoracic cavity $\to$ decrease in thoracic and alveolar pressure
• B. Relaxation of diaphragm and external intercostals $\to$ decrease in size of thoracic cavity $\to$ increase in thoracic and alveolar pressure
• C. Relaxation of diaphragm and external intercostals $\to$ increase in size of thoracic Cavity $\to$ increase in thoracic and alveolar pressure
• D. Contraction of diaphragm and external intercostals $\to$ decrease in size of thoracic cavity $\to$ decrease in thoracic and alveolar pressure

Answer 1

The correct option is A Contraction of diaphragm and external intercostals $\to$ increase in size of thoracic cavity $\to$ decrease in thoracic and alveolar pressure

The events of inspiration are: Inspiratory muscles contract and diaphragm descends, rib cage rises. Following this the thoracic cavity volume increases, plural membranes move as thoracic cavity moves, so intrapulmonary volume increases and the intrapulmonary (alveolar) pressure drops. As a result, air rushes in and fills the lungs. So, the correct option is 'Contraction of diaphragm and external intercostals $\to$ increase in size of thoracic cavity $\to$ decrease in thoracic and alveolar pressure'.