Explain how resistance in the air passages compliance elasticity of the lungs and the surface tension in the alveoli affect the flow of air in and out of the lungs?

Answer 1

Air tends to flow from high pressure to low pressure. If the pressure in the lungs exceeds that pressure at the mouth (which is usually atmospheric pressure), then air will flow out of the lungs. During normal inhalation, the diaphragm muscle contracts, pushing the abdominal contents, but increasing the volume on the thoracic cavity (the chest). Since at first the volume of the lungs increases as it is pulled with the diaphragm, the intrathoracic pressure decreases, and air moves into the lungs.

If the pressure in the lungs were to increase relative to the air at the lips then air would move out. The elasticity of the lungs tends to make the lungs want to assume a smaller size (like the elasticity of a stretched rubber band). This force in addition to the force of expiratory muscles if they are being used will increase pressure in the lung (under normal circumstances this is basically intrathoracic pressure). However, the velocity of the flow of air is limited by airways resistance. [This is similar to V=IR from electricity voltage = current x resistance] In the case of the lung, the flow is proportional to the change in pressure (delta pressure intrathoracic pressure vs atmospheric pressure) and inversely proportional to the airways resistance. So resistance in the air passages with slow the flow of air out of the lung.

Now you might think that one could easily overcome airways (air passages) resistance's negative affect on flow by increasing the pressure in the lungs. Unfortunately this cannot be done without increasing the airways resistance too. Because as I try to squeeze air out of my alveoli with more pressure that pressure is transmitted throughout the lung and to my small airways that are compressible and thus if a bear were to hug me and try to squeeze air out of me faster this would also increase the airways resistance by narrowing some of the small airways, increasing resistance and decreasing flow.