subscripts
0	reference state
$ar$	arterial blood
$air$	environment air
$blood$	blood
$CV$	control volume
d	destroyed
$diffusion$	Diffusion
e	exit
$exp$	expired air
f	fetus
$Hb$	reacted with hemoglobin
i	inlet
$lung$	lung
m	mother
M	associated with the metabolic heat
$ve$	venous blood