Physiology 8 Flashcards
What are the normal values of:
- Plasma pH
- [HCO3-] in plasma
- Arterial PCO2
- Plasma pH - 7.35-7.45
- [HCO3-] - 23-27
- Arterial PCO2 - 35-45
What are the overall process which occur to restore normal acid-base balance ?
Compensation and correction
Compensation occurs first - this is the restoration of pH irrespective of what happens to pCO2 and [HCO3-]plasma
Correction is the restoration of pH and [HCO3-]p and PCO2 to normal
What are the 4 main classes of acid-base disturbances ?
- Respiratory acidosis and alkalosis
- Metabolic acidosis and alkalosis
What is the immediate response of the body to changes in acid-base balance ?
- The body initally tries to use its buffer stores e.g. Hb, HCO3-
- BUT buffer stores are depleted quickly and need replaced
What are respiratory acid-base disturbances essentially due to ?
Thinking about the equation they are due to changes in pCO2
Specifically what is respiratory acidosis due to ?
Retention of pCO2
This is mainly due to hypoventilitation caused by:
- Chronic bronchitis
- Chronic emphysema
- Airway restriction (bronchial asthma, tumour)
- Chest injuries
- Respiratory depression
What are the values which indicate a respiratory acidosis ?
- pH < 7.35 low
- pCO2 > 45 high
- H+ and HCO3- increase
How is a respiratory acid-base disturbance compensated for ?
It is compensated for by the renal system
Specifically how is respiratory acidosis compensated for ?
Due to CO2 being the drive for H+ secretion in the tubule of the kindeys:
- H+ secretion is stimulated
- All filtered HCO3- is reabsorbed (i.e. no HCO3- excretion)
- H+ continues to be secreted and generates titratable acid (TA) and NH4+
- Acid is excreted and “new” HCO3- is added to the blood
Hence compensated will have a high HCO3-
What is respiratory alkalosis due to ?
Excessive removal of pCO2
Due to hyperventilation e.g.
- At altitude when hypoxic you hyperventilate,
- Fevers and hysterical overbreathing also cause hyperventilation
Explain the values of respiratory alkalosis
Due to shift of the equilibrium to the left Both [H+]p and [HCO3-]p fall
Decreased H+ causes alkalosis (remember that pH is only a measure of free [H+])
Uncompensated respiratory alkalosis is indicated if;
pH > 7.45 and PCO2 < 35 mmHg
How is respiratory alkalosis compensated for ?
Due to pCO2 driving H+ secretion into the tubule of the kidneys
There will be decreased H+ secretion due to decreased pCO2
- This results in insufficient H+ to reabsorb the filtered HCO3-, even though the load is lower than normal
- HCO3- is excreted and urine is alkaline
- No titratable acid (TA) and NH4+ is formed, so no “new” HCO3- is generated
What are metabolic disturbances in acid-base balance due to ?
They are due to changes in H+ conc from any other source than CO2
Specifically what is metabolic acidosis due to ?
Excess H+ from any source other than CO2
E.g.
- Ingestion of acids or acid-producing foodstuffs
- Excessive metabolic production of H+ (e.g. lactic acid during exercise or ketoacidosis)
- Excessive loss of base from the body (e.g. diarrhoea – loss of HCO3-)
What is uncompensated metabolic acidosis indicated by ?
pH < 7.35 and [HCO3-]p low
The low HCO3- is due to it being depleted as it buffers the excess H+
For metabolic acid-base disturbances what compensates for them ?
- The respiratory system as its not the problem so can compensate
- The renal system does the correction as it takes a bit longer to act than the respiratory system
Describe how metabolic acidosis is compensated for
Compensated for by the respiratory system:
- CO2 is blown off shifting the equilibrium to the left resulting in
- Decreased [H+]p and [HCO3-]p
Describe how metabolic acidosis is compensated for
- Filtered HCO3- is very low and very readily reabsorbed
- H+ secretion continues and produces TA & NH4+ to generate more “new” HCO3-
- The acid load is excreted (urine is acidic) and [HCO3-]p is restored
- Ventilation can then be normalised
What is a metabolic alkalosis due to ?
Excessive loss of H+ from the body
E.g.
- Loss of HCl from the stomach (vomiting)
- Ingestion of alkali or alkali-producing foods (e.g. Ingestion of NaHCO3 as an antacid, though not a problem with modern antacids)
What are the values which suggest metabolic alkalosis ?
pH > 7.45 & high [HCO3-]
How is metabolic alkalosis compensated for ?
- Decreased ventilation which causes CO2 retention
- Causing shift of the equilibrium to the left which increases [H+] and [HCO3-]
How is a metabolic alkalosis corrected ?
- Filtered HCO3- load is so large compared to normal that not all of the filtered HCO3- is reabsorbed
- No TA or NH4+ is generated
- HCO3- is excreted (urine is alkaline)
- [HCO3-]p falls back towards normal
