This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
Barometric pressure at 10,000 feet deep in the sea is 
Select and explain. 
A. 760 mmHg, B. 523 mmHg, C. 349 mmHg, D. 226 mmHg
B: 523 mmHg
Answer: B: 523 mmHg 
Explanation: The approximate barometric at sea level, the barometric pressure is 760 mm Hg; at 10,000 feet, it is only 523 mm Hg; and at 50,000 feet, it is 87 mm Hg. This decrease in barometric pressure is the basic cause of all the hypoxia problems in high-altitude physiology because, as the barometric pressure decreases, the atmospheric oxygen paial pressure (PO2) decreases propoionately, remaining at all times slightly less than 21 percent of the total barometric pressure; At sea level, PO2 is about 159 mm Hg, but at 50,000 feet, PO2 is only 18 mm Hg.Ref: Guyton; 13th edition; Chapter 44; Ation, High Altitude, and Space Physiology