Lecture 5 Study Guide

Lecture 5 Study Guide - Lecture 5 Ventilation/Perfusion...

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Lecture 5: Ventilation/Perfusion Relationship 1. Understand the determinants of the exchange of O2 and CO2 in the lungs. The exchange of O2 and CO2 in the lungs is determined by: A. The ratio between alveolar ventilation (Va) and pulmonary blood flow (Q) B. The rate of O2 consumption and CO2 production C. The gas tensions in inspired air and in mixed venous blood D. Chemical processes in the blood 2. Describe the differential variation in ventilation and perfusion in the zones of the lung. Because of gravitational forces the Pip are more negative at the top of the lung than at the bottom in an upright individual—due to the weight of the lung pulling down from the apex of the thoracic cavity, where it lowers Pip, and by the weight of the lung compressing the base, where Pip is increased. The result is a gradient of Pips that creates a gradient of alveolar distending pressures (Palv- Pip) from top to bottom Because of the weight of the lung, the Pip is less negative at the base of the lung than at the apex. As a consequence, the basal lung is relatively compressed in its resting state but expands more readily on inspiration that the apex. For a given change in distending pressure, ventilation will be greater at the base of the lung than at the apex. As blood flows through the pulmonary circulation, frictional forces cause energy to be lost as heat, and pressures continually drop along the arterial tree, the arterioles, capillaries, venues and veins. In terms of gravity, for each 1cm we move upward above the level the heart, the hydrostatic pressure in the blood vessels decreases by 1cm H2O. 3 Zones of the Lung: Zone 1: PA>Pa>Pv Pulmonary artery pressure may be insufficient, to maintain blood flow at the apex of the lung in the upright individual at rest. When alveolar gas pressure, PA, exceeds pulmonary arterial pressure, Pa, not only the artery but all downstream vessels at the same level are collapsed. Complete cessation of blood flow throughout the cardiac cycle does not normally occur in Zone 1 unless the arterial pressure is reduced e.g. following severe hemorrhage, or if alveolar pressure is raised e.g. during positive pressure ventilation.
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