The<unk> , with its associated<unk> , aids optimal functioning of the lungs during breathing. The<unk>  also contains<unk>  fluid, which acts as a lubricant and allows the<unk>  to slide effortlessly against each other during respiratory movements. Surface tension of the<unk>  fluid also leads to close apposition of the lung surfaces with the chest wall. This relationship allows for greater inflation of the alveoli during breathing. The<unk>  transmits movements of the ribs muscles to the lungs, particularly during heavy breathing. During inhalation the external intercostals contract, as does the diaphragm. This causes the expansion of the chest wall, that increases the volume of the lungs. A negative pressure is thus created and inhalation occurs.<unk>  fluid<unk>  fluid is a serous fluid produced by the serous membrane covering normal<unk> . Most fluid is produced by the exudation in parietal circulation (intercostal arteries) via bulk flow and reabsorbed by the lymphatic system. Thus,<unk>  fluid is produced and reabsorbed continuously. The composition and volume is regulated by mesothelial cells in the<unk>  In a normal 70 kg human, a few milliliters of<unk>  fluid is always present within the intrapleural space. Larger quantities of fluid can accumulate in the<unk>  space only when the rate of production exceeds the rate of reabsorption. Normally, the rate of reabsorption increases as a physiological response to accumulating fluid, with the reabsorption rate increasing up to 40 times the normal rate before significant amounts of fluid accumulate within the<unk>  space. Thus, a profound increase in the production of<unk>  fluid—or some blocking of the reabsorbing lymphatic system—is required for fluid to accumulate in the<unk>  space.
Pleural cavity
Lung cavity
Indium lung
Endobronchial valve
Thoracic inlet
Pulmonary bay
Thoracic diaphragm
Subclavian loop