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76 5 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, Health and the Respiratory System water vapor, and inert gases are nonphysiological participants. Nonrespiratory functions of the lungs include the delivery of odorants to the olfactory epithelium . This is important to animals that rely on sniffing to detect their environment without the danger of bringing noxious volatile agents deep into the lungs. The lungs also warm, filter, and moisturize the air. This prevents airborne pathogens from entering alveolar gas spaces, it minimizes dessication of the mucosal linings of the respiratory channels, and it protects exchange of gases by ensuring the homeostasis of temperature in the alveoli. The lungs also play an important role in immunology. Pulmonary macrophages prevent ingested and inhaled foreign substances from getting into the alveoli. The respiratory system is subject to feedback control, gradients for pressure and airflow, and other physiological regulatory phenomena mentioned in chapter . For example, there would be no exchange of blood gases between the tissues, blood, and alveoli without the appropriate partial pressure gradients for gases. Atmospheric oxygen is delivered to cellular mitochondria down a pressure gradient that decreases in each location the gases move through: the partial pressure in the atmosphere (PO) is greater than that in the alveoli, which is greater than in pulmonary capillaries, then interstitial spaces, then cytosol, then mitochondrion . For carbon dioxide, produced as a by-product of cellular metabolism , the gradient is reversed. The mitochondrial PCO is greater than the cytosolic PCO, which is greater than, in turn, the interstitial PCO, venous blood PCO, alveolar PCO, and finally atmospheric PCO. The human respiratory system is subject to central control mechanisms and negative feedback in the same way that the cardiovascular system is. For example, during inspiration as the alveoli fill with air, causing both them and the thoracic cavity to expand, mechanoreceptors in the walls of the airways and in the chest wall detect the expansion and send sensory signals to the brain stem to terminate inspiration. These signals and others from higher regions of the central nervous system get integrated in the respiratory control centers of the brain stem medulla. Collectively this stops the firing of inspiratory neurons, brings airflow and lung expansion to an end, and allows expiration to begin passively. This is one more example of a well-ordered, cyclic, negative feedback control system. Like all other major mammalian organ systems, respiratory homeostasis is ensured by a system of sensory receptors, afferent—sensory nerve—central comparators and integrators and motor—efferent nerve—and corresponding activators . This is true whether the respiratory system is regulating gas exchange or participating...

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