Acid-Base balance Flashcards
Regarding blood buffers, what is the HCO3:H2CO3 ratio at a PH of 7.4?
A 20
B 16
C 1
D 10
A
Explanation
The ratio at pH-This is taken directly out of the current textbook. In the primary exam you cannot use calculators, so the formula: pH = pka + log HCO3/H2CO3 ; 7.4 = 6.1 + log HCO3/H2CO3; 1.3 = log HCO3/H2CO3 turns out log 20 = 1.3 so ratio is 20 cannot be used.
I think that the values just needed to be learnt. I have left this question as is, as it may come up again
7.4=20
7.3=16
7.1=10
6.0=0.9
Regarding the anion gap which of the following causes a normal anion gap metabolic acidosis?
A Rhabdomyolysis
B Renal tubular acidosis type 3
C Diarrhoea
D Uraemia
C
Explanation
The anion gap refers to the difference between the concentration of cations other than Na and the concentration of anions other than Cl and HCO3 in the plasma. It is increased when the plasma concentrations of K, Ca, Mg are decreased. When the concentration of proteins increase so does the anion gap. It is decreased when cations are increased or when albumin is decreased. Albumin is a negatively charged protein and its loss from the serum results in the retension of other negatively charged ions like chloride and bicarbonate. As theses ions are used to calculate the anion gap, there is a subsequesnt decrease in the anion gap. It is not increased in hyperchloremic acidosis due to ingestion of NH4CL or carbonic anhydrase inhibitors.
Anion gap calculation: Na – (Cl + HCO3)
Gastrointestinal loss of HCO3 due to diarrhoea, causes a normal anion gap metablic acidosis. The HCO3 is lost and replaced by a chloride anion. If vomiting were to occur, it would result in a hypochloraemic alkalosis.
Note: Thre is only 3 types of RTA: type 1, 2, and 4 Uraemia and rhabdomyolysis both cause a raised anion gap metabolic acidosis
Both Uraemia and rhabdomyolysis cause a raised anion gap metabolic acidosis
Which of the following H+ concentration is compatible with life?
A 0.00000001 mol/l
B 0.00000004 mol/l
C 0.000000003 mol/l
D 0.000000005 mol/l
B
Explanation
0.00000004 mol equals a pH of 7.4 (7.3979) (the negative logarithm) pH = -log10(H+) -log(0.000 000 04) = -log(4 x 10 ^ -8) =~8 - 0.6 = 7.4
0.00000001= 8
0.000000003= 8.5
0.000000005= 8.3
Note: 0.00004 meq/l=7.4
Extra: Explanation calculation questions are difficult, better to just know the figures. Normal pH 7.35-7.45 Normal [H+] 35-45nmol/L (though remember low H+=alkalosis)
Extra: The simple rule of thumb for pH is that the number of zeros after the decimal point plus 1 is approximately the pH (add 3 more zeros if measured in millimoles). 0.00000001mol/L is pH of 8 for instance (have to multiply it by 10, 8 times, to get it to 1). A number larger than 1 at the end means a pH a bit lower, so 0.00000004 (same number of zeros) winds up being 7 and a bit, 7.4.
Which of the following statements is correct in relation to the anion gap?
A It is decreased when albumin is increased
B It is increased in hyperchloraemic acidosis due to ingestion of NH4Cl
C It consists mainly of proteins, HPO4, SO4 and organic acids
D It is the difference between cations not including Na and K and anions not including HCO3
C
Explanation
The anion gap refers to the difference between the concentration of cations other than Na and the concentration of anions other than Cl and HCO3 in the plasma. It is increased when the plasma concentrations of K, Ca, Mg are decreased. When the concentration of proteins increase so does the anion gap. It is decreased when cations are increased or when albumin is decreased. Albumin is a negatively charged protein and its loss from the serum results in the retention of other negatively charged ions like chloride and bicarbonate. As theses ions are used to calculate the anion gap, there is a subsequent decrease in the anion gap. It is not increased in hyperchloraemic acidosis due to ingestion of NH4CL or carbonic anhydrase inhibitors.
Extra:
The anion gap, actually a misnomer, refers to the difference between the concentration of cations OTHER than sodium and the concentration of anions OTHER than CL and HCO3 in the plasma. It consists for the most part of proteins in the anionic form, HPO4, SO4, and organic acids. A normal value is about 12mEq/L
Which of the following does not represent an acid load to the body?
A Chronic renal failure (CRF)
B Diabetic ketoacidosis (DKA)
C Ingestion of acid salts
D Fruit
D
Explanation
An anion gap metabolic acidosis can be caused by: Methanol, metformin, uraemia, renal failure, ketoacids, lactic acids, ethanol, salicylates, ethylene glycol, paraldehyde, toluene, iron and cyanide.
Ingestion of acid salts (eg NH4Cl and CaCl2 = acid load) Diabetic ketoacidosis (DKA) (acid load of acetoacetic acid and ß-hydroxybutyric acid) Chronic renal failure (CRF) (HPO42- excreted by amino acid breakdown from the liver is combined with H+ in the kidneys to form H2PO4- which is excreted in urine. Renal failure causes acid buildup) Fruit (the main source of dietary alkali, containing Na+ and K+ salts of weak organic acids and these anions and metabolised to CO2 leaving NaHCO3 and KHCO3 in the body)
Regarding buffers in the blood, which of the following statements is correct?
A Haemaglobin has a lower buffering capacity than plasma proteins
B The principle buffers in interstitial fluid is phosphate
C Carbonic anhydrase is present in plasma
D The carbonic acid-bicarbonate buffering system will function without carbonic anhydrase
D
Explanation
HB is a major buffer in the blood. Hb has six times the buffering capacity of the plasma proteins. This is due to the fact that Hb moleculae contains 38 histidine residues and is much more abundant in the blood. Carbonic anydrase is not present in the plasma. It is found in the red blood cells, gastric acid secreting cells and in renal tubular cells.The principle interstitial buffer is the carbonic acid-bicarbonate system (CA-B). The CA-B system will function without the carbonic anhydrase enzyme, but it will do so slowly
Extra:
In the interstitial fluid, phosphate buffers are the most significant because they are effective in low-concentration fluids. The pKa of phosphate (about 7.4) is close to the normal pH of body fluids, making it ideal for maintaining pH balance in this compartment. Secondary Buffers: The bicarbonate buffer system also plays a role in the interstitial fluid, though it is less prominent than phosphate in these low-concentration areas. Carbonic anhydrase is an enzyme that facilitates the rapid conversion of carbonic acid (H₂CO₃) to carbon dioxide (CO₂) and water (H₂O) in the lungs and kidneys. Without it, the buffering system would function very slowly, making it much less effective.
In a patient with a plasma pH of 7.1, the HCO3-/H2CO3 ratio is?
A 1
B 10
C 0.2
D 20
B
Explanation
pH HCO3/H2CO3 ratio is;
7.4=20
7.3=16
7.1=10
6.0=0.9
Note: Straight from the prescribed text
For this situtation,
pH = pKa + log HCO3/H2CO3 pKa of H2CO3 is 6.1
7.1 = 6.1 + log HCO3/H2CO3 So log HCO3/H2CO3 is 1.0, hence ratio is 10. No need to memorise tables here, just memorise pKa of H2CO3 is 6.1
Which of the following options best describes the changes found in uncompensated respiratory alkalosis?
A Decreased pH and HCO3- and normal PaCO2
B Increased pH, low HCO3-, and normal PaCO2
C Increased pH, normal HCO3-, and a low PaCO2
D Decreased pH, increased HCO3-, and normal PaCO2
C
Explanation
A decline in PaCO2 will produce a respiratory alkalosis. This will increase the pH. Uncompensated respiratory alkalosis means that the HCO3 has not had enough time to be removed (lowered) to compensate for the loss of CO2
In chronic acidosis, the major adaptive buffering system in the urine is which of the following?
A Ammonium
B Bicarbonate
C Phosphate
D Carbamino compounds
A
Explanation
Ammonia (NH3) buffering occurs via the following reaction
NH3 + H = NH4
Ammonia is produced in the proximal tubule from glutamine which is an amino acid, a reaction which is enhanced by an acid load and by hypokalaemia. Ammonia (NH3) is converted to ammonium (NH4) by intracellular H ions and is secreted into the proximal tubular lumen.
In the ascending limb of the loop of Henle, NH4 is transported into the interstitium of the medulla where it dissociates back into NH3 and H ion. The NH3 diffuses into the lumen of the collecting duct, where it is available to buffer H ions, becoming NH4 which is trapped in the lumen and excreted as the chloride salt. Every H ion buffered in this way allows HCO3 to be returned to the systemic circulation.
In chronic acidosis, the amount of NH4+ excreted at any given urine pH also increases, because more NH3 enters the tubular urine. The effect of this adaption of NH3 secretion, the cause of which is unsettled, is further removal of H+ form the tubular fluid and consequently a further enhancement of H+ secretion by the renal tubules and excretion in the urine. Because the amount of phosphate buffer filtered at the glomerulus cannot be increased, urinary excretion of acid via the phosphate buffer system is limited. The production of NH4+ by the renal tubules is the only way the kidneys can remove even the normal amount, much less an increased amount, of non-volatile produced in the body.
Which of the following options best describes these arterial blood gas results pH 7.32, pCO2-31mmHg and HCO3-20mmol/L?
A Mixed respiratory acidosis
B A picture consistent with diuretic abuse
C Compensated metabolic acidosis
D Primary metabolic acidosis
C
Explanation
The formula expressing the expected C02 level in a metabolic acidosis is;
- CO2=(1.5 x HCO3) +8
Therefore;
CO2= 30+8
CO2 =38
Note: There is some respiratory alkalosis because the CO2 is lower than the expected level of 38mmHG. All the provided options do not offer a mixed picture of metabolic acidosis and respiratory alkalosis. The metabolic acidosis is compensated for by hyperventilation as would be seen in DKA. So the option compensated metabolic acidosis is correct.
(I think that it would be better to say partially compensated or offer a mixed metabolic and respiratory option- but I don’t know if this was given as an option)
Hydrogen ions are secreted primarily in the form of:
A Bicarbonate
B H2PO4
C Free form
D NH4
C
Explanation
Hydrogen ions are secreted in the free form, and then buffered via NH4, H2PO4 and bicarbonate to facilitate further secretion.
A 40-year-old man suffers a seizure whilst driving after he was diving earlier that day. Which gas is likely responsible?
A Oxygen
B Carbon dioxide
C Nitrogen
D Carbon monoxide
A
Explanation
Oxygen is the likely cause of this deep sea underwater seizure-CNS oxygen toxicity.
Normal recreational scuba tanks contain air. If you dive below 40m-DEEP SEA, you need to use a mixture of gases. This includes oxygen and helium. An incorrect mixture of these gases can lead to a seizure. The causative gas is oxygen.
The military divers use 100% oxygen tanks and don’t dive below 10m. This avoids the need to resurface slower as there is no nitrogen (no decompression sickness) and there are no bubbles formed-closed circuit (stealth)
Oxygen at depth is a recognised cause of seizures due to CNS toxicity.
Seizure as a presentation of decompression illness is very rare
Ref: van et al lancet 2011;8377;153-64
Extra:
Divers (rather than deep sea experienced commercial divers) who resurface, a seizure would indicate a significant neurological insult. This would likely be from cerebral arterial gas embolism (CAGE) form a rapid uncontrolled ascent.
What happens to HCO3 and buffer when you add a strong acid to a solution?
A Decreased HCO3, decreased buffer
B Increased HCO3, increased buffer
C Decreased HCO3, increased buffer
D Increased HCO3, decreased buffer
A
Explanation
When a strong acid is added to the blood, the major buffer reactions are driven to the left. The blood levels of the three “buffer anions” HB, Prot, and HCO3 consequently drop. The anions of the added acid are filtered into the renal tubules. They are accompanied (“covered”) by cations, particularly NA, because electrochemical neutrality is maintained. The tubules replace the NA with H and in so doing reabsorb equimolar amounts of Na and HCO3, thus conserving the cations, eliminating acid, and restoring the supply of buffer anions to normal.
What is the major buffer in interstitial fluid?
A Hydrogen ions
B Bicarbonate
C Phosphate
D Histidine imidazole groups
B
What is the major buffer in intracellular fluid?
A Hydrogen ions
B Bicarbonate
C Phosphate
D Histidine imidazole groups
C
Explanation
The principal buffer in intracellular fluid is phosphate and proteins.
