The pathophysiology of respiration has only slowly and with difficulty entered medical thinking. It is burdened for the "clinically" minded physician with a heavy mortgage, namely abstraction and mathematics, which irritates those who are only interested in the concrete and palpable. When the physiologist speaks of alveoli, he does not associate the same meaning as the anatomist; he thinks less of a specific histological structure and more of the site of gas exchange. For him, the respiratory dead space is not simply the volume of the airways, but represents a purely functional concept, although its volume is closely related to that of the airways. Alveolar ventilation for the pathophysiologist is not merely the ventilation of certain lung areas in the anatomical sense, but a mathematical abstraction.