Acid-Base Balance Flashcards
Normal pH of arterial blood? What is the acceptable ranges?
- 4 is normal ideal value
7. 37-7.43 is the ideal range
Free [H+] in normal pH of blood?
40 x10^-9 moles/l
or
40x10^-6 mmoles/l
What sort of hydrogen ions contribute to pH?
Free
Does body make hydrogen ions? Where from?
Yes
Respiratory Acid - CO2
Metabolic Acid - inorganic (Phosphoric acid) /organic acids (lactric, fatty etc)
Major source of alkali?
Oxidation of organic anions - citrate
Difference between anion and cation?
Anion = -ve cation = +ve
What is a buffer?
Minimises changes in pH when H+ ions are added or removed
What is the HH equation measuring?
the pH in terms of the ratio of [A-]/[HA] NOT the absolute amounts
So what is the most important extracellular buffer in the body?
Bicarbonate
What is the ratio in the body between Bicarbonate:Carbonic acid.
20:1
Normal pCO2 level?
5.3 kPa / 40 mmHG
Normal HCO3 - conc
24 mmoles/L
What is the chemical reaction for the bodies buffer system?
(H+) + HCO3- H2CO3 H2O + CO2
What is the basic mechanism by which this buffer works?
Increase in ECF H+ ions drives reaction to the right to use up excess H ions and balance pH
What would normally happen in a normal buffer system in H+ was increased? Why is this ineffective in the body?
Reaction would drive to right but eventually right side would drive back and an
equilibrium will be reached
Ineffective as a buffer system in body as won’t last long
What does the body do to prevent this equilibrium occuring?
Expels the CO2 on the right side of the equation via increased ventilation
What is an important point to remember about the H ions when buffering occurs?
H ions aren’t removed - just buffered and stopped from contributing to pH changes
What happens if there is a decrease in H ions?
Reaction moves left
Where are H ions eliminated from the body?
Kidney’s
What is renal excretion of H ions coupled with?
The regulation of plasma HCO3- conc.
What are some intracellular buffers?
Organic and inorganic phospahtes
Proteins
What does buffering of H+ ions intracellularly cause and why?
Changes in plasma electrolytes since movement of H+ ions must be accompanied by Cl- or exchanged for a cation like
K+ to keep charges neutral
What causes hyperkalaemia in acidosis?
The movement if K+ out of cells by Intracellular buffering
Leads to depolarisation of excitable tissue = ventricular fibrillation and death
Why does chronic renal failure lead to wasting of bones?
Bone carbonate provides an additional store of buffer
How much more bicarbonate is needed that carbonic acid to maintain a pH of 7.4?
20x more
How does the kidney regulate bicarbonate conc.?
Reabsorbs filtered bicarbonate and generates new bicarbonate
Both of which depend on active H+ ion secretion from the tubule cells into the tubule lumen
How does the mechanism for reabsorption of bicarbonate work?
Active H+ ion secretion from tubule cells is coupled to passive Na+ reabsorption
The filtered bicarbonate reacts with secreted H+ ions to form carbonic acid
Carbonic acid then reacts in the in the presence of carbonic anhydrase to become CO2 and H2O
CO2 and H2O get absorbed into the proximal tubule cell and react with carbonic anhydrase again to form carbonic acid
Carbonic acid breaks down into H+ and Bicarbonate
Bicarbonate gets reabsorbed into peritubular capillary and the H+ ions used again and secreted out of cell actively in conjunction with passive Na+ reabsorption t
What is the minimun and maximum urine pH values in humans?
4.5
8
What acts as a buffer for urine to stop pH dropping too low?
Several weak acids and bases act as buffers. Most is done by dibasic phosphate, hydrogen phospahte also uric acid and creatinine
What is this process of buffering urine called and why is it important?
Titratable acidity
Generates bicarbonate and removes hydrogen ions
Where does titratable acidity happen?
In distal tubule
Explain titratable acidity?
Sodium hydrogen phosphate exchanges a Na+ ion with a H+ ion (Na goes into cell H+ comes out)
This new molecule is excreted from body
New bicarbonate is made in the cell by combining CO2 and H2O to make carbonic acid then into bicarbonate and 1 hydrogen ion via carbonic anhydrase
The bicarbonate enters blood with the Na+ ion given off by sodium hydrogen phosphate and the excess hydrogen ion is used in the cycle again at step 1
What is resp. acidosis?
Fall in pH due to a reduced ventilation and therefore retention of CO2
Causes pCO2 to increase
Causes of resp. acidosis?
Acute - drugs like barbiturates and opiates which suppress the medullary reps. centres
Chronic - lung diseases like bronchitis, asthma, emphysema
Bodies response to resp. acidosis?
Increased levels of bicarbonate to stabilize pH
Also increased reabsorption of bicarbonate
But this doesn’t treat the underlying cause - so blood gas values may still be abnormal but since renal system is compensating then pH is at a safe level
What is resp. alkalosis?
Increased pH due to hyperventilation and increased CO2 blow off
Causes of resp. alkalosis?
Acute - volunatry hyperventilation, aspirin or 1st ascent to altitude
Chronic - long term resistance at altitude or a decrease is pO2 below 60 mmHg stimulates peripheral chemoreceptors to increase ventilation
Bodies response to resp. alkalosis?
Bicarbonate should be lowered
Less H+ is available for secretion into tubule therefore less filtered bicarbonate is reabsorbed
What is metabolic acidosis?
A decrease of bicarbonate due to metabolic issues - either due to an increase buffering of H+ or a direct loss of bicarbonate
causes of metabolic acidosis?
Increase production of H+ ions - either due to DKA or lactic acid increase
Failure to excrete normal dietary load of H+ due to renal failure
Loss of bicarb via diarrhoea - failure to reabsob dietary bicarb
Bodies response to metabolic acidosis?
Stimulates ventilation into a kussmaul pattern to expel CO2 and increase pH
Kidneys also can help correct disturbances by restoring bicarbonate levels and excreting more H+ ions - but only if healthy
What is a problem with the way the body responds to metabolic acidosis?
The increase resp rate to expel CO2 gets rid of the bodies source for H+ ions - carbonic acid
This basically means that the resp. compensation delays renal compensation to protect the pH - but pH more important in short term
What is metabolic alkalosis?
Increase in bicarb
Bodies response to increased bicarb/metabolic alkalosis?
Increase PCO2
Causes of metabolic acidosis?
- increased loss of H+ ions via vomiting and losing gastric secretions
- Increased renal H+ ion loss due to aldosterone excess, or excess liquorice ingestion
- Excess administration of bicarbonate in a renal impaired patient (can’t compensate)
- Massive blood transfusions - blood banks contain citrate to prevent coagulation which is converted to bicarbonate in body (at least 8 units to have an effect)
What is a unwanted affect of increased PCO2?
increase of CO2 helps reabsorb bicarb
This response helps stabilize pH in short term but again delays the renal excretion of excess bicarbs
Why does liquorice cause metabolic acidosis?
contains glycyrrhizic acid which has a similar effect to aldosterone
What is the anion gap?
A measure of the difference between the principle cations (Na+/K+) and anions (Cl-/HCO3-).
Usually about 14-18mmoles/l more cations than anions.
Why is anion gap useful?
In acidosis
It increases when bicarbonate is used up by lactic acidosis/DKA
It stays the same when HCO3- is lost in the gut as its compensated by extra Cl-
Patient with pH = 7.32, [HCO-3] = 15 mM, PCO2 = 30mmHg (4kPa)
What Acid/base disturbance is this? Why?
Metabolic acidosis
HCO3 and pCO2 are both low
Patient with pH = 7.32, [HCO-3]= 33 mM, PCO2 = 60mmHg (8kPa)
What acid/base disturbance is this?
Chronic resp. acidosis
Chronic as more H+ means more HCO3 production and reabsorption - but this takes time to occur and bicarbonate would be normal in an acute attack
Patient with pH = 7.45, [HCO-3] = 42 mM, PCO2 = 50mmHg (6.7kPa)
What is wrong?
Metabolic alkalosis - both are high
pH = 7.45, [HCO-3]= 21 mM, PCO2 = 30mmHg (4kPa)
What is wrong?
Acute resp. alkalosis
pCO2 is low and bicarb is normal so not chronic
In chronic the bicarb would be low too
Patient with pH = 7.31, PCO2 = 7.7.kPa, (58mmHg), [HCO3-] =36mmoles/l.
Which of the following is true:
- It is likely that he has renal disease.
- He may have an acute respiratory infection.
- It is possible that he may have chronic bronchitis.
- There will be a decrease in his excretion of ammonium ions.
- His plasma potassium will be reduced.
3 is true as his high bicarb indicates chronic resp. acidosis
1 - can’t be renal as HCO3 is raised in an acidotic condition
2 - Can’t be acute
4 - False as ammonium extretion would increase
5 - Plasma K+ would increase due to increased H+ entering cells and K+ exiting
pH = 7.25, [HCO3-] = 12mmoles/l, PCO2 = 3.3kPa (25mmHg)
Which of the following are true?
1) They are indicative of a respiratory acidosis
2) The reduction in Pco2 is a result of under-breathing
3) The subject has probably been taking bicarbonate of soda
4) It could be related to impaired renal function
5) The subject may have been vomiting very badly
4 is true - metabolic acidosis
1 - Low pCO2 indicates a metabolic cause of acidosis
2 - acidosis causes hyperventilation
3 - Bicarb levels are low
5 - vomiting would cause loss of H+ ions via gastric secretions leading to alkalosis
