 Dear students, in this topic we shall discuss the mechanism of lung ventilation in mammals. The lung ventilation or breathing is a mechanical process. It consists of two phases, inspiration and expiration. During inspiration, fresh air moves in while during expiration, the air with low oxygen and high carbon dioxide content is moved out of the lungs. Dear students, there are certain anatomical features which help the lungs to carry out ventilation or breathing. The first feature of the lungs is that they are spongy in nature. They neither pull nor push the air. Ventilation of lungs occurs due to their elastic expansion and contraction. Dear students, the second feature of the lungs that facilitates breathing is the presence of a muscular diaphragm at the floor of thoracic cavity. The chest cavity is formed of a strong muscular structure which is diaphragm. When the diaphragm is in a relaxed state, it is dome-like. When the muscles contract, it becomes less dome-like. Because of the contraction and relaxation of the diaphragm, the volume of thoracic cavity increases or decreases. The third anatomical feature facilitating breathing is the presence of external and internal inter-costal muscles between the ribs. The muscles between the muscles are called inter-costal muscles and these are two types of external and internal coastal muscles. These two muscles play an important role in breathing because they cause the rib cage to elevate or drop as a result they expand or reduce its volume. As a result, pressure on the lungs is increased and decreased rhythmically. Dear students, now we shall discuss the mechanism of inspiration. Inspiration happens when the external inter-costal muscles contract and cause the ribs to move forward and upward, elevating the rib cage. With this, muscles of the diaphragm also contract, making it less dome-like. In both the actions, combined effect is the thoracic cavity expands, chest expands. Chest expansion is the effect that the pressure of air inside the lungs decreases. As a result, lungs expand. Air pressure in the lungs falls. This fall is below the atmospheric pressure because the air pressure in the lungs decreases and creates a vacuum-like situation. So the external air, which is at comparatively higher pressure than the pressure inside the lungs, so it rushes in from outside into the lungs and that is inspiration happens. Dear students, now we shall discuss the mechanism of expiration. During expiration, the external inter-costal muscles relax as a result, ribs move downward and inward. Saath hi saath muscles of the diaphragm also relax, making it more dome-like. The combined effect of these two actions is that the volume of thoracic cavity is reduced. This causes increase in pressure on lungs. When the air in the lungs increases in this way, the air in the lungs increases. The air inside the lungs decreases, so the air is moved out passively and that is how expiration happens.