This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System Pa 2, Subject name: Physiology
The presence of hemoglobin in normal aerial blood increases its oxygen concentration approximately how many times? 
Select and explain. 
A. 10, B. 30, C. 50, D. 70
D: 70
Answer: D: 70 
Explanation: Normal aerial blood has a PO2 of about 100 mm Hg. The concentration of oxygen in the absence of hemoglobin is the dissolved oxygen, which is 100 x 0.003, or 0.3 ml O2/100 ml blood. However, normal aerial blood contains about 15 g/100 ml of hemoglobin, and each gram can combine with 1.39 ml O2. Since the oxygen saturation of normal aerial blood is about 97%, the total oxygen concentration is given by (1.39 x 15 x 97/100) + 0.3 ml O2/100 ml blood. This is about 20.5 as opposed to the dissolved oxygen concentration of 0.3 ml O2/100 ml blood. Therefore, the presence of hemoglobin increases the oxygen concentration about 70 times