The distribution of ventilation among the hundreds of airways in the bronchial tree is not perfectly uniform. This can be either harmful or helpful to gas exchange. Any tendency to mismatch air and blood flow is deleterious to gas exchange, as described in Chapter 21. On the other hand, an uneven distribution of ventilation would be helpful if it tended to match the distribution of pulmonary blood flow, as described in Chapter 18. Fortunately, gravity is an important factor in determining the distribution of both ventilation and pulmonary blood flow, so ventilation and blood flow are fairly well matched in the lungs.
Gravity affects the distribution of ventilation by creating a gradient in Ppl between the top and bottom of the lung (Fig. 11). The increase in Ppl at the bottom relative to the top of the lung is caused by two mechanisms: (1) The weight of the column of intra-pleural fluid creates a hydrostatic column, similar to the blood in the pulmonary capillaries; and, more importantly, (2) the weight of the lung pulls the lung away from the top of the thorax and compresses the lung on the diaphragm at the bottom. This local distortion in volume results in different regions of the lung operating on different parts of the compliance curve, as shown in Fig. 11.
-10 cm H2O
-10 cm H2O
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