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KEY CONCEPTS
Normal ventilation–perfusion ratio. The function of the lungs is to maintain arterial partial pressure of oxygen (PaO2) and arterial partial pressure of carbon dioxide (PaCO2) within normal ranges. This goal is accomplished by matching 1 mL mixed venous blood with 1 mL fresh air (V̇/Q̇ = 1). Normally, ventilation (V̇) is less than perfusion (Q̇), and V̇/Q̇ ratio is 0.8.
The air in the lung is divided into four compartments: tidal volume—air exhaled during quiet breathing; inspiratory reserve volume (IRV)—maximal air inhaled above tidal volume; expiratory reserve volume (ERV)—maximum air exhaled below tidal volume; and residual volume (RV)—air remaining in the lung after maximal exhalation. The sum of all four components is the total lung capacity (TLC).
Obstructive lung disease is defined as an inability to get air out of the lung. It is identified on spirometry when forced expiratory volume in the first second of expiration (FEV1)/forced vital capacity (FVC) (total amount of air that can be exhaled during a forced exhalation) (FEV1/FVC) is less than 70% to 75% (or below the lower limit of normal (LLN) based on population studies).
Reversible airway obstruction is common in asthma and is sometimes seen in chronic obstructive pulmonary disease (COPD). An increase in FEV1 of 12% (and greater than 0.2 L in adults) after an inhaled β-agonist suggests an acute bronchodilator response.
Restrictive lung disease is defined as an inability to get air into the lung and is best defined as a reduction in TLC (usually less than 80% predicted). It is suspected when FVC is low (less than 80% predicted) and FEV1/FVC is normal.
Restrictive lung disease can be produced by a number of defects, such as increased elastic recoil (interstitial lung disease), respiratory muscle weakness (myasthenia gravis), mechanical restrictions (pleural effusion or kyphoscoliosis), and poor effort.
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The primary function of the respiratory system is to maintain normality of arterial blood gases, that is, arterial partial pressure of oxygen (PaO2) and arterial partial pressure of carbon dioxide (PaCO2). To achieve this goal, several processes must be accomplished, including alveolar ventilation, pulmonary perfusion, ventilation–perfusion matching, and gas transfer across the alveolar–capillary membrane. Alveolar ventilation is achieved by the cyclic process of air movement in and out of the lung. During inspiration, the inspiratory muscle contracts and generates negative pressure in the pleural space. This pressure gradient between the mouth and the alveoli draws fresh air (tidal volume [VT]) into the lung. Approximately one third of the inspired gas stays in the conducting airways (dead space), and two third reaches the alveoli.
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The human lung contains a series of branching, progressively tapering airways that originate at the glottis and terminate in a matrix of thin-walled alveoli. Coursing through this matrix of alveoli is a rich network of capillaries that originates from the pulmonary arterioles and ...