What happens to oxygen once it enters the bloodstream?

• A. It is dissolved directly in plasma
• B. It binds with hemoglobin
• C. It is converted to carbonic acid
• D. It remains in the alveoli

Answer: (B)

Once oxygen enters the bloodstream, it primarily binds with hemoglobin, a protein found in red blood cells. This binding is crucial because hemoglobin significantly increases the amount of oxygen that can be transported through the blood compared to what could be carried dissolved in plasma alone. Hemoglobin has a high affinity for oxygen, allowing it to pick up oxygen molecules in the lungs and release them in tissues where they're needed. This respiratory gas exchange is essential for maintaining aerobic metabolism and supporting cellular functions throughout the body.

While oxygen can be dissolved in plasma to a small extent, this method is not the primary means of transport due to its limited capacity. Oxygen is not converted to carbonic acid upon entering the bloodstream; that process occurs as carbon dioxide is transported, which is different from how oxygen is handled. Additionally, oxygen does not remain in the alveoli, as its primary role is to move into the bloodstream for distribution to body tissues.