Acid Base Balance Flashcards
What is the normal range for plasma pH?
7.35-7.45
What are the clinical effects off acidaemia?
Affects enzyme function
K+ movement out of cells -> hyperkalaemia
How does acidaemia affect enzyme function?
Denatures them
What major affect does hyperkalaemia have?
Increases cardiac excitability causing arrhythmias
When does acidaemia become severe?
Below 7.1
When does acidaemia become life threatening?
Below 7.0
What is the main effect of alkalaemia?
Causes Ca2+ to come out of solution, so free Ca2+ falls
What is the effect of alkalaemia induced hypocalcaemia?
Increased neuronal excitability causing parasthesia and tetany
What is the mortality rate if pH rises to 7.55?
40%
How does the body buffer pH?
With CO2 and HCO3-
What is normal arterial CO2?
5.3 kPa
What equilibrium is there between CO2 and bicarb in the blood?
CO2 + H2O H+ + HCO3-
What physiological process is CO2 determined by in the blood?
Respiration
Where are the chemoreceptors than internally control CO2 oncentration in the blood?
Centrally i.e. in the CNS
When CO2 is disturbed, what is usually responsible?
Respiratory disease
Which organ controls HCO3-?
The kidneys
When HCO3- is deranged, what is usually responsible?
Metabolic or renal disease
What is the equation for blood pH?
pH= pK + Log([HCO3-]/(pCO2*0.23))
Why is pCO2 multiplied by 0.23 in that equation?
Because 23% of blood co2 is dissolved and therefore available to react with the water to create H+
Is CO2 dissolved in plasma the main source of HCO3-?
No, it creates a tiny amount. It is made in RBCs and transported to plasma
What is HCO3- largely present with in ECF?
Na+ (i.e. it is disociated sodium hydrogen carbonate)
How is more HCO3- produced in RBCs?
H+ binds to Hb so the equilibrium of the co2/hco3- equation within the rbc shifts to produce more H+ and HCO3-, which is then exchanged for Cl- in the plasma
Why is plasma pH alkaline?
The large concentration of HCO3- stops nearly all the CO2 reacting so very little H+ is produced (as eqm is pushed to the left)
What is pK in the equation?
6.1 at body temperature (37 degrees)
What Is the normal range for HCO3- in the blood?
22-26 mmol/L
In the kidneys, where is most HCO3- saved?
PCT
How is HCO3- saved in the PCT?
Na is exchanged for H+. In the lumen, H+ reacts with HCO3- -> CO2 and water which is reabsorbed. Converts back to HCO3- which is transported back to ECF with Na.
Where else, apart from the PCT, is HCO3- reabsorbed?
DCT and collecting ducts
How is HCO3- reabsorbed in the DCT/CDs?
H+ is actively excreted whilst bound to ammonia nd phosphate, so no CO2 is formed in the lumen. HCO3- in the cell is then transported out into the ECF in exchange for Cl-.
Which AA is converted to ammonium in response to low pH?
Glutamine
Which ion is H+ exchanged with?
K+
What happens to K+ in acidosis?
Hyperkalaemia as more H+ is pumped out so more K+ is retained
What happens to K+ in alkalosis?
Hypokalaemia as more H+ is retained so K+ excretion increases in distal nephron
What pattern would we see on an ABG of a pt in respiratory acidosis?
High pCO2, Normal HCO3-, Low pH
What pattern would we see on an ABG of a pt in respiratory alkalosis?
Low pCO2, Normal HCO3-, rasied pH
What if we saw high co2, high hco3- and a normal pH?
Compensated respiratory acidosis
How long does compensation take to happen?
Usually 2-3 days
What is the anion gap?
Difference measured between cations and anions due to other anions that are not measured
When is the anion gap present?
If HCO3- is replaced by other anions, like in metabloic acidosis lactic acid reacts with HCO3- to replace it.
How is ventilation stimulated in acidosis?
Peripheral chemoreceptors detect pH drop
How is metabolic acidosis compensated?
Increased respiration to blow off CO2
Why cant we compensate metabolic alkalosis with respiration?
Can’t reduce breathing rate as need to maintain pO2.
What conditions cause respiratory acidosis?
COPD, severe asthma, drug overdose, neuromuscular disease
What is the difference in chronic conditions?
Respiratory acidosis may be compensated already, so pH is nearer to normal range.
When do we see respiratory alkalosis?
Hyperventilation e.g. anxiety/panic attack
What conditions cause metabolic acidosis with an anion gap?
DKA, lactic acidosis (exercsising to exhaustion, poor tissue perfusion), and uraemic acidosis in advanced renal failure
What conditions cause metabolic acidosis without an anion gap?
Renal tubulr acidosis (rare), or severe/persistent diarrhoea
In DKA what happens to K+ and why?
You would expect it to rise, being acidosis, but due to osmotic diuresis, there is a whole body depletion of K= as it is lost in urine
What conditions cause metabolic alkalosis?
Severe prolonged vomiting, mechanical drainage of stomach, K+ depletion, mineralocrticoid excess, and certain diuretics such as loop and thiazides.
