ventilation

• The flow of air in and out of the alveoli is referred to as ventilation and is composed of two stages; inspiration and expiration.
• During inspiration, the external intercostal muscles contract whereas the internal muscles relax, as a result this causes the ribs to raise upwards. The diaphragm contracts and flattens.
• During inspiration, the intercostal muscles and diaphragm cause the volume inside the thorax to increase, thus lowering the pressure. The difference between the pressure inside the lungs and atmospheric pressure creates a gradient, thus causing the air to be forced into the lungs
• During expiration, the internal intercostal muscles contract whereas the external muscles relax therefore lowering the rib cage.
• In expiration, the diaphragm relaxes and raises upwards. This action in combination decrease the volume inside the thorax, therefore increasing the pressure, forcing the air out of the lungs