Wednesday, March 20, 2013

Renal physiology (Part 4):acid-base balance



Renal physiology: acid-base balance

The normal pH of extracellular fluid (ECF) is 7.4.

The Henderson-Hasselbalch equation describes the relationship between pH and the concentration of conjugate acid and base.
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

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