Pulmonary Ventilation:Pulmonary ventilation is the process by which gasses flow between the atmosphere and lung alveoli. Air moves into the lungs when air pressure inside the lungs is less than the air pressure in the atmosphere and out of the lungs when the pressure inside the lungs is greater than the atmosphere pressure. Breathing in is called inhalation or inspiration. Just before each inhalation air pressure inside the alveoli is equal to the atmospheric pressures, 760 rnmHg, and the intrapleural pressure is 756 mmHg. With inspiration, the cavity of the thorax is enlarged as external intercostals muscles and the diaphragm contract causing a decrease in the intrapleural pressure to about 754 mmHg. The parietal pleura lining the cavity is pulled outward in all direction and the visceral pleura and lungs are pulled along with it. As the lung volume increases in this way the pressure inside the lungs i.e. 760 mmHg drops to 758 mmHg. Thus a pressure difference is established between the atmosphere and alveoli and air flows into the lungs from the atmosphere. Breathing out or exhalation (expiration) starts when the inspiratory muscles and diaphragm relaxes, the dome of the diaphragm moves up, the ribs are depressed leading to decrease in the lung volume and increase in the lung pressure (763 rnm Hg). Air flows out form the area of higher pressure to the area of lower pressure in the atmosphere. The elastic recoil of the chest wall and the lungs is due to the recoil of elastic fibers and the inward pull of surface tension due to the alveolar fluid. A thin layer of alveolar fluid coats the surface of the alveoli and exerts a force known as surface tension. Exhalation is a passive process while inhalation or inspiration is an active process.