ABSTRACT
H u m a n plasm a unde r norma l circumstance s ha s a p H o f 7.4 , w h i c h i s main -
tained b y a H C 0 3 ~ / H 2 C 0 3 bufferin g system . H 2 C 0 3 i n actualit y i s a combinatio n
of tru e H 2 C 0 3 an d dissolve d C 0 2 . Their relationshi p to H C 0 3 ~ an d t o eac h othe r
is s h o w n i n Equation (3.17):
C 0 2 + H 2 0 ^ H 2 C 0 3 ^ H C 0 3 " + H + (3.17 )
For eac h reaction , a n equilibriu m constant ca n be writte n a s
[ H C C V ] [ H + ] [ H 2 C O J K l ~ [ H 2 C 0 3 ]
A combine d pK derive d from both K x an d K 2 i s 6. 1 an d i s used i n the Henderson -
Hasselbalch equatio n involvin g bloo d bicarbonat e buffe r system . T h e conjugat e
base i s H C 0 3 ~ , an d the aci d is designate d H 2 C 0 3 . Bot h are expressed i n terms o f
meq/L. Th e H C 0 3 ~ i s als o terme d th e metabolic componen t o f th e buffe r system ,
whereas H 2 C 0 3 i s referred to a s the respiratory component . T h e latter depends o n
the partia l pressur e o f C 0 2 , or pC0 2. Unde r normal conditions, w e ca n write th e
Henderson-Hasselbalch expression fo r the bicarbonat e system a s s h o w n i n E q u a -
tion (3 .19):
I n re d bloo d cells , th e enzym e tha t catalyze s th e conversio n o f C 0 2 an d H 2 0 t o
H C 0 3 ~ an d H + i s carbonic anhydrase, an d i t plays a crucia l rol e i n maintaining th e
p H o f th e organis m a t a constan t level .
