Health and the Respiratory System
Breathing in the human includes both ventilation and respiration. The rib cage, diaphragm, and intercostal muscles constitute a bellows-like system in which the lungs are found. Neurogenically controlled movements of the thoracic cavity cause the expansions and contractions of the lung that respiratory physiologists call ventilation. During the inspiratory and expiratory phases of each respiratory cycle, atmospheric air moves into and out of the lungs in a rhythm that is analogous to the flow of ocean tides. In both cases, air and water flow over the same path during each cycle. Because of this analogy, respiratory physiologists, pulmonologists, and respiratory therapists call the cyclic flow of air in and out of the lungs the tidal volume. The number of respiratory cycles in a minute multiplied by the tidal volume is called the minute ventilation.
Respiration concerns the fate of the gases that are carried in and out of the lungs with each tidal volume. The primary respiratory gases are oxygen and carbon dioxide. The mechanisms by which these gases get from the lungs to the tissues, and from the tissues to the lungs, respiratory physiologists call external respiration. It involves mainly the exchange and transport of gases between lungs and blood and between blood and tissues. Biochemists call the mechanisms by which oxygen gets used by the cells tissue or internal respiration. This involves use of oxygen by mitochondria and the processes of electron transfer. Therefore, to fully understand the human respiratory system, the student must grasp the physiological concepts of ventilation, gaseous exchange and transport, and uptake and release of gases by cells and subcellular organelles such as the mitochondria.
The human respiratory system serves both respiratory and nonrespiratory purposes. Physiologically, the respiratory system delivers fresh ambient air to the blood and releases gaseous metabolic by-products to the atmosphere. The physiologically important ingredient of fresh ambient air is oxygen. The significant gaseous component released to the atmosphere is carbon dioxide. Nitrogen,