Carefully study the diagram of the human respiratory system with labels A, B, C and D. Select the option which gives correct identification and main function and /or characteristic.
(A) (i) Trachea: It is supported by bony rings for conducting inspired air.
(B) (ii) Ribs: When we breathe out, ribs are lifted.
(C) (iii) Alveoli: Thin-walled sac-like structures for exchange of gases.
(D) (iv) Diaphragm: It is pulled up when we breathe in.
In the figure:
(i) is Trachea
The trachea serves as passage for air, moistens and warms it while it passes into the lungs , and protects the respiratory surface from an accumulation of foreign particles.
(ii) is Ribs
The ribs are the bony framework of the thoracic cavity. The ribs form the main structure of the thoracic cage protecting the thoracic organs, however their main function is to aid respiration .
(iii) is Alveoli.
The alveoli provide a surface where the exchange of gases can take place . When we breathe in air, we lift our ribs and flatten our diaphragm, and the chest cavity becomes larger as a result.
(iv) is Diaphragm
Diaphragm is a dome-shaped muscle that contracts rhythmically and continually, and most of the time, involuntarily. Upon inhalation, the diaphragm contracts and flattens and the chest cavity enlarges .
So, the correct answer is (c) - Alveoli: Thin-walled sac-like structures for exchange of gases.