Which of the following sequenes is correct to intite inspiration ?
(i) the contraction of external intercostal muscles raises the ribs and sternum
(ii) Volume of thorax increases in the dorso-ventral axis
(iii) intrapulmonary pressure decreases
(iv) Diaphragm contraction
(v) Air rushes into lungs
(vi) Volume of thorax increases in the anterior-posterior axis

Options

A. `(i), (ii), (iv),(v),(iii), (vi)`
B. `(i), (ii), (iii), (iv), (vi), (v)`
C. `(i), (ii), (iv), (vi), (iii), (v)`
D. `(vi), (v), (i), (ii), (iii), (iv)`

Answer

To determine the correct sequence of events that initiate inspiration, we can break down the process step by step:

1. **Contraction of External Intercostal Muscles**: The process of inspiration begins with the contraction of the external intercostal muscles. This contraction pulls the ribs upward and outward.

2. **Raising of Ribs and Sternum**: As the external intercostal muscles contract, the ribs and sternum are raised, which contributes to an increase in the volume of the thoracic cavity.

3. **Increase in Volume of Thorax**: The raising of the ribs and sternum leads to an increase in the volume of the thoracic cavity. This increase occurs in both the dorso-ventral (front to back) and anterior-posterior (top to bottom) axes.

4. **Diaphragm Contraction**: Simultaneously, the diaphragm, which is a dome-shaped muscle at the base of the thoracic cavity, contracts and moves downward. This further increases the volume of the thoracic cavity.

5. **Decrease in Intrapulmonary Pressure**: As the volume of the thoracic cavity increases, the pressure inside the lungs (intrapulmonary pressure) decreases. This pressure drop creates a vacuum effect.

6. **Air Rushes into Lungs**: Finally, due to the decreased intrapulmonary pressure, air rushes into the lungs to equalize the pressure difference between the atmosphere and the lungs.

Now, let's arrange these steps in the correct order based on the original statements provided:

1. (i) The contraction of external intercostal muscles raises the ribs and sternum.
2. (iv) Diaphragm contraction.
3. (ii) Volume of thorax increases in the dorso-ventral axis.
4. (vi) Volume of thorax increases in the anterior-posterior axis.
5. (iii) Intrapulmonary pressure decreases.
6. (v) Air rushes into lungs.

Thus, the correct sequence is: (i), (iv), (ii), (vi), (iii), (v).