Acid/Base

• Henderson-hasselbalch equation can be used to calculate and estimate blood pH levels
• water and CO2 form carbonic acid in a reversible reaction which then dissociates to H and bicarbonate ions
• kA is the dissociation constant of how readily bicarbonate dissociates
• kA derived from the law of mass action where the product of concentration of dissociated substance is divided by the concentration of non-dissociated substance is a constant
• the concentration of bicarbonate ions is proportional to the concentration of CO2 as water is a constant
• the logarithm of kA is then taken and rearranged to give a new equation

  $pH =$$pK_A +$$log([HCO_3^-] / [CO_2])$
• henry's law states that the concentration of a gas in a liquid is proportional to its partial pressure therefore the concentration of CO2 is the constant concentration times the partial pressure of CO2 which is 0.03
• the henderson-hasselbalch equation is:

  $pH=$$pK_A +$$log ([HCO_3^-]/0.03*$$PCO_2)$
• blood pH can be calculated knowing that:

  • pKA = 6.1
  • normal plasma [HCO3-] = 24mM
  • normal arterial PCO2 = 40mM
• normal plasma pH = 7.4
• plasma pH depends on the ratio of bicarbonate ions to PCO2 - anything which alters either of these two factors will alter plasma pH
• the acid-base balance is a homeostatic mechanism to maintain a constant internal environment with very tight regulation of plasma and body fluid pH
• pH influences enzyme function (change/denature in the wrong pH) which affects metabolic processes (enzyme catabolize) and cellular functions (eg binding O2 to Hb)
• the Bohr effect explains how a change in pH effects how Hb binds
• plasma pH is dependent on anything which alters HCO3- or PCO2 which will alter pH
• plasma pH is proportional to [HCO3-]/PCO2 and its changes are best seen by plotting the change on a graph of plasma [HCO3-] against pH in a davenport diagramme
• patient with difficulty breathing (respiratory problems - asthma, blockage etc) will have an increased material PCO2 shifting the balance from 40mmHg to 60mmHg. As PCO2 increases the ratio value will decrease it is classed as respiratory acidosis
• a decrease in pH is not desirable and so the body attempts to restore the ratio by increasing the concentration of bicarbonate ions - the kidneys absorb more HCO3 in the renal tubules and excrete less of it in the urine
• when the body increases HCO3 in response to respiratory acidosis and the pH is back to the correct level it is called compensated respiratory acidosis
• a diabetic with no access to blood glucose due to poor regulation will use fats for energy causing ketones to build up
• acids are normally buffered by plasma bicarbonate ions and when acids are produced in excess bicarbonate ions become overwhelmed and cannot cope causing the ratio of bicarbonate ions to PCO2 to drop causing the pH to also drop
• when the pH of plasma decreases due to an increase in acidic ketones it is metabolic acidosis, specifically diabetic ketoacidosis if the metabolic acidosis is due to diabetes
• compensation for metabolic acidosis includes decreasing PCO2 through hyperventilation and kussmal breathing (respiratory compensation)
• when metabolic acidosis is compensated through kussmal breathing the pH becomes normal again is it called compensated metabolic acidosis
• shifting right on the Davenport diagram means that the patient is experiencing alkalosis - this happens as the overall value of the ratio increases rather than decreasing
• PCO2 decreases due to hyperventilation from panic attacks, asthma attacks, or high altitudes and is then compensated by decreasing plasma bicarbonate ion concentration through increasing its renal excretion
• plasma bicarbonate ion concentration increases as excess acid is lost from the body (excess vomiting, and cancer patients) this is compensated by increasing PCO2 by hypoventilation (breathing less)
• you must know the clinical biological history to diagnose the patients in regards to their acid-base status
• causes of metabolic acidosis include:

  • renal disease
  • diabetic ketoacidosis
  • lactic acidosis
  • chronic diarrhoea
  • poisoning/overdose
• renal disease causes the impaired excretion of H ions causing them to be retained and the increased excretion of bicarbonate ions causing metabolic acidosis
• diabetic ketoacidosis causes the production of keto-acids as a byproduct of fatty acid metabolism causing metabolic acidosis
• lactic acidosis causes an increased anaerobic metabolism as a result of tissue anoxia causing metabolic acidosis
• chronic diarrhoea causes the loss of bicarbonate-rich body fluids as the pancreas secretes bicarbonate ions to combat stomach acid, causing metabolic acidosis
• poisoning or overdosage causes the production of acidic metabolites i.e. lactic acid produced from salicylate overdose (aspirin), formic acid and oxalic acid produced with alcohol poisoning. causing metabolic acidosis
• vomiting causes the loss of hydrogen ions in gastric fluid leading to metabolic alkalosis
• ingestion of alkali (sodium bicarbonate) is usually delt with by renal mechanisms but if there are renal problems then it may cause metabolic alkalosis if ingested in large volumes
• potassium deficiency causes hydrogen ions to be excreted by the kidneys in response to the low blood potassium concentration causing metabolic alkalosis
• acute respiratory impairment (obstruction in the respiratory tract, sudden asthma attack, bronchopneumonia) can cause respiratory acidosis as kidneys have no time to initiate compensatory mechanisms therefore pH can drop quickly leading to coma or death
• increasing bicarbonate reabsorption can take up to 48-72 hours to become effective
• chronic obstructive airway disease (COAD) such as emphysema and bronchitis requires long-term renal adjustments meaning that the blood pH tends towards normal but it is compensated and does not represent a normal acid-base status