Decrease in arterial PO2 strongly stimulates breathing when PO2 falls below:
Correct option is D, 50 mmHg. In Lung Volumes, Capacities and Regulation, the accurate description is 50 mmHg, and that is what you should commit to memory. Keep the definition tight and the role clear — 50 mmHg is the option that correctly names the structure, process, or principle asked for in Lung Volumes, Capacities and Regulation. Then 50 mmHg is the answer you can defend. Check yourself by covering the options and writing the answer first, then matching the letter — that builds confidence. If the wording feels dense, translate it into everyday language first, then map that plain sentence back onto 50 mmHg. Remember that physiology questions reward precise language: name the process, the location, and the outcome, then match that to the option text.
Ref: Animal physiology is Unit 7 of the CSIR NET Life Science Syllabus, covering core body systems. It accounts for roughly 10% of the total marks in Sections B and C.