Chapter 9 - Flow in the Lungs

Abstract

The respiratory system provides the means for gas exchange between the atmospheric air and the cardiovascular system. It is divided into a conducting pathway composed of the nasal cavity, the pharynx, the larynx, the trachea, and the bronchi, and a branching pathway, which eventually diverges into the alveolar sacs. The alveoli are the locations for gas exchange and provide approximately 70 square meters of area for exchange. The respiratory boundary is composed of the alveoli epithelial cell, the capillary endothelial cell, and the fused laminae between these cells. Each lung is surrounded by a pleural membrane that facilitates the changes in lung volume to account for the movement of gases into and out of the lungs. The lungs go through an approximate 20% change in volume, which significantly affects the blood vessel area within the lungs. Due to these large stresses, the Kirchhoff stress and the Green's strain are most applicable to relate the forces to the deformation. The pressure and the volume of the lungs are altered based on the movement of the ribs and the diaphragm. As the diaphragm moves downward and the ribs move outward the lungs expand and experience a temporal reduction in pressure. The reverse movements of the diaphragm and lungs cause the lungs to shrink and experience an increase in pressure. Oxygen and carbon dioxide diffusion is based on the solubility of the gas within blood. Oxygen diffuses into the red blood cell and becomes associated with the heme groups of hemoglobin. Carbon dioxide, however, is modified into plasma soluble molecules by proteins carried within the red blood cells and is therefore not directly transported by them.