Gunn: Acid/Base Physiology 1

Question 1

What is the normal blood pH and what are its extreme ranges?

Answer 1

• Normal blood pH is 7.37 - 7.42
• Extreme Ranges of pH are from 7.0 - 7.8
• Large amounts of acid produced through…
  
  • Oxidative metabolism which produces CO₂.
  • Protein Catabolism.

Question 2

What is oxidative metabolisms role in acid production?

Answer 2

Oxidative Metabolism produces 13, 000 to 20, 000 mmol CO₂ daily.

H₂O + CO₂ ⇔ H₂CO₃ ⇔ H⁺ + HCO₃^-

As the carbonic acid is a weak acid some dissociates to form H+. However, as it is in equilibrium with the dissolved CO₂ , it is often referred to as a volatile acid.

Question 3

How is protein catabolism linked to acid production?

Answer 3

• Acts to produce 40-60 mmol of noncarbonic acid daily.
• Oxidation of sulphur containing amino acid residues produce sulphuric acid.
• Because noncarbonic acids are not in equilibrium with a volatile component they are known as non-volatile or fixed acids.
• Nonvolatile acid may increase markedly…
  
  • In ischaemia or extreme exercise due to lactic acid formation.
  • In diabetes due to the formation of acetoacetic acid and beta hydroxybutyric acid.

Question 4

What is the difference between pH change in plasma and pH change in distilled water?

Answer 4

With the addition of an acid - such as HCl - to plasma pH drops gradually from 7.4 to 7.14. The addition of the same amount of HCl to an equivalent volume of distilled water produces a drop in pH that would be fatal.

• This is because plasma has a bicarbonate buffer while distilled water does not.

Question 5

What is the bicarbonate buffer system?

Answer 5

H⁺ + HCO₃^- ⇔ H₂CO₃ ⇔ H₂O + CO₂

Question 6

What concentration of HCO₃^- and P_{CO2 keep} pH regulated at 7.4?

Answer 6

HCO₃^- regulated at 24 mmol.L^-1 by the kidney.

P_CO2 regulated at 40mmHg by the respiratory system.

Question 7

Over what time period is the bicarbonate buffer effective? Other buffer systems?

Answer 7

The bicarbonate system is the main buffer in the extracellular fluid (about 45%) (plasma and interstitial fluid). With blood and ECF providing effective short-term buffering (seconds to minutes).

However, H⁺ also combines with intracellular proteins and organic phosphates in tissue and bone. Here H⁺ is transported across the cell membrane in exchange for Na+ and K+. The time course of this process takes hours to days and therefore the bicarbonate system is not effective.

Question 8

What is the isohydric principle?

Answer 8

This is the idea that all equilibriums are equivalent and thus pH can be evaluated from the status of any buffer system. This is reasonably accurate for blood and interstitial phases but less so for the intracellular phase, which is not homogenous with the extracellular fluid.

Question 9

How is volatile acid buffered?

Answer 9

1. CO₂ is rapidly hydrated in the red blood cells by carbonic anhydrase forming H₂CO₃.
2. HCO₃^- diffuses into the plasma.
3. H⁺ is retained in red blood cells as cell membrane is impermeable to cations - Therefore, Cl^- moves into RBCs to equilibrate charge (Chloride shift).
4. Additional H⁺ formed as CO₂ is combined with haemoglobin to form carbaminohaemoglobin.
5. H⁺ formed binds to haemoglobin facilitating the release of oxygen from deoxyhaemoglobin.

Question 10

What “effect” does the buffering of volatile acids result in?

Answer 10

The Bohr Effect because as CO₂ concentration increases the dissociation of O₂ increases - as haemoglobins affinity for O₂ decreases.

In capillaries O2 falls and Hb becomes reduced making it a better base and therefore a better buffer

Ther Haldane effect: Increaqsed ability of blood to carry CO2 when there is a decreased amount of oxyHb. (70% deoxyHb is 3.5x more effective than oxyHb in forming carbamino compounds AND 30% deoxyHB is a better buffer than oxyHb thus improving CO2 carriage as bicarbonate)

Question 11

How is the blood buffer characterised?

Answer 11

When whole blood with a haemoglobin concentration of 148g.L^-1 is fully saturated with oxygen and exposed to P_CO2 values between 23 and 85 mmHG the resultant pH values are linearly related to HCO_3^- on the pH-bicarbonate diagram. This line is termed the normal buffer line and it reflects the buffering capacity of whole blood for CO₂ .

Question 12

What does the blood buffer line show?

Answer 12

The blood buffer line reflects the buffering capacity of whole blood for carbon dioxide.

Question 13

Can the blood buffer line and the isobars of the bicarbonate-system be overlaid?

Answer 13

Yes as both systems must be in equilibrium.

• The fact that the red line is at an angle shows the buffering of the red cells as if this didn’t occur it would be flat and for the same increase of PCO2 we would see a much larger drop in pH.

Question 14

What does a reduced haematocrit result in?

Answer 14

Reduced haematocrit decreases the buffering capacity of the body as the number of red blood cells present in the body decrease.

• Reduced bicarbonate production at the same concentration of CO_2.
• Drop in pH for same concentration of CO₂ is more.

Question 15

How can base excess and base deficit be defined?

Answer 15

• Base excess is measured by titration of a blood sample with strong acid to pH 7.40 at a P_CO2 of 40mmHg at 37ºC.
• Base deficit is measured by titration of a blood sample with NaOH to pH 7.4 at a P_CO2 of 40mmHg ar 37ºC.

Question 16

Answer 16

Question 17

Which is which?

Answer 17

I - hyperventilation

II - Diarrhoea

III - Asthma

IV - Vomiting