Acid Base Balance 1 and 2 Flashcards
Why is ECF pH so closely monitored?
Metabolic reactions are extremely sensitive to the pH of their environment, so a change in ECF pH causes severe metabolic disturbance
What is the normal ECF pH?
7.4
In what form does hydrogen contribute to ECF pH?
Only free H+ ions contribute
What are the two sources of H+ for the body?
- Respiratory acid
2. Metabolic acid via metabolism of organic and inorganic acids
What is the major source of alkali for the body?
Oxidation of organic anions such as citrate
What is the role of buffers in relation to H+ ions?
They minimise changes in pH when H+ ions are added
What is the name of the equation which gives the pH of a buffer solution?
Henderson-Hasselbalch equation
What is the most important extracellular buffer?
Bicarbonate buffer
What determines the amount of bicarbonate in the body?
The amount of CO2 dissolved in the plasma, which depends on the solubility of CO2 and PCO2
What is the ‘standard bicarbonate’?
24mmoles/l
What is the normal pKa and mmHg value for ECF pCO2?
5.3 kPa (4.8-5.9)
40mmHg (36-44)
What is the special significance of the bicarbonate buffer system versus an ordinary buffer system?
There is involvement with ventilation - this IS NOT a closed system. When there is an increase in H+, the reaction is pushed to the right, but ventilation increases and therefore there is higher expulsion of CO2, eliminating some of this product from the equilibrium, rather than simply pushing the equation back towards the left as the H+ increase prompts an increase in products, this H+ expulsion greatly INCREASES THE BUFFERING CAPACITY OF THE BICARBONATE SYSTEM
The reverse occurs when there is a decrease in H+, the reaction pushes to the left and ventilation drops, increasing the amount of CO2, further pushing the reaction towards the left
The change in ventilation allows the chemical effect of the change in equilibrium to be magnified to allow greater control over the ECF equilibrium
What is the overall aim of the acid base balance system?
To maintain arterial pH
Where does elimination of H+ from the body take place?
The kidneys
What are the two linked factors which determine pH? How are the two regulated?
HCO3 - renal regulation, linked with H+ elimination
PCO2 - respiratory regulation
What is the effect on pH if the amount of bicarbonate decreases?
It becomes more acidic/decreases
What are the main intracellular buffer systems?
Plasma proteins
Describe the changes which need to take place if there is movement of protons from the ECF to the ICF
The electrochemical neutrality needs to be maintained to prevent a gradient, so the protons must either travel with chlorine or the proton must be exchanged for a cation - always K+. If this takes place, there must be a compensatory amount of K+ moved out of the cell in order to maintain the chemical K+ balance
When a proton moves from the ECF to the ICF, what cation may it be replaced with?
K+
What happens to H+ and K+ in acidosis? What results?
The increase in H+ in the ECF creates a gradient which causes it to enter the cell, forcing a cation departure from the cell of K+. This results in hyperkalaemia
Dysfunction to one or both of which two organ systems is likely to impact on the body pH?
Lungs
Kidneys
What is it important to deal with free H+ in the body? How is this done?
Is is very dangerous and an excess will cause extreme acidosis well outwith the ranges compatible with life.
To protect the body they are bound and buffered to reduce their effect, and then excreted by the kidney to correct the acid load
How does the kidney regulate bicarbonate? What are both of these dependant on (think tubule)?
Reabsorbing filtered HCO3
Generating new HCO3
Active H+ secretion from the tubule cells into the lumen
How does the mechanism for the reabsorption of HCO3 function? (one of the two parts of the renal regulation of HCO3 to influence pH)
Does H+ secretion take pace during the reabsorption of HCO3?
There is active secretion of H+ from tubule cells which is coupled to passive Na+ reabsorption. Filtered HCO3 reacts with the secreted H+ to form H2CO3. In the presence of carbonic anhydrase on the luminal membrane dissociate to CO2 and H20
CO2 is freely permeable and enters the cell from the tubule, where it is reconverted to H2CO3 (bicarbonate) in the presence of carbon anhydrase which then dissociates to H+ and HCO3. The H+ ions are the source of the secreted H+ and the HCO3 ions pass into the peritubular capillaries with Na+(90% at the proximal tubule)
see diagrams
No, this is very significant
Comment on the quantity of bicarbonate filtered per day and the importance of its reabsorption
Vast quantities are filtered per day and the majority must be reabsorbed to avoid acidation
What is the minimum urine pH in humans?
4.5-5.0
What is the maximum urine pH in humans?
8.0
What is the main buffer in the urine?
Dibasic phosphate (inorganic phosphate)
What is the process of suffering the H+ in the urine called?
Titratable acidity
What is the net result of titratable acidity?
Generation of new HCO3 and excretion of H+
When is titratable activity used by the body?
Only in situations of acid load
Where does titratable activity take place?
In the distal tubule
What is ammonium excretion and when is it used by the body?
Excretion of ammonia as a major adaptive response to an acid load
What is the end result of ammonium excretion?
New HCO3 is generated and H+ is excreted
How is NH3 produced and how does this go on to govern ammonium excretion?
By deamination of amino acids, primarily glutamine - the most important metabolic AA - this then combines with H+ in acid load, combining with Cl to be excreted and excreting the acid load
Do respiratory disorders affect pH via PCO2 or HCO3?
PCO2
Do renal disorders affect pH via PCO2 or HCO3?
HCO3
Describe respiratory acidosis
A pH fall due to a respiratory change, so PCO2 has to have increased. It results from reduced ventilation and therefore retention of CO2
Any respiratory disorder will affect PCO2
What are some acute causes of respiratory acidosis?
Drugs which depress the medullary respiratory centres, such as barbiturates and opiates
Obstructions of major airways
What are some chronic causes of respiratory acidosis?
Lung disease e.g. bronchitis, emphysema, asthma
Are PCO2 values able to be corrected in patients with lung disease?
No - renal compensation to increase HCO3 protects the pH but does not correct the original disturbance
Describe respiratory alkalosis
Increasing pH of respiratory origin, so there must be a fall in pCO2 and this must have happened due to increased ventilation causing CO2 blow-off
What are some acute causes of respiratory alkalosis?
Voluntary hyperventilation
Aspirin
First ascent to altitude
What are some chronic causes of respiratory alkalosis?
Long term resistance at altitude
What is the effect of respiratory alkalosis on HCO3?
It needs to be decreased, which occurs because there is less PCO2 and H+ available for secretion and therefore less of the filtered load of HCO3 is reabsorbed, so HCO3 is lost in the urine, decreasing the alkaline load
Describe metabolic acidosis
An acidosis due to a decrease in HCO3
Either due to increased buffering of H+ or direct loss of HCO3
What must be done in metabolic acidosis to protect the pH?
PCO2 must be decreased
What causes metabolic acidosis?
Any increased acid load in relation to metabolism e.g.
Increased H+ production in DKA
Failure to excrete the normal H+ as a result of renal failure
Loss of HCO3 in diarrhoea
Describe metabolic alkalosis
HCO3 increase and a resulting PCO2 increase to protect the pH
What are some causes of metabolic alkalosis?
H+ ion loss - vomiting loss of gastric secretions
Renal H+ loss - aldosterone excess
State whether there is an increase or decrease in the following; H+, pH, and what the primary disturbance is and what changes to compensate in;
- Respiratory Acidosis
- Respiratory Alkalosis
- Metabolic Acidosis
- Metabolic Alkalosis
- RAcid - Increased H+, decreased pH
Initially increased PCO2 then increases HCO3 to compensate - RAlk - Decreased H+, increased pH
Initially decreased PCO2 then decreased HCO3 to compensate - MAcid - Increased H+, decreased pH
Initially decreased HCO3 then decreased PCO2 to compensate - MAlk - Decreased H+, increased pH
Initially increased HCO3 then increased PCO2 to compensate
If its respiratory, the PCO2 is the initial insult and the HCO3 changes to compensate
If its metabolic, the HCO3 is the initial insult and the PCO2 changes to compensate
What is the difference in the change of pH for a given increase in PCO2 between acute and chronic RAcid?
There is a smaller decrease in pH in chronic RAcid than in acute RAcidosis
What is the danger of sustained hyperkalaemia?
VF and death
What is important to remember regarding the volume drop vs metabolic alkalosis?
The drop in BV will always take precedence but may well exacerbate metabolic alkalosis like in severe cases of vomiting
Why do patients with vomiting and/or diarrhoea become alkalotic even though they lose both acid and alkali?
The ECF volume has decreased so aldosterone msg released and this increases sodium reabsorption in the distal tubule, which is exchanged for H+, so is excreted, creating and alkaline load
What is the anion gap?
The difference between the sum of the principal Cations (Na+ and K+) and the principal Anions in the plasma (Cl- and HCO3). It is normally 16mmoles
