ChemPath 3S: Acid-Base Flashcards
Summarise the production of H+ ions by cells of the body
- Metabolism of proteins, carbohydrates and fats produce CO2, H2O and H+ ions
- 50-100 mmol/day of H+ is produced
- Some of this is buffered (see image)
- Most of the H+ is excreted by the kidneys
What are the limitations of buffering H+ ions with HCO3- ions in the ECF? How is this overcome?
Limitation:
- As you buffer the H+, you use up the [HCO3-] ions
- The [HCO3-] buffer is only effective in the short term
Overcome by:
- To maintain normal homeostasis, the kidney needs to excrete H+ ions and regenerate bicarbonate
- Bicarbonate is regenerated through the production of carbonic acid → [HCO3-] is reabsorbed back into the blood
- H+ ions CANNOT pass through the membrane by itself, so a transport system is necessary (Na+/H+ exchange)
Summarise the production of CO2 by cells of the body
- Metabolism of proteins, carbohydrates and fats produce CO2, H2O and H+ ions
- 20,000-25,000 mmol/day of CO2 is produced and then excreted by the lungs (in any one day)
Summarise the how CO2 is sensed and excreted
- Respiration is controlled by chemoreceptors in the hypothalamic respiratory centre
- In health, any increase in CO2 stimulates respiration (CO2 is excreted via lungs) → maintain stable CO2 concentration
What is the role of RBCs in buffering?
- The buffer in RBCs is Hb
- CO2 will be taken up by RBCs and it is buffered by Hb, thereby controlling the concentration of H+ ions
- Produces HCO3- ions as a by product (alongside HHb)
What is the interrelationship between the lungs and the kidneys in the excretion of products of metabolism?
What are the normal values in an ABG of:
- pH
- PaCO2
- serum bicarbonate
- PaO2
How are serum bicarbonate values reported in an ABG?
not directly… it calculates it using the information in photo
What is the primary acid base abnormality seen in metabolic acidosis?
What may be the causes of metabolic acidosis?
How does respiratory compensation in metabolic acidosis work?
- As soon as [H+] increases, your body will try to compensate by increasing RR and blowing off more CO2
- In a compensated metabolic acidosis, you will see a low pCO2
What is the primary acid base abnormality in respiratory acidosis?
N.B. a slight increase in bicarbonate is due to slight ‘compensation’ of the body to correct acidosis
What are the possible causes of respiratory acidosis?
How does metabolic compensation in respiratory acidosis work?
- Over a few days, this leads to increased renal excretion of H+ combined with generation of bicarbonate
- H+ may return to near normal but pCO2 and bicarbonate remain elevated
Which compensation mechanism is faster? Respiratory or metabolic?
Metabolic compensation is slower
- as kidneys can’t respond as fast as the lunch (which can be immediate)
What is the primary abnormality in metabolic alkalosis?
The primary abnormality is decreased H+ (increased pH) with increased HCO3
What is the compensatory mechanism of metabolic alkalosis?
This tends to inhibit the respiratory centre (identified by a rise in pCO2)
- H+ may then return towards normal (as CO2 rises in the blood, and thus H+ levels)
What primary abnormality is seen in respiratory alkalosis?
What can occur in chronic respiratory alkalosis? (compensatory mechanism)
If this mechanism is prolonged (chronic resp. alkalosis),
- → this can lead to decreased renal excretion of H+ and less bicarbonate generation
- → H+ may return to normal but pCO2 and bicarbonate will remain low
severe metabolic acidosis with partial respiratory compensation
- low pH shows acidosis
- low CO2 shows respiratory compensation
- high O2 shows lungs are functioning well (and is on O2!)
- low bicarbonate shows metabolic acidosis
What may be the cause of a patient’s metabolic acidosis?
What is the acid base disturbance in this case?
64 yo female with 3 week Hx of:
- intermittent vomiting
- abdo pain
- weight loss
O/E:
- dehydrated
- jaundiced
- hypotensive
- oliguric
metabolic alkalosis with partial respiratory compensation
What may be the cause of a patient’s metabolic alkalosis?
What is the acid-base abnormality seen here?
Acute respiratory alkalosis
- no metabolic component (compensation), hence no bicarbonate change
What is the acid-base abnormality seen here?
- 72yo man
- long Hx of COPD
- on diuretics for heart failure
- serum K = 2.6 (3.5-5.5)
Respiratory acidosis + metabolic alkalosis
What is the acid-base abnormality seen here?
Respiratory alkalosis + metabolic acidosis
- classic aspirin overdose picture
- stimulates resp centre → hyperventilation → low CO2
- reduced H+ excretion by kidney (hence bicarbonate is low as used up)
What is the acid-base abnormality seen here?
SEVERE mixed respiratory acidosis + metabolic acidosis
- metabolic acidosis because CO2 and bicarbonate levels are moving in opposite directions! (low bicarbonate)
- O2 therapy (very high O2 level)
