Where does the transfer of oxygen into the bloodstream primarily take place during gas exchange in humans?

• A. Esophagus
• B. Trachea
• C. Bronchioles
• D. Alveoli of the lungs

Answer: (D)

Verified step by step guidance

1. Understand the process of gas exchange in humans: Gas exchange primarily occurs in the lungs, where oxygen is transferred into the bloodstream and carbon dioxide is removed.
2. Identify the structure responsible for gas exchange: The alveoli are tiny air sacs in the lungs that are surrounded by capillaries. They provide a large surface area for efficient gas exchange.
3. Explain the mechanism: Oxygen from inhaled air diffuses across the thin walls of the alveoli into the surrounding capillaries, where it binds to hemoglobin in red blood cells. Simultaneously, carbon dioxide diffuses from the blood into the alveoli to be exhaled.
4. Clarify why other options are incorrect: The esophagus is part of the digestive system and does not play a role in gas exchange. The trachea and bronchioles are part of the respiratory system but serve as air passageways rather than sites of gas exchange.
5. Conclude with the correct answer: The alveoli are the primary site of oxygen transfer into the bloodstream during gas exchange in humans.