Pulmonary ventilation and perfusion
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) ratio of 1 in a situation of continuous countercurrent flow of gas to blood, with a blood-to-gas exposure of 0.75 sec. Human lung, by contrast, is only relatively efficient, showing a range of 
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ratios for its many alveoli, determined by the distribution of 
and 
throughout the lungs.Ventilation
within the lungs. The right lung is larger than the left lung, receiving approximately 52% to 53% of a tidal breath in the supine position, during both spontaneous breathing and with mechanical ventilation. These percentages change under the influence of gravity with change in posture. Anesthesia, paralysis, and mechanical ventilation introduce further changes.At functional reserve capacity, in each slice of lung, from nondependent (apex in sitting position, anterior lung in supine, up lung in lateral decubitus position) to most dependent portion, the alveolar volume decreases. Basal alveoli are one quarter the volume of apical alveoli at end expiration. This puts the basal alveolar characteristics on a steeper portion of their pressure-volume (P-V) curve (Figure 29-1); although the basal alveoli are smaller than apical alveoli at functional reserve capacity, the basal alveoli expand more than do the apical alveoli during inspiration. Therefore, in an awake, spontaneously breathing patient, in all positions, ventilation per unit of lung volume is smallest at the highest portion (e.g., the apex in an upright patient) and increases with vertical distance down the lung.
In the supine patient, general anesthesia with paralysis and mechanical ventilation decreases the difference between the ventilation of the dependent and nondependent alveoli, causing nearly uniform distribution of ventilation throughout the lung. This is attributed to a decreased functional reserve capacity, shifting alveolar characteristics downward on their P-V curves (see Figure 29-1). When the patient is in the lateral decubitus position, anesthesia reverses the distribution of ventilation so that the nondependent (upper) part of the lung receives more ventilation than does the dependent (lower) part of the lung. This arrangement holds for both spontaneous and mechanical ventilation and is clinically significant because the dependent lung has greater perfusion, which causes increased 
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mismatch. The change in distribution of 
to lung regions in the lateral decubitus position is attributed to (1) decreased functional reserve capacity, causing a shift along the P-V curve (which can be partially reversed by positive end-expiratory pressure); (2) more compression of the dependent lung by the mediastinum and abdominal contents; and (3) increased compliance of the nondependent hemithorax.
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mismatching. Anesthesia may reverse bronchoconstriction and favorably impact this factor.Pulmonary blood flow
The two major determinants of distribution of pulmonary blood flow (
) within the lung are (1) gravity and (2) hypoxic pulmonary vasoconstriction (HPV). Pulmonary artery pressure (PPA) decreases by 1 mm Hg or 1.35 cm H2O for every cm of vertical distance up the lung. Because the pulmonary circulation is a low-pressure system, this causes significant differences in 
between the lower and higher regions of the lung, with greater 
going to the lower lung regions. The actual 
to an alveolus also depends on the alveolar pressure (PALV), which opposes the PPA and pulmonary venous pressure (PPV). This interaction is summarized in Figure 29-2. All of these relationships are dynamic, varying throughout the cardiac and respiratory cycles. There are four defined zones of blood flow in the lung. In zone 1, at the apex of an upright lung, PALV is greater than PPA, preventing any blood flow and thereby creating alveolar dead space. Zone 1 is negligible in healthy lungs. In zone 2, PPA is greater than PALV, which is greater than PPV, so that 
depends only on PPA minus PALV. In zone 3, PPA is greater than PPV, which is greater than PALV, and 
is a function of PPA minus PPV independent of PALV
