There are many distinct, separate, and interrelated parts to the broncho-pulmonary-circulatory system that make up the complex organs of respiration. We can delineate the function of each portion and determine how these are related to each other. By this means is created a composite picture of respiratory function.
If, however, we wish to view respiratory physiology from the standpoint of what is essential to normal, healthy living; what causes impaired function and the symptoms of such pathology, and how to treat these pathological conditions, then we must consider the entire respiratory unit as an adaptive and compensatory mechanism. This, as we shall see, is true in all of the major divisions of respiratory function: ventilation through tracheobronchial channels, gas interchange through the alveolar capillary membrane, circulation of blood through the pulmonary capillary bed from the right heart to the left auricle, and the systemic circulation with gas exchange in the
LEVINE ER. Some Fundamentals of Respiratory Physiology. AMA Arch Intern Med. 1955;96(3):357-359. doi:10.1001/archinte.1955.00250140079007