Acid Base Balance 1&2 Flashcards
What is the normal pH of arterialised blood?
7.4
What is the concentration of free hydrogen ions in blood that has a pH of 7.4?
40x10^ -6 mmoles/l
Do bound hydrogen ions contribute to the pH?
No - only free ions contribute to the pH
What are the sources of hydrogen ions in the body?
Respiratory acid: not normally a factor unless lung function is impaired
Inorganic acids e.g. S-containing amino acids
Organic acids: fatty acids and lactic acid
What is the net gain of hydrogen ions in the body per day?
50-100
What is the major source of alkalis in the body?
Oxidation of organic anions e.g. citrate
What is the normal bicarbonate level in the body?
24 mmoles/l (22-26)
What is the normal CO2 level in the blood?
1.2 mmoles/l
What is the normal range of pH in the blood?
7.37-7.43
What is the normal value of pCO2?
5.3kPa (4.8-5.9) or 40mmHg (36-44)
How does a buffer system work?
It minimises changes in the pH when hydrogen ions are added or removed
What is the ratio of bicarbonate to CO2 when the pH is 7.4?
20:1
How does the bicarbonate buffer system decreases the hydrogen ion concentration?
The excess hydrogen ions drive the equation to the right so that some of the hydrogen ions are removed from solution
What effect does the buffer system have on CO2 and ventilation?
It increases ventilation and decreases CO2
Are the extra hydrogen ions removed from the body?
No - the bicarbonate has buffered the hydrogen ions and prevented them contributing to the pH
How are hydrogen ion removed from the body?
They are excreted by the kidneys
Which organ controls the bicarbonate concentration?
Kidneys
What other buffer exist in the ECF?
Plasma proteins and dibasic phosphate
What are the primary intracellular buffers?
Proteins, organic/inorganic phosphates and haemoglobin (in erythrocytes)
What is the effect of buffering the hydrogen ions using the ICF buffers?
It causes changes in plasma electrolytes since to maintain electrical neutrality movement of hydrogen ions must be accompanied by chloride ions or exchanged for potassium
What is the consequence of moving potassium out of cells into the plasma?
Hyperkalaemia: depolarisation of excitable tissues leading to VF and death
Why is there wasting of bones in chronic renal failure?
Bone carbonate provides an additional store of buffer
How do the kidneys regulate bicarbonate levels?
Reabsorbing filtered bicarbonate and generating new bicarbonate
How is bicarbonate reabsorbed?
1) Active hydrogen ion secretion from the tubule cells
2) Couple to passive sodium reabsorption
3) Filtered bicarbonate reacts with the hydrogen to form HC2CO3 which is converted to CO2 and H2O in the presence of carbonic anhydrase
4) CO2 is freely permeable and enters the cell
5) CO2 is converted back to H2CO3 which dissociates to form hydrogen and bicarbonate
6) The hydrogen is then secreted
Where does the majority of bicarbonate reabsorption occur?
Proximal tubule
Are any hydrogen ions excreted during bicarbonate reabsorption?
No
What are the minimum and maximum urine pH values?
Min: 4.55.0
Max: 8.0
Which weak acids and bases act as buffers in the urine?
Dibasic phosphate, HPO4, uric acid and creatinine
How does Na2HPO4 work in the lumen of the distal tubule?
One sodium ion is reabsorbedin exchange for secreted hydrogen. The monobasic phosphate removes the hydrogen from the body
Are ammonia or ammonium lipid soluble?
Ammonia is
Ammonium is not
How is ammonia produced?
Deamination of amino acids, mainly glutamine
How does ammonia reduce the hydrogen ion concentration?
It combines with the secreted hydrogen ions to form ammonium which combines with chloride ions to form NH4CL which is secreted
What happens to renal glutaminase activity when the pH falls?
Renal glutaminase activity increases and more ammonium is produced and excreted
How long does it take the kidney to maximally respond to acid loads?
4-5 days because of the requirements of increased protein synthesis
What is the max increase in hydrogen ion loss per day in severe acidosis?
250mmoles/l
What is a drop in pH called?
Acidosis
What is a rise in pH called?
Alkalosis
What is respiratory acidosis?
pH has fallen due to a respiratory change which has increased pCO2. Results from reduced ventilation and therefore retention of CO2
What are the acute causes of respiratory acidosis?
Drugs which depress the medullary respiratory centres e.g. barbiturates and opiates
Obstruction of major airways
What are the chronic cause of respiratory acidosis?
Lung disease e.g. bronchitis, emphysema and asthma
What effect will increases pCO2 cause?
Increased secretion of hydrogen ions and increase bicarbonate. NH3 production will also start but it takes time
How effective is the renal compensation in respiratory acidosis?
It will protect the pH but it wall not correct the original disturbance
What happens to the blood gas values in chronic respiratory acidosis?
They are never normalised
What is respiratory alkalosis?
An increase in pH due to a fall in pCO2 - increased ventilation and blowing off CO2
What are the acute causes of respiratory alkalosis?
Voluntary hyperventilation, aspirin and first ascent to altitude
What is the chronic cause of respiratory alkalosis?
Long term residence at altitude
How do the kidneys respond to respiratory alkalosis?
Less bicarbonate is reabsorbed to more bicarbonate is lost in the urine
What is metabolic acidosis?
An acidosis due to a decrease in bicarbonate conc. either due to increased buffering of hydrogen or direct loss of bicarbonate
What are the causes of metabolic acidosis?
Increased hydrogen production e.g. in diabetic ketoacidosis or in lactic acidosis, failure to excrete the normal dietary load of hydrogen as in renal failure or loss of bicarbonate e.g. as in diarrhoea
What happens to ventilation in metabolic acidosis?
Ventilation increases in depth rather than rate = Kussmaul breathing
How do the kidneys correct metabolic acidosis?
By restoring bicarbonate conc. and excreting hydrogen ions
What effect does respiratory compensation have in the renal correction of metabolic acidosis?
It delays the renal correction but protects the pH (much more important)
What happens in metabolic alkalosis?
Bicarbonate levels increase and pCO2 will also have to increase
What are the causes of metabolic alkalosis?
Vomiting: loss of hydrogen ions
Increased renal loss: aldosterone excess/excess liquorice ingestion
Excess administration of bicarbonate
Massive blood transfusions (citrate is used to prevent coagulation but converts to bicarbonate)
What happens to the excess bicarbonate in metabolic alkalosis?
It is excreted in the urine
What effect does high acidity have on potassium levels?
It causes hyperkalaemia as the hydrogen ions are buffered intracellularly in exchange for potassium
How is the hyperkalaemia treated?
1) Give insulin (+ glucose if non-diabetic)
2) Calcium resonium (exchanges calcium ions for potassium ions)
3) Calcium gluconate (IV) - reduces the excitability of the heart and stabilizes the cardiac muscle cell membrane
What takes precedence, restoration of volume or correction of metabolic alkalosis?
Restoration of volume
What is the anion gap?
The difference between the sum of the principal cations and the principal anions (usually 14-18mmoles/l)
What happens to the anion gap in acidosis due to a loss in bicarbonate?
There would be no change
What happens to the anion gap in lactic acidosis or diabetic ketoacidosis?
The anion gap is increased
