1. Hydrogen ion homeostasis and blood gases

2. Overview Hydrogen homeostasis Sources of hydrogen in the body
Acid base control
Assessment of acid base status
Disorders of hydrogen ion homeostasis
Acidosis
Alkalosis

3. Hydrogen homeostasis Hydrogen ion homeostasis is essential for life.
Examples:
Mitochondrial functioning
Charge and shape of proteins
Ionization of Ca++ and Mg++

4. Normal [H+] Concentrations
35 – 45 nmol/L in the ECF
40 – 40,000 nmol/L in urine
1,000,000,000 nmol/L in gastric acid

5. Sources of hydrogen ion
Carbon dioxide is the major source of acid in the body.
CO2 + H2O ↔ H2CO3
H2CO3 ↔ HCO3- + H+

6. Other sources of Hydrogen
Glycolysis  lactic acid
Lipolysis  Free fatty acid
Ureagenesis  Urea synthesis
Ketogenesis  Ketoacids
Renal excretions of buffered acids  H+

7. The Body and pH:
Homeostasis of pH is controlled through extracellular & intracellular buffering systems
Respiratory: eliminate CO2
Renal: conserve HCO3- and eliminate H+ ions
Electrolytes: composition of extracellular (ECF) & intracellular fluids (ICF)
- ECF is maintained at 7.40

8. Respiratory Control Mechanisms
Works within minutes to control pH; maximal in hours
Only about 50-75% effective in returning pH to normal
Excess CO2 & H+ in the blood act directly on respiratory centers in the brain
CO2 readily crosses blood-brain barrier reacting w/ H2O to form H2CO3
H2CO3 splits into H+ & HCO3- & the H+ stimulates an increase or decrease in respirations

9. Renal Control Mechanisms:
Don’t work as fast as the respiratory system; function for days to restore pH to, or close to, normal
Regulate pH through excreting acidic or alkaline urine; excreting excess H+ & regenerating or reabsorbing HCO3-
Excreting acidic urine decreases acid in the EC fluid & excreting alkaline urine removes base

11. Assessment of acid base status
Direct measurements: H+ PH
PCO2
PO2
Derived measures:
Bicarbonate (HCO3)

12. Use heparinized blood and measure within 10 minutes

15. Metabolic Disturbances:
Alkalosis: elevated HCO3- (>26 mEq/L)
Causes include: Cl- depletion (vomiting, prolonged nasogastric suctioning), Cushing’s syndrome, K+ deficiency, massive blood transfusions, ingestion of antacids, etc.
Acidosis: decreased HCO3- (<22 mEq/L)
Causes include: DKA, shock, sepsis, renal failure, diarrhea, salicylates (aspirin), etc.

16. Acid base disorders
Metabolic acidosis (↓ HCO3, ↓ pCO2) Metabolic alkalosis (↑ HCO3, ↑ pCO2) Respiratory acidosis (↑ pCO2, ↑ HCO3) Respiratory alkalosis (↓ pCO2, ↓ HCO3)

22. Acid-Base w/o Compensation:
Parameters: pH
PaCO2
HCO3-
Metabolic
Alkalosis
Normal
Acidosis
Respiratory

24. Interpretation Practice:
pH: 7.31
PaCO2: 48
HCO3-: 24
pH: 7.47
PaCO2 : 45
HCO3- : 33
Normal values
Arterial pH: 7.35 – 7.45
HCO3-: 22 – 26 mEq/L
PaCO2: 35 – 45 mmHg

25. Acid-Base Fully Compensated:
Parameters: pH
PaCO2
HCO3-
Metabolic
Alkalosis
Normal
>7.40
Acidosis
<7.40
Respiratory

26. Interpretation Practice:
pH: 7.36
PaCO2: 56
HCO3-: 31.4
pH: 7.43
PaCO2 : 32
HCO3: 21
Normal values
Arterial pH: 7.35 – 7.45
HCO3-: 22 – 26 mEq/L
PaCO2: 35 – 45 mmHg

27. Acid-Base Partially Compensated:
Parameters: pH
PaCO2
HCO3-
Metabolic
Alkalosis
Acidosis
Respiratory

28. Interpretation Practice:
pH: 7.47
PaCO2: 49
HCO3-: 33.1
pH: 7.33
PaCO2 : 31
HCO3- : 16
Normal values
Arterial pH: 7.35 – 7.45
HCO3-: 22 – 26 mEq/L
PaCO2: 35 – 45 mmHg

36. Case Study 4:
Mrs. D is admitted to the ICU. She has missed her last 3 dialysis treatments. Her ABG reveals the following:
pH: ( )
PaCO2: 32 ( 35-45mmHg)
HCO3-: 18 (22-26mEq/L)
Assess the pH, PaCO2 & HCO3-. Are the values high, low or WNL?

37. References
presentation
interactive-tutorial
Marshall, W. and Bangert, S. (2008). Clinical chemistry (6th ed.). Edinburgh, London: Mosby Elsevier. ISBN (chapter 3)
Gaw, A. et al. (2004). Clinical Biochemistry (3rd ed.)
Beckett, G. et al. (2008). Clinical Biochemistry (8th ed.)
Bishop., et al. (2000). Clinical Chemistry (4th ed.)