CHEMPATH: Acid-base handling Flashcards
- What is the normal H+ concentration?
- What is normal pH?
- What is the definition of pH?
- [H+] 35-45 nmol/l in ECF
- pH 7.35 -7.46
- pH = log 1/ [H+]
How is H+ produced? How much? Where is it excereted?
By metabolism of protein, carbohydrates and fats which make CO2, water and H+.
Around 50-100mmol/day of H+ is made
Excretion mainly by kidney
Why does addition of H+ ions not cause increase in overall H+ concentration in the body?
Buffering systems are at play
Which buffering systems are used to regulate H+ cocentration? What is the disadvantage of these systems?
- Bicarbonate H++HCO3- H2CO3
- (ECF, Glomerular Filtrate)
- Haemoglobin H+ + Hb- HHb
- (Red Cells)
- Phosphate H++HPO4- H2PO4
- (Renal tubular Fluid / Intracellular)
- Also Protein and Bone
Disadvantage: eventually kidneys need to excrete the H+ and regenerate bicarbonate as buffering is only effective in the short term
How is bicarbonate replenished?
Reabsorption in the proximal tubule - virtually all bicarbonate is reabsorbed
Regeneration through carbonic acid i.e. water and CO2, although H+ is produced as a biproduct and this needs to be excreted through exchange with Na+
How is CO2 excreted? How much is produced each day?
NB: also produced by metabolism of proteins, carbohydrates, fats –> CO2, water and H+.
20,000-25,000 mmol/day of CO2 produced
Excreted by the lungs through expiration
What is a buffer?
A buffer is a weak acid with a base that can be used to mop up extra H+ ions (allows excess H+)
What is the acid-base equation?
CO2 + H2O H2CO3 H+ + HCO3-
NB: all parts are interlinked, if one side changes then the other changes too.
What is the buffering system used in blood?
Haemoglobin
What do blood gas machines measure? What is not measured but rather calculated?
Blood gases measure partial pressure of O2 and CO2 and [H+].
Bicarbonate is calculated by the equation below (which uses k coefficient which relates to the solubility of CO2 in blood)
What is the primary abnormality in metabolic acidosis? Name 3 causes of metabolic acidosis.
Primary abnormality is increased H+ (decreased pH) with decreased bicarbonate
Causes of metabolic acidosis:
- Increased H+ production e.g. diabetic ketoacidosis
- Decreased H+ excretion e.g. Renal tubular acidosis
- Bicarbonate loss e.g. intestinal fistula
How is metabolic acidosis compensated?
Respiratory centre is stimulated
Reduction of CO2 occurs by increased respiration
Picture: If you did not have respiratory compensation you could go up to about pH6.9. However, with respiratory compensation the rise should be limited to about pH7.1
What is the primary abnormality in acute respiratory acidosis? What are 3 causes?
Primary abnormality is increased CO2 producing increased H+ (decreased pH) and a slight increase in bicarbonate (2-4 mmol/L).
May be due to:
- Decreased Ventilation
- Poor Lung Perfusion
- Impaired Gas Exchange
How is chronic respiratory acidosis compensated?
Increased renal excretion of H+ combined with generation of bicarbonate
H+ may return to normal but pCO2 and bicarbonate will be elevated
How is metabolic alkalosis compensated?
There is inhibition of the respiratory centre –> rise in pCO2 –> H+ returns to normal
NB: this is limited as pCO2 is required to stimulate respiratory drive