1. Introduction to Physiology
Acid & Base
Dr. Barney Ward - Consultant Anaesthetist

2. Acid & Base What are acids and bases ( for non-chemists)
Why is keeping acid / base balance important
How does the body manage this balance
How can we look at what is going on in the body?

3. What are acids? Acids donate protons (H+ ions)
Eg. Hydrochloric acid contains 1 hydrogen atom and 1 chlorine atom
In solution HCl splits into ions (charged particles) HCl  H+ + Cl-
HCl is a strong acid, and almost all dissociates into H+ and Cl-

4. What are bases? Bases can accept protons (H+ ions)
Eg. sodium hydroxide (NaOH) splits in solution to form Na+ and OH-
NaOH  Na+ + OH-
OH- ions can accept protons (H+) to form water
OH- + H+  H20

5. Strong vs. weak In chemistry, we meet many strong acids & bases
In biology, most acids and bases are much weaker
Strong acids dissociate almost completely
Weaker acids do not…
CH3-COOH  CH3COO- + H+ (cf. HCl  H+ + Cl-)

6. Measuring acidity Concentration of H+ ions in solution - [H+]
Actual numbers not helpful!
Range M
Taking negative base10 logarithm gets rid of zeroes
Number known as pH & gives range of

7. Measuring acidity For example - water has [H+] = 0.0000001 M
- log10 ( ) = 7 (neutral pH)
Acids have pH values less than 7
Bases have pH values greater than 7

8. Acids have pH values less than 7
Bases have pH values greater than 7

9. Why is maintaining pH important?
Changes in acidity profoundly affect proteins
Protein function relies entirely on their shape
If pH changes, so does the shape of proteins
Narrow limits of normal pH
Remember a 1 unit pH change = 10 fold [H+] change

10. What disturbs the body’s pH?
CO2 production (respiration)
CO2 is a weak acid when in solution
Produced when glucose metabolysed via glycolysis
Occurs in all cells, all the time

11. What disturbs the body’s pH?
Metabolic acids
Under strenuous exercise, anaerobic metabolism occurs
Lactic acid produced
Also commonly accumulates in shock (poor perfusion state)
Protein breakdown produces acids

12. How does the body prevent pH changes?
Buffers
Buffers allow acid to be ‘stored’ temporarily
Bicarbonate / CO2 is the most important:
H+ + HCO3 -  CO2 + H2O
Excess acid (H+) drives the reaction to the right
CO2 produced can be excreted by the lungs

13. How does the body prevent pH changes?
Buffers
Other buffer systems:
Proteins - can reversibly accept H+ ions
Intracellular phosphate - HPO42- + H+  H2PO4-

14. How does the body prevent pH changes?
Short term buffering requires a final solution
Respiratory acid (CO2) is excreted by the lungs
Metabolic acids are excreted by the kidneys

15. Acid & Base in practice
Within the body, excess acid produces an acidosis
Excess base produces an alkalosis
Acidosis & alkalosis can be produced from either a respiratory or metabolic source

16. Acid & Base in practice CO2 too high = respiratory acidosis
CO2 too low = respiratory alkalosis
Metabolic acids too high = metabolic acidosis
Metabolic acids too low = metabolic alkalosis

17. Acid & Base in practice - Blood gases
Arterial blood gas analysis reveals oxygenation & acid
base status of the body
The analysis usually includes: pH
PaO2
PaCO2 BE
Pa = Partial arterial pressure of O2 or CO2
BE = Base Excess (-ve values = acidosis, +ve values = alkalosis)

18. Acid & Base in practice - Blood gases
Some normal values
pH – 7.45 (no units)
PaO2 10 – 13 KPa
PaCO2 4 – 4.5 KPa
BE mmol/l ve = acidosis, +ve=alkalosis

19. Blood gases 1 - Oxygenation
To interpret PaO2 result, you need to know what oxygen concentration the patient is breathing
PaO2 9 (10 – 13 KPa)
pH (7.35 – 7.45)
PaCO2 (4 – 4.5 KPa)
BE ( mmol/l)
%age O2 is often given as Fractional inspired O2
eg. 21% O2 = FiO2 0.21

20. Blood gases 2 - Acid & Base
This patient has severe sepsis
PaO (10 – 13 KPa)
pH (7.35 – 7.45) low pH= acid
PaCO (4 – 4.5 KPa) high CO2= acid
BE ( mmol/l) -ve = acid
First look at pH - it tells you what the overall problem is
Next look at the PaCO2 - it tells you what the respiratory side of things is doing
Then look at the base excess (BE) which tells you about the metabolic component
Here there is a metabolic acidosis (eg. severe sepsis)

21. Blood gases 3 - Compensation
This patient has diabetic ketoacidosis
PaO (10 – 13 KPa)
pH (7.35 – 7.45) low pH= acid
PaCO (4 – 4.5 KPa) high CO2= acid
BE ( mmol/l) -ve = acid
The body NEVER over compensates for disturbances
Here there is a respiratory alkalosis trying to compensate for a metabolic acidosis

22. Blood gases 4 - Compensation
The lungs can compensate for disturbances QUICKLY
The kidneys can only compensate for metabolic disturbances SLOWLY
BUT…
When things go wrong in the body, a metabolic acidosis can build up FAST

23. Acid / Base Don’t worry if this is clear as mud!
More blood gas work in group session