Question

Which of the following statements truely explains the respiratory tract?
I. The right lung is larger than the left
II. Expiration is predominantly a passive phenomenon
III. Air enters the lungs because of created negative pressure.

• A. $I$ only
• B. $I$ and $II$ only
• C. $II$ and $III$ only
• D. $I,II$ and $III$

Answer 1

Answer: (D)

The two stages of breathing include inspiration and expiration. During inspiration atmospheric air is drawn into the lungs while during expiration alveolar air is released into the atmosphere. It is favourable for inspiration to take place if the intra-pulmonary pressure (pressure within the lungs) is lesser than the atmospheric pressure and for expiration to take place when the intra-pulmonary pressure is higher than the atmospheric pressure. In order to create this condition of negative pressure in the lungs with respect to atmospheric pressure during inspiration, the pulmonary volume is increased by (a) the contraction of the diaphragm which leads to an increase in the volume of the thoracic cavity in the anteroposterior axis (vertical dimension), (b) the contraction of external intercostal muscles which elevate the ribs and sternum, leading to an increase in the volume of the thoracic cavity in the dorso-ventral axis (upward and outward). The rise in the volume of the thoracic cavity causes the lungs to enlarge, which obviously increases the pulmonary volume and consequently, reduces the intrapulmonary pressure as compared to the atmospheric pressure. So, at this point air flows into the lungs from outside i.e. down the pressure gradient from higher to lower pressure.
The pressure gradient during expiration is exactly the opposite of that of inspiration. Expiration starts when the relaxation of the diaphragm and the inter-costal muscles return the diaphragm and the sternum to their normal positions, reduces the volume of the thoracic cavity and thereby the pulmonary volume. This, in turn, leads to a slight increase in intra-pulmonary pressure as compared to the atmospheric pressure, causing the expulsion of air from lungs to the outside, i.e. expiration. Normal expiration during quiet breathing is a passive process as no contraction of muscles is involved, unlike inspiration which involves contraction of muscles and is an active process. Another way of justifying expiration is a passive phenomenon is that it involves the elastic recoil of the lungs to a smaller size when air moves out of them. Expiration can become active only at the time of forceful breathing, during which the abdominal and internal intercostal muscles are involved. So, the statements II and III are factually correct.
The right lung has three lobes and two fissures as opposed to the left lung which has two lobes and one fissure. Therefore, the right lung is broader, larger, and heavier compared to the narrower, smaller, and lighter left lung. So, the statement I is also factually correct.