70. | Principles of Surgery, Companion Handbook

Principles of Surgery Companion Handbook

FIGURE 12-1 Model to explain the uneven distribution of blood flow in the lung based on the pressures affecting the capillaries. In zone 1, alveolar pressure (Pa) exceeds pulmonary artery (Pa) and venous (Pv) pressures so that the collapsible vessels are held closed and there is no flow. In zone 2, pulmonary arterial pressure exceeds alveolar pressure, but alveolar pressure exceeds venous pressure. Under these conditions, there is a constriction at the downstream end of each collapsible vessel. In zone 3, pulmonary arterial and venous pressures exceed alveolar pressure, and the colapsible vessels are held open. (From: West JB, Dollery CT, Naimark A: Distribution of blood flow in isolated lung: Relation to vascular and alveolar pressures. J Appl Physiol 19: 713, 1964, with permission.)