Gaseous exchange in the alveoli
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.
In the atmospheric air there is a high concentration of oxygen 20-95% (PO2 140mm Hg) while the proportion of carbon dioxide is low (0.04%).
The alveolar PO2 is about 100mm Hg and the PO2 of venous blood is about 40mm Hg. This pressure gradient is sufficient for the transfer of O2. The PCO2 of venous blood is 46mm.Hg and that of alveolar air is only 6mm.Hg (1/10th of O2), it is adequate for CO2 transfer by diffusion. CO2 diffuses 20 times faster than O2.
Regulation of respiration :
In the brain the medulla oblongata contains a respiratory center. This controls breathing. The respiratory center consists of an inspiratory center and an expiratory center. The axons from the nerve cells of these centres lead to the intercostals muscle through the intercostals nerves and the diaphragm via the phrenic nerves. These nerve fibres transmit impulses to the external intercostal muscles and internal intercostal muscles alternately. The walls of the alveoli have sense endings which are stimulated by changes in the tension of alveolar walls.
Mechanism of Breathing :
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.