Explain: Diffusion of $O_2$ from alveoli to tissues and of $CO_2$ from tissues to alveoli.

(N/A) The exchange of gases occurs primarily by diffusion based on pressure gradients. The following table summarizes the partial pressures (in $mm Hg$) of respiratory gases:

Respiratory Gas	Atmospheric Air	Alveoli	Blood (Deoxygenated)	Blood (Oxygenated)	Tissues
$O_2$	$159$	$104$	$40$	$95$	$40$
$CO_2$	$0.3$	$40$	$45$	$40$	$45$

The data indicates a concentration gradient for $O_2$ from alveoli to blood and from blood to tissues. Similarly, a gradient exists for $CO_2$ in the opposite direction, from tissues to blood and from blood to alveoli.
Since the solubility of $CO_2$ is $20-25$ times higher than that of $O_2$, the amount of $CO_2$ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher than that of $O_2$.
The diffusion membrane is composed of three major layers:
$(1)$ Thin squamous epithelium of alveoli.
$(2)$ The endothelium of alveolar capillaries.
$(3)$ The basement membrane in between them.
The total thickness of this membrane is much less than a millimeter, facilitating rapid diffusion. Thus, all factors are favourable for the diffusion of $O_2$ from alveoli to tissues and $CO_2$ from tissues to alveoli.