Renal physiology:
acid-base balance
The normal
pH of extracellular fluid (ECF) is 7.4.
 |
So,
Acid-base disorders are due to alterations in bicarbonate [HCO3-]
or CO2
·
Metabolic acid-base disorders are
due to change in bicarbonate.
·
Respiratory acid-base disorders are
due to change in CO2.
Buffering
systems:
limit [H+] fluctuation in the blood by eliminating excess acid produced
by the body.
1. Bicarbonate
buffer system (main): H+ + HCO3-
↔ H2CO3 ↔ H2O + CO2
·
Bicarbonate buffering system is
the main buffering of ECF acids and is regulated by both the kidneys and
lungs.
·
In kidney, 85% of HCO3
is reabsorbed by PCT into blood in exchange of Cl- buffering ECF H+.
2. Phosphate
system: H+ + HPO42- ↔H2PO4-
3. Protein
buffers: H+ + Protein- ↔ HProtein
[Abbreviation: H2O = water; CO2
= carbon dioxide; HCO3- = bicarbonate; H2CO3
= carbonic acid; H+ = hydrogen ions; HPO42- =
phosphate ions; H2PO4- = phosphoric acid].
Normal
Arterial blood gases values:
PH = 7.35-7.45
pCO2
= 35-45 mm
Hg
pO2
= 80-95 mm
Hg
|
O2
saturation = 95-99%
HCO3
= 22-26
mEq/L
|
[Abbreviation: pCO2 = pressure of
arterial carbon dioxide; pO2 = pressure of arterial O2; HCO3-
= bicarbonate].
5 - Steps Guide to ABG Analysis
1. Is pO2 normal (sample arterial or
mixed)? It may decrease with hypoxia.
2. Is the pH normal, acidotic or
alkalotic?
3. Are the pCO2 or HCO3
abnormal? Which one appears to influence the pH?
4. If both the pCO2 and HCO3 are
abnormal, the one which deviates most from the norm is most likely causing an
abnormal pH.
5. Check the pO2. Is the
patient hypoxic?
Diagnosis of
various acid-base balance disorders
Respiratory Acidosis
|
|||
pH
|
pCO2
|
HCO3
|
|
Acute
|
< 7.35
|
> 45
|
Normal
|
Partly
Compensated
|
< 7.35
|
> 45
|
> 26
|
Compensated
|
Normal
|
> 45
|
> 26
|
Respiratory Alkalosis
|
|||
pH
|
pCO2
|
HCO3
|
|
Acute
|
> 7.45
|
< 35
|
Normal
|
Partly
Compensated
|
> 7.45
|
< 35
|
< 22
|
Compensated
|
Normal
|
< 35
|
< 22
|
Metabolic Acidosis
|
|||
pH
|
pCO2
|
HCO3
|
|
Acute
|
< 7.35
|
Normal
|
< 22
|
Partly
Compensated
|
< 7.35
|
< 35
|
< 22
|
Compensated
|
Normal
|
< 35
|
< 22
|
Metabolic Alkalosis
|
|||
pH
|
pCO2
|
HCO3
|
|
Acute
|
> 7.45
|
Normal
|
> 26
|
Partly
Compensated
|
> 7.45
|
> 45
|
> 26
|
Compensated
|
Normal
|
> 45
|
> 26
|
