149 - Acid-Base Balance Flashcards
Plasma buffer systems
1
2
HCO3- + H+ H2CO3 (carbonic acid)
H2CO3 CO2 + H2O
Features of HCO3- + H+ H2CO3 / H2CO3 CO2 + H2O
Equilibrium is far in favour of CO2.
Very slow reaction, that is fast in the presence of carbonic anhydrase.
Location of carbonic anhydrase
In the cytoplasm of all cells.
Particularly high in RBCs
Ratio of HCO3- + H+ and H2CO3
H2CO2- and H+ = 26mM
H2CO3 = 3 micromolar (H2CO3 is mostly in the form of CO2 + H2O, rapidly converted by carbonic anhydrase)
From the action of carbonic anhydrase, what form a buffer system in the blood?
CO2, bicarbonate.
These are effectively an acid-base pair.
pK of CO2 + H2O HCO3- + H+
6.1 (when 50% of reaction is on one side)
Henderson-Hasselbach equation
For any acid-base pair, pH = pKa + log([base]/[acid])
Example of Henderson Hasselbach equation for bicarbonate/carbon dioxide system
pH = pKa + log([base]/[acid])
- 1 + log([HCO3-]/[CO2])
- 1 + log([HCO3-]/[0.03 x pCO2])
[HCO3-] is often 24mM
pCO2 is often 40mmHg
With these values, pH ~= 7.4
Example of plasma buffering systems, other than CO2/HCO3-
1
2
3
1) Plasma proteins (~10mEq).
2) Phosphate (~2mEq), as H PO42- or H2PO4- or H3PO4
3) Intracellular haemoglobin (Hb + H+ HbH+)
CO2 conversion to HCO3-
CO2 formed in mitochondria diffuses into blood and into RBC. High carbonic anhydrase in RBC converts it to bicarbonate.
Organs that can alter pH
1 a
2 a, b
• Lungs can
– alter pCO2 through changes in ventilation
• Kidneys can
– alter HCO3- by changes in production & excretion
– alter pH by changes in H+ excretion
Acids produced in the body, dealt with by the kidneys 1 2 3 4
- Sulphuric & phosphoric acids from proteins & lipids
- Lactic acid anaerobic metabolism
- Keto acids from fatty acids
- 70 mmol of strong acid per day
How are acids initially buffered?
By HCO3-.
Some buffering also provided by Hb
Example of a state where keto acids can be very high
Diabetes
First thing to show up when there is an increase in non-volatile acids
Decreased HCO3-
Non-volatile acids
Acids that aren’t H2CO3. Not breathed out by the lungs
Acid/base Gi secretions
Acid in stomach acid, HCO3- in pancreatic secretions
Anion gap
Anions and cations in body should all add together for a net charge of 0.
All ions can’t be realistically measured
When commonly-measured anions and cations (Na+, K+, HCO3-, Cl-) are measured and added together, unmeasured anions leave a gap of 12mmol/L.
What contributes most to anion gap?
Albumin contributes ~80%
Acidoses associated with a high anion gap
1
2
3
1) Lactic acidosis (from ischaemia, anaerobic exercise)
2) Diabetic ketoacidosis
3) Renal injury
Effect of hypoventilation on pH
Decrease in pH from retention of CO2.
Bicarbonate will increase in response, but not so much as to negate decreased pH.
How does HCO3- increase when there is a respiratory acidosis?
Increased production, reabsorption of HCO3- in the kidney.
Equilibrium will favour HCO3-
Renal response to respiratory acidosis
Increased production and reabsorption of HCO3-.
Increased H+ excretion.
Renal response is quite limited.
Example of a metabolic alkalosis, and body response
1
2
3
1) Vomiting leads to loss of H+.
2) Slightly decreased ventilation (but can’t do this much, as you need to breathe).
3) Kidneys increase HCO3- filtration, reduces the amount of H+ excretion. This is a stronger compensation than that of the lungs.
