The correct option is
B. To facilitate the exchange of gases between air and blood.
Detailed Explanation:
- Understanding Alveoli:
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Alveoli are tiny, balloon-like structures located at the end of the bronchioles in the lungs. They are surrounded by a network of capillaries (small blood vessels) and are the primary site for gas exchange in the respiratory system.
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Gas Exchange Process:
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When we inhale, air enters the alveoli, where oxygen from the air diffuses across the thin walls of the alveoli into the blood in the capillaries. Simultaneously, carbon dioxide (a waste product of metabolism) diffuses from the blood into the alveoli to be exhaled. This process is driven by the concentration gradient of gases (oxygen and carbon dioxide) between the air in the alveoli and the blood in the capillaries.
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Why Option B is Correct:
- The primary function of the alveoli is indeed to facilitate this exchange of gases. The large surface area provided by millions of alveoli, along with their thin walls, maximizes the efficiency of gas exchange, making it the most critical function of these structures.
Why the Other Options are Incorrect:
- Option A: To transport oxygen to the bloodstream:
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This option is misleading because the alveoli do not transport oxygen; they are the site where oxygen enters the bloodstream. The actual transport of oxygen is carried out by red blood cells, which pick up oxygen from the alveoli and carry it throughout the body.
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Option C: To produce mucus for trapping pathogens:
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While mucus is important in the respiratory system for trapping pathogens and particles, it is primarily produced by goblet cells and mucous glands in the airways, not by the alveoli. The alveoli are designed for gas exchange, not mucus production.
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Option D: To regulate blood pH levels:
- Although the respiratory system plays a role in regulating blood pH through the control of carbon dioxide levels (since CO2 can form carbonic acid in the blood), this is not the primary function of the alveoli. The alveoli are specifically adapted for gas exchange, not for direct regulation of blood pH.
Summary of Key Points:
- Alveoli are the primary site for gas exchange in the lungs.
- They allow oxygen to diffuse into the blood and carbon dioxide to diffuse out.
- The large surface area and thin walls of alveoli enhance the efficiency of gas exchange.
- Other options (A, C, D) misrepresent the functions of alveoli or attribute functions to them that belong to other parts of the respiratory system.
This understanding of alveoli is crucial for grasping how the respiratory system functions as a whole and its role in maintaining homeostasis in the body.