The actual sites of gas exchange within the lungs are the
bronchioles.
terminal bronchioles.
spaces between the parietal and visceral pleura.
blood air barrier of the alveoli.
interlobular septa.
The Correct Answer and Explanation is:
Correct Answer: Blood air barrier of the alveoli
Explanation
The actual sites of gas exchange within the lungs are the blood air barrier of the alveoli. This is where oxygen from inhaled air diffuses into the blood, and carbon dioxide from the blood diffuses out into the alveoli to be exhaled.
The alveoli are tiny, balloon-like air sacs located at the end of the respiratory tree. Each human lung contains about 300 million alveoli, providing a vast surface area (~70 square meters) for gas exchange. Alveoli are surrounded by an extensive network of pulmonary capillaries. The walls of the alveoli and the capillaries are extremely thin and are composed of a single layer of epithelial and endothelial cells, respectively. These layers, along with a thin interstitial space between them, make up the blood air barrier.
The blood air barrier is crucial because it allows for efficient diffusion of gases while maintaining a separation between air and blood. Oxygen must pass through the alveolar epithelium, the interstitial fluid, and the capillary endothelium to enter the bloodstream. Similarly, carbon dioxide moves in the opposite direction for exhalation.
The other options listed do not play a direct role in gas exchange:
- Bronchioles and terminal bronchioles are airways that conduct air to the alveoli but do not themselves participate in gas exchange.
- The spaces between the parietal and visceral pleura (pleural cavity) contain pleural fluid, which reduces friction during breathing but has no role in gas exchange.
- The interlobular septa separate different lobules in the lungs and support lung structure but are not involved in gas exchange.
Therefore, the blood air barrier of the alveoli is the site where vital gas exchange between the respiratory and circulatory systems occurs.
