Normal serum pH
7.4
How is serum [K+] connected to acid/base balance?
Because K+ and H+ are exchanged for one another (acutely anyway)
Organs involved in acid-base balance (2)
Lungs: regulate CO2
Kidneys: regulate H+ and HCO3-
H+ removal mechanisms (4)
Lungs: hyperventilation to remove CO2
Kidneys:
-HCO3- reabsorption in the PCT
-Titratable acid excretion in the collecting tubule (H+ and phosphate)
*Generates HCO3- too
-Ammonium (NH4+) excretion - most efficient
Bicarb buffer system
H+ + HCO3- ⇌ H2CO3 ⇌ H2O + CO2
What organ is responsible for restoring HCO3- levels when they are altered?
The kidney
Potential causes of metabolic acidosis (4)
↓pH, ↓HCO3-
-Shock
-Diarrhea
-Kidney disease
-DKA
How does shock cause metabolic acidosis?
Hypoperfusion → tissues not getting enough O2 → lactate production (acidic)
Anion gap equation
Na+ - (Cl- + HCO3-)
Normal anion gap (AG)
8-12
Definition of anion gap mediated acidosis (AGMA) vs non-anion gap mediated acidosis (NAGMA)
AGMA = AG >12 - acidosis is due to excess acid production (shock, DKA)
NAGMA = AG <12 (normal) - acidosis is due to HCO3- depletion and replacement with Cl- (diarrhea, kidney disease)
Mechanism of AGMA vs NAGMA
Na+ - (Cl- + HCO3-)
AGMA: excess H+ is buffered by HCO3-, causing HCO3- levels to fall
NAGMA: HCO3- is lost from the body but Cl- rises to correct the electrical balance (no change in AG)
How does excess albumin affect the AG?
It decreases it because it’s negatively charged (-)
Potential causes of metabolic alkalosis (3)
↑pH, ↑HCO3-
-Vomiting
-Diuretics
-Excess aldosterone
How to assess resp compensation to metabolic acidosis or alkalosis
Use this equation and think about how pCO2 needs to change to correct/maintain [H+]
[H+] = 24 + (pCO2 / HCO3-)
Approach to acid/base problems
- Clinical picture - vomiting, diarrhea, shcok, etc.
- pH
<7.4 = acidemia
>7.4 = alkalemia - Determine if metabolic or respiratory - look at HCO3- and pCO2 levels
*If metabolic - assess for resp compensation - Calculate AG (if acidotic)
