Lec. 6
Hydrogen ion homostasis and
blood gases
The normal processes of metabolism result
in the net formation of 40-80mmol/24h of
hydrogen which excreted by the kidney in
the urine.
 Incomplete oxidation of energy substrates
generates acids e.g. Lactic acid.
 Accumulation of lactic acid it will produce
imbalance in the metabolic rate.
 Carbon dioxide is generated during
oxidative metabolism which is excreted by
the lungs.

CO2 itself is not an acid, in the presence of
water it can hydrated to form a weak acid
(carbonic acid )
CO2+ H2O ↔ H2CO3
 Pulmonary ventilation is controlled so that
CO2 excretion exactly matches the rate of
formation
 Hydrogen ion concentration of the body is
maintained in ECF
 The intracellular hydrogen ion conc. is
slightly higher but controlled.

Buffering of hydrogen ions
As hydrogen ions are generated they are
buffered to limit the rise of hydrogen ion
conc. (formation of weak acid)
H+ + HCO3- ↔ H2CO3
 If the hydrogen ion concentration falls,
carbonic acid dissociates, generating
hydrogen ions.
 The bicarbonate buffer system is the most
important in the ECF.

The capacity of the bicarbonate system
enhanced by CO2 and disposed of by
conversion into CO2 and water.
 To maintain the capacity of the buffer
system the bicarbonate must be
regenerated.

Bicarbonate formation can be continue if
these hydrogen ions are removed.
 This process occurs in the cells of the renal
tubules where the H+ ions secreted in to
the urine and bicarbonate is generated and
retained in the body.

Bicarbonate reabsorption and
hydrogen ion excretion
The glomerular filtrate contains the same
conc. of bicarbonate as the plasma.
 If bicarbonate were not reabsorbed, a
plenty of it would excreted in the urine and
causes acidosis.

 The
luminal surface of renal tubular
cells is impermeable to bicarbonate.
 Carbonic acid is formed within the
renal tubular cells from CO2 and
H2O in the presence of carbonate
dehydrates.
 Then the carbonic acid dissociates in
to H+ and HCO3-.
The bicarbonate ions pass across into the
interstetial fluid while the H+ ions
secreted across the luminal membrane in
exchange for Na+ ions.
 Then in tubular fluid, H+ ions combine
with bicarbonate to form carbonic acid
which dissociate into CO2 and H2O.
 Some of the CO2 diffuses back into renal
tubular cells while the reminder secrete in
the urine.

Hydrogen ions are secreted into the
tubular fluid during bicarbonate
reabsorption.
 The principal urinary buffer is phosphate.
HPO4-- + H + ↔ H2PO4 Ammonia, an important urinary buffer
which produced by the deamination of
glutamine in renal tubular cells in the
presence of glutaminase.
NH3 + H+ ↔ NH4+

Transport of carbon dioxide
CO2 produced by aerobic metabolism,
diffuses out of cells and into the ECF.
 In red blood cells metabolism is anaerobic
and little carbon dioxide is produced.
 CO2 diffuses into red cells down a
concentration gradient and carbonic acid is
formed facilitated by carbonic dehydratase.

The overall effect of this process is that
carbon dioxide is converted to bicarbonate
in red blood cells which diffuses out of the
cells.
 In the lungs Liberating hydrogen ions
combine with bicarbonate to form CO2 to
be excreted in the expired air while
bicarbonate diffuses into the cells from the
plasma.
