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

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?

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-

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

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-)

Measuring acidity Concentration of H+ ions in solution - [H+] Actual numbers not helpful! Range 0.1 - 0.00000000000001 M Taking negative base10 logarithm gets rid of zeroes Number known as pH & gives range of 0 - 14

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

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

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 7.35 - 7.45 Remember a 1 unit pH change = 10 fold [H+] change

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

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

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

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

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

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

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

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)

Acid & Base in practice - Blood gases Some normal values pH 7.35 – 7.45 (no units) PaO2 10 – 13 KPa PaCO2 4 – 4.5 KPa BE -2 - +2 mmol/l -ve = acidosis, +ve=alkalosis

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 (-2 - +2 mmol/l) %age O2 is often given as Fractional inspired O2 eg. 21% O2 = FiO2 0.21

Blood gases 2 - Acid & Base This patient has severe sepsis PaO2 10 (10 – 13 KPa) pH 7.0 (7.35 – 7.45) low pH= acid PaCO2 4.2 (4 – 4.5 KPa) high CO2= acid BE -19 (-2 - +2 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)

Blood gases 3 - Compensation This patient has diabetic ketoacidosis PaO2 10 (10 – 13 KPa) pH 7.3 (7.35 – 7.45) low pH= acid PaCO2 2.1 (4 – 4.5 KPa) high CO2= acid BE -19 (-2 - +2 mmol/l) -ve = acid The body NEVER over compensates for disturbances Here there is a respiratory alkalosis trying to compensate for a metabolic acidosis

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

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