OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

During gas exchange, what happens to carbon dioxide?

1. It is produced and stored

2. It diffuses into the alveolus

3. It combines with oxygen

4. It is expelled through digestion

The correct answer is: It diffuses into the alveolus

During gas exchange in the lungs, carbon dioxide plays a critical role as it diffuses from the bloodstream into the alveoli. This process occurs because of the differences in concentration between the carbon dioxide in the blood and the air in the alveoli. Blood returning from the body's tissues is rich in carbon dioxide, which is a byproduct of cellular respiration. As this blood passes through the capillaries surrounding the alveoli, carbon dioxide diffuses across the alveolar membrane into the alveoli, where its concentration is lower. This diffusion is essential for maintaining proper gas levels in the blood and facilitating the removal of carbon dioxide from the body, allowing for efficient oxygen uptake in return. Consequently, the process of carbon dioxide diffusing into the alveolus is a fundamental aspect of respiratory physiology, ensuring the body can effectively rid itself of excess carbon dioxide while absorbing oxygen. The other options do not accurately describe what happens to carbon dioxide during gas exchange. While carbon dioxide is indeed produced during metabolism, it does not combine with oxygen during gas exchange or get expelled through digestion, which is a completely separate process involving the digestive system.