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In order to understand ventilation, we must first look at the relationship between pressure and volume. Pressure is caused by gas molecules striking the walls of a container. The pressure exerted by the gas molecules is related to the volume of the container. Let’s see how volume affects pressure. This large sphere contains the same number of gas molecules as the original sphere. Notice that in this larger volume, the gas molecules strike the wall less frequently thus exerting less pressure. Let’s see what happens when it is decreased in size. In this small sphere, the gas molecules strike the wall more frequently thus exerting more pressure. Notice that the number of gas molecules has not changed. These demonstrations illustrate Boyle’s Law, which states that the pressure of a gas is inversely proportional to the volume of its container. Thus if you increase the volume of a container, the pressure will decrease and if you decrease the volume of a container, the pressure will increase. The volume of the thoracic cavity is changed by muscle contraction and relaxation. Take a few seconds to notice the movements of your chest during your own quiet breathing. During quite inspiration, the diaphragm and the external intercostal muscles contract slightly enlarging the thoracic cavity. Notice how the diaphragm flattens and moves inferiorly, while the external intercostal muscles elevate the rib cage and move the sternum anteriorly. These actions enlarge the thoracic cavity in all dimensions. As we learned from Boyle’s law, increasing the volume decreases the pressure within the thoracic cavity and the lungs. Intrapleural pressure is the pressure within the pleural cavity. Intrapleural pressure is always negative, which acts like a suction to keep the lungs inflated. The negative intrapleural pressure is due to three main factors. The surface tension of alveolar fluid, the elasticity of the lungs and the elasticity of the thoracic wall. Let’s observe the effect of the surface tension. The surface tension of the alveolar fluid tends to pull each of the alveoli inward and therefore pulls the entire lung inward. Surfactant reduces this force. Let’s see the effect of the lung’s elastic tissue. The abundant elastic tissue in the lungs tends to recoil and pull the lung inward. As the lung moves away from the thoracic wall, the cavity becomes slightly larger. The negative pressure this creates acts like a suction to keep the lungs inflated. Let’s observe the effect of the thoracic wall’s elasticity. The elastic thoracic wall tends to pull away from the lung, further enlarging the pleural cavity and creating this negative pressure. The surface tension of pleural fluid resists the actual separation of the lung and thoracic wall. What do you think will happen to a lung if you cut through the thoracic wall into its pleural cavity? Air enters the pleural cavity as it moves from high pressure to low pressure. This is called a pneumothorax. Normally, there is a difference between the intrapleural and intrapulmonary pressures, which is called transpulmonary pressure. The transpulmonary pressure creates the suction to keep the lungs inflated. In this case, when there is no pressure difference, there is no suction and the lung collapses. Let’s see why the other lung did not collapse. The lungs are completely separate from one another, each surrounded by its own pleural cavity and pleural membranes. Therefore, changes in the intrapleural pressure of one lung do not affect the other lung. Several factors change airway resistance by affecting the diameter of the airways. They do this by contracting or relaxing the smooth muscle in the airway walls, especially the bronchioles. Let’s see how these factors affect airflow. Parasympathetic neurons release the neurotransmitter acetylcholine, which constricts bronchioles. As you can see in the equation, increased airway resistance decreases airflow. Histamine, released during allergic reactions, constricts bronchioles. This increases airway resistance and decreases airflow, making it harder to breathe. Epinephrine, released by the adrenal medulla during exercise or stress, dilates bronchioles thereby decreasing airway resistance. This greatly increases airflow, ensuring adequate gas exchange.