What factors determine the volume of oxygen entering lung capillary blood?

• A. Heart rate, blood glucose level, alveolar moisture, and humidity.
• B. Alveolar ventilation, respiratory minute volume, surface area of the membrane, and the oxygen pressure gradient.
• C. Age, sex, height, and body mass index.
• D. Genetic factors and stress levels.

Answer: (B)

The volume of oxygen entering lung capillary blood is governed by diffusion across the alveolar–capillary membrane, driven by the partial pressure gradient. The best answer includes four interrelated factors: alveolar ventilation (how well air reaches the alveoli), respiratory minute volume (the total air moved per minute), the surface area of the alveolar–capillary membrane (more area means more space for diffusion), and the oxygen pressure gradient between alveolar air and capillary blood (the driving force for diffusion). Together, these determine how much O2 crosses into the blood per unit time: adequate ventilation keeps alveolar PO2 high and maintains a strong gradient, sufficient surface area allows more diffusion to occur, and a larger gradient speeds transfer.

Other factors listed don’t directly control this diffusion process. For instance, heart rate or blood glucose don’t set the amount of O2 that diffuses across the alveolar membrane, and while age, sex, height, or BMI influence overall lung capacity, they’re not the immediate determinants of diffusion rate. Genetic factors and stress levels aren’t direct drivers of O2 transfer across the membrane.