Biology EXCHANGE OF OXYGEN

### KEY TOPICS

star Exchange of gases
star Transport of Gases
star Transport of Oxygen

### EXCHANGE OF GASES

● 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.")

### TRANSPORT OF GASES

● 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.")

### TRANSPORT OF OXYGEN

● 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.")