The trachea serves as a conduit for ventilation and the clearance of tracheal and bronchial secretions and has an average length of 10-13 cm. The trachea bifurcates at the carina into the right and left main stem bronchi. The right main stem bronchus lies in a more vertical orientation relative to the trachea, whereas the left main stem bronchus lies in a more horizontal orientation.
The periodic exchange of alveolar gas with the fresh gas from the upper airway reoxygenates desaturated blood and eliminates CO2. This exchange is brought about by small cyclic pressure gradients established within the airways. During spontaneous ventilation, these gradients are secondary to variations in intrathoracic pressure; during mechanical ventilation, they are produced by intermittent positive pressure in the upper airway.
The lung volume at the end of a normal exhalation is called functional residual capacity (FRC). At this volume, the inward elastic recoil of the lung approximates the outward elastic recoil of the chest (including resting diaphragmatic tone).
Closing capacity is normally well below FRC, but it rises steadily with age. This increase is probably responsible for the normal age-related decline in arterial O2 tension.
Whereas both forced expiratory volume in 1 sec (FEV1) and forced vital capacity (FVC) are effort dependent, forced midexpiratory flow (FEF25-75%) is more effort independent and may be a more reliable measure of obstruction.
Changes in lung mechanics due to general anesthesia occur shortly after induction. The supine position reduces the FRC by 0.8-1.0 L, and induction of general anesthesia further reduces the FRC by 0.4-0.5 L. FRC reduction is a consequence of alveolar collapse and compression atelectasis due to loss of inspiratory muscle tone, change in chest wall rigidity, and upward shift of the diaphragm.
Local factors are more important than the autonomic system in influencing pulmonary vascular tone. Hypoxia is a powerful stimulus for pulmonary vasoconstriction (the opposite of its systemic effect).
Because alveolar ventilation () is normally about 4 L/min and pulmonary capillary perfusion () is 5 L/min, the overall
ratio is about 0.8.
Shunting denotes the process whereby desaturated, mixed venous blood from the right heart returns to the left heart without being resaturated with O2 in the lungs. The overall effect of shunting is to decrease (dilute) arterial O2 content; this type of shunt is referred to as right-to-left.
General anesthesia commonly increases venous admixture to 5% to 10%, probably as a result of atelectasis and airway collapse in dependent areas of the lung.
Note that large increases in Paco
2 (>75 mm Hg) readily produce hypoxia (Pao
2 <60 mm Hg) at room air, but not at high inspired O2 concentrations.
The binding of O2 to hemoglobin seems to be the principal rate-limiting factor in the transfer of O2 from alveolar gas to blood.
The greater the shunt, ...
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