Explain the transport of $O_2$ and $CO_2$ between alveoli and tissues with a diagram.

(N/A) Alveoli are the primary sites of exchange of gases. Exchange of gases also occurs between blood and tissues.
$O_2$ and $CO_2$ are exchanged in these sites by simple diffusion,mainly based on pressure/concentration gradients.
Solubility of the gases as well as the thickness of the membranes involved in diffusion are also important factors that affect the rate of diffusion.
Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as $pO_2$ for oxygen and $pCO_2$ for carbon dioxide.
Partial pressures of gases (in $mmHg$) at different sites are given in the table below:

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

[Diagram: Diagrammatic representation of exchange of gases at the alveolus and the body tissues with blood and transport of oxygen and carbon dioxide]