Acid-Base Balance Flashcards
What is the difference between volatile and fixed acids?
Volatile acids can be eliminated from the body as a gas, e.g. CO2.
Dietary acids and acids produced by anaerobic respiration are ‘fixed’ and cannot be converted to CO2.
Name the 2 ways the kidneys regulate acid-base balance?
- Reabsorb filtered HCO3.
- Secreted fixed acids;
- Titrate non-HCO3 buffer in urine (primarily PO4)
- Secrete NH4 to urine.
Where does H+ secretion occur in the kidney?
PCT
DCT
Collecting duct
Where does HCO3 reabsorption occur in the kidney?
90% in PCT
10% in intercalated discs of DCT/Collecting duct.
How is HCO3 reabsorbed?
Carbonic anhydrase is found in the brush border of the lumenal side of the cells. This generates CO2 + H2O from filtered HCO3 + H+, which moves into the cell and is converted back by carbonic anhydrase to produce HCO3 which moves out the cell via the basolateral membrane to the interstitium. The H+ ion is transported back across the tubular membrane to join with HCO3.
1:1 bicarbonate in and out.
How are fixed acids secreted?
Similar but results in formation of a new bicarbonate.
Either titration of filtered PO4 or secreted as NH4 into the urine.
Filtered phosphate (HPO4) combines with H+ to make H2PO4 which is excreted in the tubule.
The H+ comes from H2O + CO2 being catalysed to H+ and HCO3 by carbonic anhydrase in the cell.
HCO3 leaves the cell to the interstitum.
The ammonia system uses glutamine metabolism which releases NH3 which combines with the H+ ion to give NH4 which is excreted in the urine.
How would the H+, HCO3 and pCO2 appear in metabolic acidosis?
What could cause metabolic acidosis?
Increased [H+].
Decreased [HCO3-] to buffer H+.
Decreased pCO2 due to increased ventilation to blow off CO2.
Build up of acid through metabolism (e.g. muscle metabolism leads to lactic acid build up).
Ingestion of acid (e.g. methanol).
Failure to excrete acid (e.g. renal tubular acidosis).
Loss of HCO3- in the stool (diarrhoea) or urine (renal tubular acidosis).
What would H+, HCO3 and pCO2 be like in metabolic alkalosis?
What could cause it?
Decreased [H+]
Increased [HCO3]
Increased pCO2 (due to decreased ventilation to raise CO2).
Vomiting - stomach acid is highly acidic.
Increased aldosterone production - causes increase K+ secretion in the urine and H+ in used to compensate for K+ lost via the H+/K+ antiporter.
Describe H+ and pCO2 in Respiratory acidosis?
Describe compensatory mechanisms?
Increased pCO2 due to hypoventiltion, causing increased [H+].
Most increased [H+] is buffered intracellularly.
Renal compensation - H+ ions are secreted and HCO3 is reabsorbed more (Increased [HCO3]?).
Respiratory compensation - stimulation of arterial and CSF chemoreceptors causes an increase in breathing rate.
Describe H+ and pCO2 in respiratory alkalosis?
Describe compensation mechanisms?
Decreased pCO2 due to hyperventilation, leading to decreased [H+].
Renal compensation - H+ ions are generated and HCO3 is secreted (decreased [HCO3]?).
Resiratory compensation - stimulation of arterial and CSF chemoreceptors causes an increase in breathing rate.
What is the carbonic acid buffering equation?
CO2 + H2O H2CO3 HCO3- + H+
How is the anion gap calculated?
[Na+] - [Cl-] + [HCO3-] = AG
What is the normal anion gap?
6-12 mmol/L
What causes a rise in the anion gap?
Decrease HCO3 which is not compensated for by Cl-. May be compensated for by another anion.
What is the anion gap used for?
To diagnose causes of metabolic acidosis.
If AG goes up could be lactic acidosis, ketoacidosis or ingestion of acid (poisoning).
If AG doesnt go up could be diarrhoea.
