Why is anatomic and physiologic dead space the same in a normal lung?

In a normal lung, the anatomic dead space and the physiologic dead space are not the same. The anatomic dead space refers to the portion of the respiratory tract that is not involved in gas exchange, including the nose, mouth, pharynx, larynx, trachea, and bronchi. The physiologic dead space, on the other hand, includes the anatomic dead space as well as any alveoli that are not perfused or are poorly perfused. In a normal lung, the physiologic dead space is usually about 20-30% of the tidal volume.