acid base balance

Question 1

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?

Answer 1

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

Question 2

What is pKa?

What is carbonic acid?

What are its constituent parts?

What is the pKa of Carbonic acid?

Answer 2

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

Question 3

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?

Answer 3

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

Question 4

what is the calculation of plasma pH defined by?

Answer 4

• it will be defined by the Henderson-Hasselbalch equation so,

Question 5

at pH 7.4, is there more H2CO3 or more HCO3-?

Answer 5

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.

Question 6

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?

Answer 6

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

Question 7

What may cause metabolic acidosis example

Answer 7

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.

Question 8

What is considered acidosis?
What is considered alkalosis?

Answer 8

pH < 7.35 acidosis
pH >7.45 alkalosis

Question 9

describe this diagram?

Answer 9

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

Question 10

what are the 4 different causes of acid base disturbances?

Which ones are respiratory and metabolic disorders?

Answer 10

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

Question 11

what 2 things can an acidosis be caused by?

what 2 things can alkalosis be caused by?

Answer 11

acidosis:
- rise in pCO2
- fall in HCO3-

alkalosis:
- fall in pCO2
- rise in HCO3-

Question 12

describe this diagram

Answer 12

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

Question 13

what is respiratory acidosis caused by? (3)

Answer 13

• hypoventilation (less CO2 being blown off)
• ventilation perfusion mismatch
• reduced lung diffusing capacity

Question 14

How does increase PCO2 lead to acidosis according to the Henderson-Hasselbalch equation?

Answer 14

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.

Question 15

what is the renal compensation for an increase in pCO2?

Answer 15

increased HCO3-, reabsorption and increased HCO3- production - raises pH towards normal.

Question 16

what are 5 conditions/ drugs that will cause respiratory acidosis ?

Answer 16

• COPD
• blocked airway (foreign body or tumour)
• lung collapse
• injury to chest wall
• drugs reducing respiratory drive eg - morphine

Question 17

describe this diagram

Answer 17

The Davenport Diagram is a graphical tool to Interpret acid-base Issues and is the diagram used in the photo

Normal to point A on Davenport diagram
Lower pH and an increase in bicarbonate.

No respiratory compensation.
Kidneys secrete more bicarbonate to buffer the extra H+ (indicated by the shift A→A2) The kidneys excrete more acid.
Note that the PCO2 has not changed during compensation.

Question 18

what does respiratory alkalosis result from?

Answer 18

it results from a decrease in pCO2 generally caused by: alveolar hyperventilation (more CO2 being blown off)
- this will cause a decrease in H+ ion concentration and thus a rise in pH.

Question 19

what is renal compensation for respiratory alkalosis?

Answer 19

• reduced HCO3- reabsorption and reduced HCO3- production
• thus plasma HCO3- levels fall, compensating for lower H+, moving pH back towards normal.

Question 20

what are the causes of respiratory alkalosis?

Answer 20

-increased ventilation, from hypoxic drive in pneumonia, diffuse interstitial lung diseases, high altitude, mechanical ventilation.

• hyperventilation- brainstem damage, infection driving fever.

Question 21

what is metabolic acidosis a result of?

what does it cause?

Answer 21

• metabolic acidosis is a result of excess H+ in the body
• this reduces HCO3- levels (shifts equation to the left )
• addition of acid decreases pH, ventilation is unaffected so pCO2 initially normal.

Question 22

what is the respiratory compensation for metabolic acidosis?

Answer 22

• the lower pH is detected by peripheral chemoreceptors, causes an increase in ventilation which lowers pCO2
• the bicarbonate equation is driven further to the left, lowering H+ ions and HCO3- concentration further
• the decrease in hydrogen ion concentration moves pH towards normal
• respiratory compensation cannot fully correct the pH, HCO3 and H+, so excess H+ need to be removed or HCO3- restored (by slow renal compensation)

Question 23

what are the causes of metabolic acidosis?

Answer 23

• loss of HCO3- (eg from gut in diarrhoea)
• exogenous acid overloading (aspirin overdose)
• endogenous acid production
• failure to secrete H+

Question 24

what are causes of metabolic alkalosis?

Answer 24

• vomitting (loss of HCL from stomach)
• ingestion of alkali substances
• potassium depletion

Question 24

what is metabolic alkalosis a result from?

what does it cause?

Answer 24

• metabolic alkalosis results from an increase in HCO3- concentration or a fall in H+ ions.
• removing the H+ ions from equation drives the reaction to the right, increasing the HCO3-
• this will lower the H+, which raises the pH with pCO2 remaining initially normal.

Question 25

respiratory compensations for metabolic alkalosis?

Answer 25

• increase in Ph detected by peripheral chemoreceptors causing a decrease in ventilation which raises pCO2
• the equation is driven further to the right, increasing H+ and HCO3-
• increase in H+ moves pH towards normal
• respiratory compensation os often small (or even absent), ventilation cannot reduce enough to correct imbalance
• renal response is to secrete less H+

Question 26

What are ABGs?

How can they be analysed using an acid-base (ABG) nomogram?

Describe the different disturbances that can be seen on an ABG nomogram?

Answer 26

ABGs are arterial blood gases

ABGs can be analysed using an acid-base (ABG) nomogram

By plotting the PaCO2 and H+/pH values on the ABG nomogram, most ABGs can be analysed

If the plotted point lies outside the designated areas, this implies a mixed disturbance