p H b u f f e r s y s t e m s in t h e b o d y •
F i g u r e 1 5 . 1 4
H
H
When H+ builds up, causing pH to decrease, the
amine group combines with H+.
When H+ decreases, causing pH to rise, the
carboxylic acid group releases H+.
b. Carbonic acid-bicarbonate buffer system
H
2
CO
3
--------- ►
H+
+
HCO3-
Carbonic acid
Hydrogen ion
Bicarbonate ion
(weak acid)
When pH rises, the reaction will shift to the
right to release additional H+ into the blood.
c.
Phosphate buffer system
+
I
0
H
2
PO4-
-----►
H2O
+
HPO42-
Hydroxide ion
Dihydrogen
Water
Monohydrogen
(strong base)
phosphate
phosphate
(weak acid)
(weak base)
Dihydrogen phosphate releases H+.
H+
+
HCO
3
-
--------- ►
H
2
CO
3
Hydrogen ion
Bicarbonate ion
Carbonic acid
(weak base)
If the pH drops, the reaction will shift to the left,
using up some additional H+ that is in solution.
The carbonic acid-bicarbonate buffer system is an
important and dynamic buffer that can be changed
by actions of the lungs and kidneys.
H+
+
HPO42-
--------- ►
H
2
PO4-
Hydrogen ion
Monohydrogen
Dihydrogen
(strong acid)
phosphate
phosphate
(weak base)
(weak acid)
Monohydrogen phosphate binds H+.
Because phosphates are the major anions in the
intracellular fluid, the phosphate buffer system is
important in regulating the pH of the cytosol.
It is the major buffer in the urine.
as hemoglobin in red blood cells and albumin in blood
plasma either release or combine with
H+ to maintain
normal pH.
The carbonic acid-bicarbonate buffer system is an im-
portant system for the blood. It is based on the bicarbonate
ion (HCO3- ), which can act as a weak base, and carbonic
acid (H2CO3), which can act as a weak acid. This buffer sys-
tem works with mechanisms of the kidneys and lungs, as we
will see next. Carbonic acid can dissociate or form, depend-
ing on whether there is an excess or shortage of H+.
The phosphate system regulates the cytosol and is the
most important buffer in the urine. Its mechanism is simi-
lar to the mechanism for the carbonic acid-bicarbonate
buffer system.
The Kidneys Help Maintain the Acid-Base Balance of Body Fluids
459
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