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Several land animals such as snails, some spiders, scorpions and vertebrates, breathe through lungs.
These are air pockets located inside the body. The oxygen gas in the air that enters the lungs diffuses into the blood or hemolymph, distributing itself into the circulation.
Snails, spiders and scorpions have no special mechanism to force air in and out of the lungs; gas renewal occurs by simple diffusion. Already vertebrates have pulmonary ventilation mechanism that ensure the constant renewal of air in the lungs.
Gas Exchange and Gas Transportation
Gas diffusion on respiratory surfaces
Gaseous exchanges between the environment and respiratory surfaces occur through diffusion. Broadly speaking, diffusion is the movement of particles from one region where they are in the highest concentration to another where they are in the lowest concentration.
For oxygen gas to diffuse from water to a fish's gills, for example, there must be a higher concentration of gas in water than in the blood circulating in the gills. On the other hand, for carbon dioxide to diffuse from the gills into the surrounding water, the concentration of this gas must be higher in the blood than in the surrounding environment.
The concentration of a particular gas, whether in air or water, is expressed in terms of its partial pressure. Oxygen gas partial pressures (PO2) and carbon dioxide (PCO2) in atmospheric air are, respectively, on the order of 160 mmHg and 0.23 mmHg.
These values are obtained from the percentage at which each gas is present in the air by atmospheric pressure at sea level. See on the board:
|Gas||Air concentration||Fraction X Atmospheric Pressure||Partial pressure|
|O2||21%||0.21 X 760 mmHg||= 160 mmHg|
|CO2||0,03%||0.003 X 760 mmHg||= 0.23 mmHg|
The air we inspire has POWDER2 equal to 160 mmHg and PCO2 equal to 0.23 mmHg. Inside the lungs the inhaled air mixes with the residual air present there, so that the partial pressures of oxygen and carbon dioxide become, respectively, 104 mmHg and 40 mmHg.
Venous blood that reaches the blood capillaries of the lungs in turn has POWDER2 40 mmHg and PCO2 equal to 45 mmHg. Like the PO2 pulmonary air (104 mmHg) is greater than that of the pulmonary capillary blood (40 mmHg), there is diffusion of oxygen gas from the pulmonary air to the blood. On the other hand, as PCO2 capillary blood (45 mmHg) is greater than the PO2 pulmonary air (40 mmHg), oxygen gas diffuses from the blood to the lungs. As it passes through the capillaries of body tissues, the blood ceases the oxygen gas obtained in the lungs and acquires carbon dioxide.