The respiratory system facilitates the exchange of oxygen and carbon dioxide between the external environment and the human body. The airways (trachea and bronchi), lungs, and alveoli and pulmonary surfactant provide the biochemical and biophysical basis for this exchange. Acid–base balance in the body also relies on these tissues as well as on the carbonic anhydrase and other enzyme systems. Various diseases of the respiratory system affect either the structure or the biochemistry of the lungs; medications and treatments for these diseases rely on knowing their biochemical basis.
BASIC ANATOMY AND DEVELOPMENT
The respiratory system, composed of the nose, nasal cavity, larynx, trachea, bronchiole tubes, and lungs, is derived from the endoderm, although the splanchnic mesoderm contributes to the visceral pleura, cartilage, and connective tissue and also to the immediately associated smooth musculature and capillaries (Figure 17-1). Other closely associated structures include the ribs and diaphragm and the central cardiovascular system, which provides pulmonary arteries and veins.
Overview of Lung Structure. The basic structure of lungs (A) includes the trachea and repetitively branching bronchi/bronchioles leading to a terminal bronchiole. Pulmonary arteries and veins provide circulation to this lung tissue. At the terminal bronchiole/alveoli (B), the exchange of oxygen and carbon dioxide between alveoli and the surrounding vasculature is conducted as discussed later in this chapter. [Reproduced with permission from Barrett KE, et al.: Ganong’s Review of Medical Physiology, 23rd edition, McGraw-Hill, 2010.]
The respiratory system facilitates the transport of oxygen (O2) to the body’s tissues and enables elimination of gaseous waste products of metabolism. Muscles of the ribs, diaphragm, neck, shoulders, and abdomen generate a negative pressure gradient, which promotes the flow of external air into a system of branching, conducting airways and peripheral terminal bronchioles.
Exchange via passive diffusion through the cell membranes of the type I epithelial cells of the alveoli and the pulmonary capillaries is responsible for the uptake and removal of O2 and carbon dioxide (CO2), respectively (Figure 17-2). Pulmonary arteries from the right ventricle of the heart deliver deoxygenated, CO2-laden blood through the branching blood vessels, to the capillaries; pulmonary veins return the freshly oxygenated blood with decreased CO2 to the left auricle and ventricle for delivery to the body’s tissues. Finally, positive pressure, along with the elastic walls of the alveoli, springing back from full expansion, expels the waste gas, including the CO2. Although exhalation is normally a passive process, active expulsion is also possible, utilizing the same muscle groups noted above. Defects or injuries to any of the parts of the respiratory system or to those that affect the musculature and/or cardiovascular system can lead to respiratory disease.
Structure of the Alveolus. (...