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Gli scambi respiratori

metabolic exchanges take place at the walls of the pulmonary alveoli consist of cells covered by a thin veil fluid and the blood is separated by two thin layers of cells: dell'alveolo the wall and the endothelium of the capillary.

The gas exchange is due to the pressure difference between the oxygen of the air and that of blood (phenomenon of gas diffusion ).

more concentrated oxygen in the air (21%) than in blood (16%) tends to go from first to second. Dissolved in blood plasma, oxygen is determined by hemoglobin in red blood cells, whose heme (prosthetic group containing Fe) binds oxygen via reversible bonds, resulting in oxyhemoglobin, again because of the pressure difference, the greater the in blood alveoli, carbon dioxide passes from this to the cavity of the alveoli, from which is expelled through exhalation.
Oxygen is poorly soluble in water to which 98% of the oxygen carried by the blood stream is bound to hemoglobin.

Carbon dioxide is much more soluble and, once dissolved, most of it reacts with water to form carbonic acid in the blood, according to the reaction

CO2 + H2O H2CO3 ----»

This breaks down into H + ions and bicarbonate ions HCO3-So in the end is 9% in physical solution in plasma and 64% are present in plasma and red blood cell cytosol under form of bicarbonate ion, 27% is bound to hemoglobin in the form of carbodiossiemoglobina. In the lungs the carbon dioxide bound to hemoglobin is dissociated from it and diffuses into the alveoli together with that dissolved in the plasma in the form of carbonic acid is released by the action of the enzyme carbonic anhydrase that catalyzes the reaction: H2CO3
----» CO2 + H2O

and then spreads as well.

Nitrogen, present for 4 / 5 in the air, an inert gas that does not react with hemoglobin: is inhaled and exhaled as such, only in small quantity is dissolved in plasma, where, however, does not no reaction.