Introduction to Physiology Acid & Base

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 • PaCO24.2 (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 • PaCO22.1 (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

Introduction to Physiology Acid & Base