Physical principles of respiratory and diffusion of gases

> movement of gases from air to blood
Diffusion, and then oxygen binding to hemoglobin, and more diffusion, etc.

> blood to air
Diffusion, and then some carbon dioxide unbinding from hemoglobin and some bicarbonate ion releasing carbon dioxide, and more diffusion, etc.

whole mechanism of gas exchange is carried by the simple phenomenon of pressure difference. When the atmospheric pressure is low outside, the air from lungs flow out. When the air pressure is low inside, then the vice versa.

For inhalation the low pressure in the lungs is achieved by the contraction of the external intercostal muscles and the relaxation of the internal intercostals which acts to pull the ribcage upwards and outwards, hence, increasing the volume of the thoracic cavity. The diaphragm also contracts to extend the cavity further downwards. Due to the function of the pleural membrane, the size of the lungs is increased which, in turn, creates an area of lower air pressure inside the lungs; hence, air is drawn into the lungs.


Exhalation is the process of expelling air out of the lungs. For this to occur, the air pressure inside the lungs must be higher than that of the external atmosphere as air flows from areas of higher pressure to ones of lower pressure. This is achieved by the relaxation of the external intercostal muscles which bring the rib cage inwards and the diaphragm bulges upwards (recoiling to its original shape) respectively. The internal intercostal muscles contract so as to pull the rib cage further inwards, hence, decreasing the volume of the thoracic cavity and that of the lungs which, in turn, creates high pressure in the lungs; hence air is pushed out of lungs.

Upon inhalation, gas exchange occurs at the alveoli, the tiny sacs which are the basic functional component of the lungs. The alveolar walls are extremely thin (approx. 0.2 micrometres). These walls are composed of a single layer of epithelial cells close to the pulmonary capillaries which are composed of a single layer of endothelial cells. The close proximity of these two cell types allows permeability to gases and, hence, gas exchange.