Ans. is 6b' i.e., 150 mlo In healthy individuals, physiological (total) dead space is equal to anatomica dead space, which is about 150 ml.Anatomical dead spaceo When air is breathed in, only the air that reaches the alveoli participates in gas exchange. The air in the airways (Trachea, bronchi) is breathed out unchanged as air in them does not participate in gaseous diffusion. Hence the airway space behaves as functionally dead so far as gas exhange is concerned. This is called 'anatomical dead space'. The volume of this space is 150 ml. That means only the first 350 ml of the 500 ml inspired with each breath (tidal volume) at rest mixes with the air in the alveoli. Conversely, with each expiration, the first 150 ml expired is gas that occupied the dead space, and only the last 350 ml is gas from the alveoli. So, normally dead space (Vd): tidal volume (Vt) ratio is 0[?]30 (150 : 500).Physiological dead space (total dead space)o Normally, 350 ml of air enters the alveoli, with each breath. However, not all the air that enters the alveoli necessarily participates in alveolar gas exchange. In disease states, no diffusion (exchange) may take place between the gas in some of the alveoli and blood, and some of the alveoli may be overventilated. Such spaces in the alveoli constitues 'alveolar' dead space. The sum of anatomical dead space and alveolar dead space is called the 'physiological' dead space. Hence, the physiological dead space is the volume of inspired air, in airways (anatomical dead space) and in the alveoli (alveolar dead space), that does not participate in gaseous exchange. In healthy individuals, anatomical dead space and physiological dead space are identical because all the air entering into the alveoli participates in gaseous exchange (as alveolar diffusion, ventilation and perfusion are normal in healthy individuals).