This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Anatomical dead space is equivalent to: 
Select and explain. 
A. 1/3rd of tidal volume, B. 2/5th of tidal volume, C. 10 ml/kg of body weight, D. 15 ml/kg of body weight
A: 1/3rd of tidal volume
Answer: A: 1/3rd of tidal volume 
Explanation: Anatomic dead space refers to the volume of the conducting airways and is around 130 - 180ml (2ml/kg throughout life) in the adult i.e about 1/3rd of tidal volume. Physiologic dead space: It refers to the propoion of the tidal volume that does not paicipate directly in gas exchange. It consists of the anatomic dead space and the alveolar dead space. It comprises 25 - 35% of of the normal tidal volume.