Note that we are using differently than in Chapter 27, where represented flow through an airway at a particular instant in time. Here, V is the volume of air exiting the lungs during a series of breaths. Total ventilation (V T) is the volume of air moved out of the lungs per unit of time: Finally, in the third part of this chapter, we see that the ratio of ventilation to perfusion-and the distribution of ventilation-perfusion ratio among alveolar units-is critically important for gas exchange and thus for the composition of the arterial blood gases: P O2, P CO2, and pH.Ībout 30% of total ventilation in a respiratory cycle is wasted ventilating anatomical dead space (i.e., conducting airways) In the second part of this chapter, we examine the special properties of the pulmonary circulation and see that, like ventilation, perfusion varies in different regions of the lung. In Chapters 17 to 25 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24, we discussed the cardiovascular system. Perfusion is the convective movement of blood that carries the dissolved gases to and from the lung. In the first part of this chapter, we consider the importance of ventilation for determination of alveolar P O2 and P CO2 and also see that ventilation varies from one group of alveoli to the next. In Chapter 27, we discussed the mechanics of ventilation. Ventilation is the convective movement of air that exchanges gases between the atmosphere and the alveoli. Ventilation and perfusion-both of which require energy-are critical because they set up the partial pressure gradients along which O 2 and CO 2 diffuse. VENTILATION AND PERFUSION OF THE LUNGSĪlthough diffusion is at the very heart of the gas exchange, as we discussed in Chapter 30, two other parameters are also extremely important. Medical Physiology A Cellular and Molecular Approach, Updated 2nd Ed.
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