Similarly, carbon dioxide is delivered to the alveolus in the mixed venous blood and diffuses into the alveolus in the pulmonary capillary. Oxygen is delivered to the alveolus by alveolar ventilation, is removed from the alveolus as it diffuses into the pulmonary capillary blood, and is carried away by blood flow. would be 1.0, but there would be no gas exchange because there could be no gas diffusion between the ventilated alveoli and the perfused pulmonary capillaries. For instance, suppose that all 5 L/min of the cardiac output went to the left lung and all 5 L/min of alveolar ventilation went to the right lung. for the whole lung is really of interest only as an approximation of the situation in all the alveolar-capillary units of the lung. However, ventilation and perfusion must be matched on the alveolar-capillary level, and the Image not available. for the whole lung is in the range of 0.8 to 1.2. and Image not available., as well as in gas delivery to or removal from the lung.Īlveolar ventilation is normally about 4 to 6 L/min and pulmonary blood flow (which is equal to cardiac output) has a similar range, and so the Image not available. Alterations in the ratio of ventilation to perfusion, called the Image not available., will result in changes in the alveolar Image not available. are thus determined by the relationship between alveolar ventilation and pulmonary capillary perfusion. Similarly, the mixed venous blood brings carbon dioxide into the lung and takes up alveolar oxygen. These concentration differences must be maintained by ventilation of the alveoli and perfusion of the pulmonary capillaries.Īlveolar ventilation brings oxygen into the lung and removes carbon dioxide from it. Diffusion of oxygen and carbon dioxide occurs passively, according to their concentration differences across the alveolar-capillary barrier. Gas exchange between the alveoli and the pulmonary capillary blood occurs by diffusion, as will be discussed in the next chapter.
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