The development of complex cellular systems such as the vertebrates requires the availability of large amounts of oxygen for the metabolic needs of the cells. The respiratory and circulatory systems are the specialized and hierarchically organized flow systems that meet this need. The respiratory system carries oxygen from the air to the pulmonary veins, whereas in the circulatory system oxygen is carried by the blood and is delivered to the cells by the thinnest vessels, the capillaries. A countercurrent of oxygen and carbon dioxide crosses the capillary wall, such that carbon dioxide is extracted from the blood stream and is discharged from the lungs into the atmosphere during exhalation.
KeywordsReflection Coefficient Capillary Wall Capillary Network Airway Wall Bronchial Tree
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