This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System, Subject name: Physiology
Least amount of co2 is in 
Select and explain. 
A. Anatomical dead space-end inspiration phase, B. Anatomical dead space-end expiration phase, C. Alveoli-end inspiration phase, D. Alveoli-end expiration phase
A: Anatomical dead space-end inspiration phase
Answer: A: Anatomical dead space-end inspiration phase 
Explanation: Ans. (a) Anatomical dead space-end inspiration phase(Ref: Ganong, 25th ed/p.630)Anatomical dead space containing atmospheric air at the end of inspiration will have the least amount of CO2 - because atmospheric air has negligible CO2 Anatomical dead space-end expiration phase: Here alveolar gas rich in CO2 occupies the Anatomical dead space dead space areaAlveolar air contains a mixture of O2 from the inspired air and CO2 which is continuously added to it to exhale out