The process of inspiration and expiration happens due to pressure changes in the thoracic cavity. The thorax is an airtight compartment bounded by the sternum in front, the vertebral column at the back, the ribs encircling the sides and the diaphragm found below. The rib bones are provided with the two sets of muscles namely external and internal intercostal muscles. By the contraction and expansion of these muscles the volume of the thoracic cavity is reduced or increased. The floor of the thoracic cavity is completely closed by the diaphragm. The act of breathing is performed by expansion and contracton of the thoracic cavity.
During quiet respiration, contraction of external intercostal muscle causes the ribs to move anteriorly and outwardly. This movement enlarges the cavity of the thorax by increasing it side to side and in dorso-ventral dimensions.
The contraction of radial muscles of the diaphragm leads to flattening of inelastic, dome shaped central part of the diaphragm. As a result of these muscular movements, the volume of the thoracic cavity is increased. This causes the air pressure within the lungs to fall below the atmospheric pressure. So air (tidal air) from outside passes through the air passage into the lungs to equalize the pressure.
1.The diaphragm relaxes and rises to resume the original dome shape.
2. The ribs take their original position as a result of contraction of the internal intercostal muscles.
Once the air is within the lungs the process of gaseous exchange begins. Capillaries of the pulmonary artery remains close to the wall of the alveloli. This enhances the exchange of gases.
Oxygen and carbon-di-oxide are exchanged across the alveolar membrane by diffusion from the site of higher to low partial pressure until the partial pressure of the two regions are equal. This process is a simple physical one which does not involve any secretary or active transport mechanism.