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.
What is equal to bicarbonate creation?
Acid excretion
Bicarbonate creation and acid excretion 1 2 3 4 5 6 7 8
1) Na/H+ antiport secretes H+
2) H+ in filtrate combines with filtered HCO3- to form CO2
3) CO2 diffuses into cell and combines with water to form H+ and CO3- (under influence of carbonic anhydrase)
4) H+ is secreted again and excreted
5) HCO3- is reabsorbed
6) Glutamine is metabolised to NH4+ and HCO3-
7) NH4+ is immediately excreted
8) HCO3- is reabsorbed
What happens to HCO3- in the kidneys if concentrations of it exceed those of H+?
It remains in the tubule, is excreted
What happens if H+ in the kidneys exceeds concentrations of HCO3-?
Binds other buffers (EG phosphate and creatinine).
Number of days of acidosis before renal tubule cells begin making more ammonia?
About 2 days of acidosis before more ammonia is made to buffer
How is acidic urine formed?
When H+ secretion is in excess of bicarbonate.
Leads to acid excretion.
Event that accompanies passing of acidic urine
HCO3- production.
Bicarbonate reabsorption into capillary is associated with formation of bicarbonate within cell
Cell type in the kidneys associated with control of acidosis
Intercalated cell type A
How do intercalated cell type A respond to acidosis?
H+/K+ ATP ase secrete H+, take in K+.
