Acid Base Homeostasis in Respiration Flashcards
Identify a formula to calculate plasma pH.
pH = pKa + log([HCO3-] / [CO2])
where HCO3- is in mmol/L and CO2 is calculated from PCO2 and a solubility constant (40 mmHg because we are using the value of PCO2 in arterial blood)
Define pKa.
pH at which 50% is ionised and 50% is unionised in the reaction.
Write the equation corresponding to pKa for bicarbonate/carbonic acid.
H2CO3 ↔ HCO3- + H+
where,
50% of particles are present as H2CO3 (unionised) and 50% as HCO3- (ionised)
Write the equilibium reaction of bicarbonate. How could we shift this to the left ? to the right ?
H2CO3 ↔ HCO3- + H+
If H + rises, the equation is driven to the left If H + falls, the equation is driven to the right
What is the pKa of bircarbonate ? What is normal pH ?
pKa for bicarbonate/carbonic acid: 6.1
normal pH = 7.4
At normal pH is there more bicarbonate, or more carbonic acid ?
6.1 is pH where 50% carbonic acid and 50% bicarbonate. Decreasing H+ (i.e. increasing pH to 7.4) would shift the equilibrium to the right in this equation:
H2CO3 ↔ HCO3- + H+
so there would be more bicarbonate in normal pH.
What is the ratio of bicarbonate to carbonic acid at pH 7.4 ?
pH = pKa + log ([H+][A-]:[HA])
7.4 = 6.1 + log ([bicarbonate]/[carbonic acid]) 1 = log ([bicarbonate]/[carbonic acid]) reverse log (1) = bicarbonate / carbonic acid bicarbonate / carbonic acid = 10
How can we change absolute levels of bicarbonate ?
The absolute levels of bicarbonate can be changed by changes to breathing.
Given that H2O + CO2 → H2CO3, increased CO2 leads to more H2CO3.
This increase in H2CO3 in turn leads to an increase in bicarbonate. Vice versa.
Write the equation for the bicarbonate buffer system ? Show where respiratory, and metabolic disturbances affect the equation.
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
- If disturbance in CO2, respiratory disturbances
- If disturbance in H+ or HCO3, metabolic disturbance s
Define acidosis and alkalosis.
pH < 7.35 acidosis
pH >7.45 alkalosis
Draw a Davenport diagram, and explain its contents.
Refer to slide 8 in lecture on “Acid Base Homeostasis”
AB shows plasma pH change as CO2 changes
CD shows plasma pH change when non-volatile acid is added/ removed (static PCO2)
Line at concentration of HCO3- at 24 mmol/l
is the buffer line
Identify the main kinds of acid-base disturbances. What are the main causes of each of these acid-base disturbances ?
RESPIRATORY DISORDERS (alkalosis and acidosis)
☺ Increased CO2 (hypoventilate)
☺ Decreased CO2 (hyperventilate)
METABOLIC DISORDERS (alkalosis and acidosis) ☺ Increased non-volatile acid/decreased base ☺ Increased base/decreased non-volatile acid
ALKALOSES (can be respiratory or metabolic)
♦ Fall in PCO2
♦ Rise in HCO3-
ACIDOSES (can be respiratory or metabolic)
♦ Rise in PCO2
♦ Fall in HCO3-
In general, how does the body respond to alkaloses or acidoses ?
♠ The lungs and kidneys may try to return any disturbance towards normal (i.e. back to pH 7.4) – compensation.
♠ Two ways:
- the respiratory system alters ventilation – (happens quickly)
- the kidneys alter excretion of bicarbonate – (takes 2-3 days)
Identify the main causes of respiratory acidosis.
♣ Results from an increase in PCO2 caused by:
- Hypoventilation (less CO2 being blown away)
- Ventilation:perfusion mismatch
♣ This can specifically be due to:
- COPD
- Blocked airway (tumour or foreign body)
- Lung collapse
- Injury to chest wall
- Drugs reducing respiratory drive (e.g. morphine, barbituates, general anaesthetics)
♣ From Henderson-Hasselbach equation, an increase in PCO2 causes an increase in H+, so a lowering of pH.
Explain the compensation which occurs as a result of respiratory acidosis.
♣ Compensation occurs through
a) “Plasma HCO3- levels increase to compensate for increased H+ concentration (i.e. increased [HCO3-] will counteract the effect on the pH of an increased pCO2 because it returns the value of the [HCO3]/0.03 PCO2 ratio towards normal)”
b) Renal compensation – increased HCO3- reabsorption and increased HCO3- production raises pH towards normal (but pH return often incomplete)