Respiratory Physiology - CO2 handling

Question 1

Where does CO2 come from?

Answer 1

CO2 is a metabolic waste product, produced in the TCA cycle in the micochondrion in metabolically active cells

Roughly 200ml/minute is produced at rest, and around 120L of CO2 is present in the body at any given time (1.8L/kg of body weight)

Question 2

How is CO2 transported around the body?

GRAPH/Link to chloride shift vid/Diagram
IMAGE of CO2+Hb equilibrium

Answer 2

As bicarbonate, dissolved directly in the plasma, and as carbamino compounds
Dissolved in plasma (5% of arterial CO2, 10% of venous CO2)
CO2 is 20x more soluble in plasma than O2
Higher PaCO2 means more CO2 dissolved in blood (Henry’s Law)

Bicarbonate (90% of arterial CO2, 60% of venous CO2)
CO2 diffuses freely into RBC, where carbonic anhydrase converts it into HCO3-
Accumulation of bicarbonate inhibits carbonic anhydrase, so it is swapped with extracellular chloride (Chloride (Hamburger) shift
Oxyhaemoglobin buffers the excess hydrogen, releasing O2 in the process.
Chloride entering the RBC encourages water to follow the osmotic gradient, engorging the RBC, and making the venous Hct higher than arterial, as it reduces the apparent plasma volume, while enlarging each RBC, without changing RBC count.

Carbamino compounds (5% arterial, 30% venous)
Hb and plasma proteins can bind CO2 directly, where it no longer influences partial pressure

Question 3

What is the haldane effect?

Might be worth quickly mentioning what pKa is/what it means for Hb in this context

Answer 3

DeoxyHb is able to carry more CO2 than oxyHb for two reasons:
1. DeoxyHb forms carbamino complexes more easily (3x higher affinity for CO2)
2. DeoxyHb is better able to buffer hydrogen ions, allowing more HCO3 to be formed (although this relies on the chloride shift)

DeoxyHb has a pKa of 8.2
OxyHb has a pKa of 6.6

Question 4

Draw and explain the CO2 dissociation curve

IMAGE
Needs a tiny bit of expansion

Answer 4

CO2 is far more soluble than O2
Graph shape is more hyperbolic than sigmoid

Question 5

How is CO2 removed from the body?

GRAPH - effect of MV on CO2

Answer 5

CO2 is tranported to the pulmonary arteries, where it then dissolves down its partial pressure gradient into the alveolus

PaCO2 is inversely proportional to alveolar MV

During acute hyperventilation, PaCO2 drops, and CO2 is mobilised form other areas of the body (such as fat and bone), down the partial pressure gradient. The opposite happens in acute hypoventilation

During apnoea, CO2 rises by approximately 0.5kPa/minute