Efficient lung function depends on (1) the airways (trachea and bronchi) being free from obstruction; (2) good inspiration and expiratory movements; any interference with the movement of the thoracic cage such as occurs with paralysis of the chest-wall muscles or from the splinting effect of obesity, will affect inspiration, whereas expiration is impaired when there is loss of elasticity of the lung tissue itself such as occurs in emphysema; (3) the absence of any pathological process in and around the air sacs interfering with the exchange of gases. The air sacs, or alveolar, are surrounded by thin-walled capillaries (the alveolocapillary membrane).
The close proximity of these structures allows oxygen to diffuse readily into the blood from the atmospheric air, and carbon dioxide to pass in the reverse direction. In certain diseases such as pneumonia or heart failure, the outpouring of fluid into the air sacs seriously interferes with this gaseous exchange and in an attempt to compensate for this the rate of respiration is increased.
The control of respiration
Respiration is partly under voluntary control, but in addition the rate and depth of breathing are automatically adjusted in accordance with the changing requirements of the body by a control unit in the medulla of the brain (the respiratory center) which responds to changes in the arterial carbon dioxide pressure and hydrogen ion concentration. A rise in carbon dioxide pressure, such as occurs with exercise or with obstruction of the airways or in certain diseases affecting the lungs, stimulates the respiratory center to effect a compensatory increase in the rate and depth of respiration. Similarly, any increase in the blood hydrogen ion concentration as occurs ni uremia and in diabetic coma results in very deep sighing respiration (Kussmaul's breathing).
The close proximity of these structures allows oxygen to diffuse readily into the blood from the atmospheric air, and carbon dioxide to pass in the reverse direction. In certain diseases such as pneumonia or heart failure, the outpouring of fluid into the air sacs seriously interferes with this gaseous exchange and in an attempt to compensate for this the rate of respiration is increased.
The control of respiration
Respiration is partly under voluntary control, but in addition the rate and depth of breathing are automatically adjusted in accordance with the changing requirements of the body by a control unit in the medulla of the brain (the respiratory center) which responds to changes in the arterial carbon dioxide pressure and hydrogen ion concentration. A rise in carbon dioxide pressure, such as occurs with exercise or with obstruction of the airways or in certain diseases affecting the lungs, stimulates the respiratory center to effect a compensatory increase in the rate and depth of respiration. Similarly, any increase in the blood hydrogen ion concentration as occurs ni uremia and in diabetic coma results in very deep sighing respiration (Kussmaul's breathing).
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