acid base balance Flashcards
what is an acid?
what is a base?
what is pH a measure of?
what is the formula for pH?
what is the H+ concentration at pH 7?
An acid is defined as any chemical substance that can donate a proton, H+
A base (alkali) is defined as any chemical substance that can accept a proton, H+
pH is a measure of H+ concentration
Because H+ concentration can vary over a large range in solutions, the pH scale was created
pH = -log10 [H+]
[H+] of 10-7M = pH 7.0
What is pKa?
What is carbonic acid?
What are its constituent parts?
What is the pKa of Carbonic acid?
pKa (aka the dissociation constant) is the pH at which 50% of a compound is ionised and 50% is unionised
Carbonic acid (H2CO3) is a weak acid that almost instantaneously dissociates into its constituent parts (HCO3- and H+ ions)
The pKa of carbonic acid is 6.1
What % of carbonic acid will be ionised and unionised below and above a pH of 6.1?
What form is primarily it in at pH 7.4?
How do we shift the dissociation reaction to the left or right?
This means above pH 6.1, there will be more than half of carbonic acid ionised
Below a pH of 6.1, there will be more than half of carbonic acid unionised
This means at pH 7.4, more of carbonic acid will be ionised than unionised
To shift the dissociation reaction left towards formation of H2CO3, we increase the concentrations of H+ and or HCO3-
To shift the dissociation reaction right towards the formation of H+ and HCO3-, we increase H2CO3 concentration
what is the calculation of plasma pH defined by?
- it will be defined by the Henderson-Hasselbalch equation so,
at pH 7.4, is there more H2CO3 or more HCO3-?
there will be more bicarbonate
- the pH suggests that there will be less H+ ions which will move the reaction to the right, making more HCO3.
How are the absolute levels of bicarbonate changed by changes to respiration?
What are respiratory and metabolic acid-base disturbances (in picture)?
What may cause metabolic acidosis?
Absolute levels of bicarbonate can be changed by changes to respiration because the by-products of aerobic respiration (H2O+CO2) can be used to create carbonic acid (H2CO3)
This means more CO2 (PCO2) generates more H2CO3, which will dissociate into more HCO3- and H+ ions
What may cause metabolic acidosis example
An example of metabolic acidosis is ketoacidosis due to uncontrolled diabetes, where acids start to build in the blood stream, due to a lack of insulin.
What is considered acidosis?
What is considered alkalosis?
pH < 7.35 acidosis
pH >7.45 alkalosis
describe this diagram?
Start is physiological pH, bicarbonate concentration, and PCO2
From start to A shows an increase in PCO2 and decrease in pH (acidosis)
From start to B shows a decrease in PCO2 and an increase in pH (alkalosis)
From start to C shows plasma pH change when non-volatile acid is added/base is decreased – the PCO2 is static, meaning this is not a respiratory acid-base disturbance, and either acid is being generated from another course (such as ketoacidosis) or there is less bicarbonate ion production
From start to D shows plasma pH change when non-volatile acid is removed/base is increased – the PCO2 is static, meaning this is not a respiratory acid-base disturbance, and either acid is being removed or there is more bicarbonate ion production
what are the 4 different causes of acid base disturbances?
Which ones are respiratory and metabolic disorders?
1) Increased CO2
2) Decreased CO2
3) Increased non-volatile acid/decreased base
4) Increased base/decreased non-volatile acid
Where primary change is to the CO2 levels - respiratory disorders
Where primary change is to bicarbonate levels - metabolic disorders
what 2 things can an acidosis be caused by?
what 2 things can alkalosis be caused by?
acidosis:
- rise in pCO2
- fall in HCO3-
alkalosis:
- fall in pCO2
- rise in HCO3-
describe this diagram
1) 1st Step: examine the pH.
* A pH less than 7.4 indicates acidosis, whereas a pH greater than 7.4 indicates alkalosis.
2) 2nd Step: examine the plasma PCO2 and HCO3− concentration.
* The normal values for PCO2 is about 40 mm Hg, and for HCO3− is 24 mEq/L.
* Expected values for a simple respiratory acidosis would be reduced plasma pH, increased PCO2, and increased plasma HCO3− concentration after partial renal compensation.
* Expected values for a simple metabolic acidosis would be a low pH, a low plasma HCO3− concentration, and a reduction in PCO2 after partial respiratory compensation.
3) The lungs and kidneys may try to return any disturbance towards normal – compensation
* The ways in which the two systems compensate are
* the respiratory system alters ventilation – this happens quickly
* the kidneys alter excretion of bicarbonate – this takes 2-3 days
what is respiratory acidosis caused by? (3)
- hypoventilation (less CO2 being blown off)
- ventilation perfusion mismatch
- reduced lung diffusing capacity
How does increase PCO2 lead to acidosis according to the Henderson-Hasselbalch equation?
from the Henderson Hasselbalch equation, an increase in pCO2 causes an increase in H+, so lowering of pH.
Thus, plasma HCO3- levels increase to compensate for increased H+ concentration.
what is the renal compensation for an increase in pCO2?
increased HCO3-, reabsorption and increased HCO3- production - raises pH towards normal.