● `color{brown}("Alveoli")` are the primary sites of exchange of gases.

● Exchange of gases also occur between `color{violet}("blood and tissues.")`

● `color{brown}(O_2 "and" CO_2)` are exchanged in these sites by `color{violet}("simple diffusion")` mainly based on `color{brown}("pressure/concentration")` gradient.

● `color{violet}("Solubility")` of the gases as well as the thickness of the membranes involved in `color{violet}("diffusion")` are also some important factors that can affect the `color{violet}("rate of diffusion.")`

● Pressure contributed by an individual gas in a mixture of gases is called `color{brown}("partial pressure")` and is represented as `color{violet}(pO_2)` for oxygen and `color{violet}(pCO_2)` for carbon dioxide.

● `color{violet}("Partial pressures")` of these two gases in the `color{violet}("atmospheric air")` and the two sites of diffusion

● The data given in the table clearly indicates a concentration `color{violet}("gradient for oxygen")` from `color{violet}("alveoli")` to `color{violet}("blood and blood to tissues.")`



● Similarly, a gradient is present for `color{violet}(CO_2)` in the opposite direction, i.e., from `color{violet}("tissues")` to `color{violet}("blood")` and `color{violet}("blood to alveoli.")`

● As the solubility of `color{violet}(CO_2)` is `color{brown}("20-25 times higher")` than that of `color{violet}(O_2)`, the amount of `color{violet}(CO_2)` that can diffuse through the diffusion membrane per unit `color{violet}("difference in partial pressure")` is much higher compared to that of `color{violet}(O_2)`.

● The `color{violet}("diffusion")` membrane is made up of three major layers, namely, the `color{brown}(" thin squamous epithelium")` of `color{violet}("alveoli,")` the `color{brown}("endothelium")` of `color{violet}("alveolar capillaries")` and the `color{brown}("basement substance")` in between them.

● However, its total thickness is much less than a `color{violet}("millimetre.")`

● Therefore, all the factors in our body are favourable for `color{violet}("diffusion")` of `color{brown}(O_2)` from `color{brown}("alveoli")` to `color{brown}("tissues")` and that of `color{violet}(CO_2)` from `color{violet}("tissues to alveoli.")`

● `color{violet}("Blood")` is the medium of transport for `color{violet}(O_2)` and `color{violet}(CO_2)`.

● About `color{brown}("97% of " O_2)` is transported by `color{violet}("RBCs")` in the `color{violet}("blood.")`

● The remaining `3 %` of `color{violet}(O_2)` is carried in a `color{brown}("dissolved state")` through the `color{violet}("plasma.")`

● Nearly `color{brown}("20-25 % of " CO_2)` is transported by `color{violet}("RBCs")` whereas `color{brown}("70 %)` of it is carried as bicarbonate.

● `color{brown}("About 7 %")` of `color{violet}(CO_2)` is carried in a `color{violet}("dissolved state")` through `color{violet}("plasma.")`

● `color{brown}("Haemoglobin")` is a red coloured iron containing pigment present in the `color{violet}("RBCs.")`

● `color{violet}(O_2)` can bind with `color{violet}("haemoglobin")` in a reversible manner to form `color{brown}("oxyhaemoglobin.")`

● Each `color{violet}("haemoglobin")` molecule can carry a maximum of `color{brown}("four molecules")` of `color{violet}(O_2)`.

● `color{violet}("Binding of oxygen")` with `color{violet}("haemoglobin")` is primarily related to partial pressure of `color{violet}(O_2)`.

● Partial pressure of `color{violet}(CO_2)`, `color{violet}("hydrogen")` ion concentration and temperature are the other factors which can interfere with this `color{violet}("binding.")`

● A `color{brown}("sigmoid")` curve is obtained when percentage saturation of `color{violet}("haemoglobin")` with `color{violet}(O_2)` is plotted against the `color{violet}(pO_2)`.

● This curve is called the `color{brown}("Oxygen dissociation curve")` and is highly useful in studying the effect of factors like
`color{violet}(pCO_2, H^+)` concentration, etc., on `color{violet}("binding of" O_2 "with haemoglobin")`.

● In the `color{violet}("alveoli,")` where there is high `color{violet}(pO_2)`, low `color{violet}(pCO_2,)` lesser `color{violet}(H^+)` concentration and lower temperature, the factors are all favourable for the `color{violet}("formation of oxyhaemoglobin")`.

● Whereas in the `color{violet}("tissues,")` where low `color{violet}(pO_2)`, high `color{violet}(pCO_2)`, high `color{violet}(H^+)` concentration and higher temperature exist, the conditions are favourable for `color{violet}("dissociation of oxygen")` from the `color{violet}("oxyhaemoglobin.")`

● This clearly indicates that `color{violet}(O_2)` gets bound to `color{violet}("haemoglobin")` in the lung surface and gets dissociated at the `color{violet}("tissues.")`

● Every 100 ml of `color{violet}("oxygenated blood")` can deliver around 5 ml of `color{violet}(O_2)` to the `color{violet}("tissues")` under normal `color{violet}("physiological conditions.")`