1. Mechanism of respiration:
Breathing or ventilation is the mechanical process of inhalation of fresh air (oxygen) and exhalation of foul air (carbon dioxide). It consists of the following two steps:
Process of inhaling air into the lungs is called inspiration. At first, the muscles between the ribs contract. When external muscles between these ribs contract, the ribs are pulled out and the chest cavity expands and secondly, a muscular partition wall between the chest cavity and the lower abdominal cavity, called diaphragm, contracts in such a way that it is brought down a little bit (Convexed). This also expands the chest cavity, called diaphragm, contracts in such a way that it is brought down a little bit. This also expands the chest cavity. Thirdly, there is a contraction of muscles of the stomach. All these events create a partial vacuum in the chest cavity and the atmospheric air rushes in the lung by respiratory passage.
When the exchange of gases in the lungs has taken place, the air has to be expelled. Expulsion of the air from the lungs is called expiration. Reversal of events that had occurred in inspiration makes foul air go out of lungs in atmosphere.
2. Exchange of gases in lungs:
The air inhaled by inspiration comes to alveoli. All around the alveoli there is a network of blood capillaries. These capillaries are extremely fine tubes with only one layered wall. The deoxygenated blood collected from different parts of the body is at first brought to the heart, and from here pumped out to the lungs. This blood which may also be called venous blood is sent to the lungs where it has to pass through the network of capillaries around the alveoli. The oxygen from the alveolus diffuses out into the blood capillary due to difference in partial pressures of oxygen and is picked up by the hemoglobin molecules present inside the red blood corpuscles. Again, carbon dioxide, which is in greater amount in the venous blood, comes from the capillary into the alveolus.