6.3 Mechanism of Breathing

. Mechanism of Breathing The movement of air between the atmosphere and the lungs is known as ventilation or breathing. Inspiration and expiration are the two phases of breathing. Inspiration is the movement of atmospheric air into the lungs and expiration is the movement of alveolar air that diffuse out of the lungs. (Figure . ) Lungs do not contain muscle fibres but expands and contracts by the movement of the ribs and diaphragm. The diaphragm is a sheet of tissue which separates the thorax from the abdomen. In a relaxed state, the diaphragm is domed shaped. Ribs are moved by the intercostal muscles. External and internal intercostal muscles found between the ribs and the diaphragm helps in creating pressure gradients. Inspiration occurs if the pressure inside the lungs (intrapulmonary pressure) is less than the atmospheric pressure likewise expiration takes place when the pressure within the lungs is higher than the atmospheric ­pressure. SURFACTANTS are the thin non– cellular films made of protein and phospholipids covering the alveolar membrane. The surfactant lowers the surface tension in the alveoli and prevents the lungs from collapsing. It also prevents pulmonary oedema. Premature Babies have low levels of surfactant in the alveoli may develop the new born respiratory ­distress syndrome (NRDS) because the synthesis of surfactants begins only after the th week of gestation. XI Std Biology-Zoology Chapter- Figure . Mechanism of Breathing Air inhaled. Air exhaled. Rib cage expands. Rib cage gets smaller. Lung Diaphragm Inspiration Expiration Inspiration Expiration Respiratory centre initiates the stimuli during inspiration. The diaphragm and exspiratory muscles contract. The thoracic volume increases as the chest wall expands. The intra pulmonary pressure is ­reduced. The alveolar pressure decreases than the atmospheric pressure Air is taken inside due to expansion of alveoli. Air flows into the alveoli until the alveolar pressure equalizes the atmospheric pressure and the alveoli get inflated. Respiratory centre terminates the stimuli during expiration. The diaphragm relax but internal intercostal muscles contract. The thoracic volume decreases as the chest wall contracts. The intra pulmonary pressure is increased. The alveolar pressure increases than