Carriage of CO2 Flashcards
what are the bi- products of metabolism?
The bi-products products of metabolism are:
0.16 g H2O / min
1.22 kcal heat / min
200 ml CO2 / min (resting state)
They are considered as ‘waste’ products, but they actually are not.
The removal of CO2 (out of the above products) produces the greatest problem
200ml of CO2 equates to 40 ml CO2 / litre of blood
This would have significant impact on blood pH
CO2 is removed through diffusing from the cells its produced in 🡪 capillary blood 🡪 veins 🡪 right heart 🡪 lungs 🡪 excretion
CO2 is carried in blood in 3 ways:
Physically dissolved as CO2 (exerting a ‘partial pressure’)
Bound to proteins (in the plasma or Hb in RBC) forming carbamino compounds (NHCOO-)
As bicarbonate ion (HCO3-)
Physically dissolved: 5% of total
The amount that is dissolved in blood depends on the partial pressure and solubility coefficient for CO2.
Solubility coefficient (α) for CO2 = 5.20 ml of CO2 L-1 kPa-1 (much greater than for oxygen)
According to Henry’s Law:
At 5 kPa (arterial blood): there are 26 ml of CO2 dissolved per litre of blood
NB arterial blood still has quite a lot of CO2 🡪 helps regulate pH
At 6 kPa (venous blood) there are 31 ml of CO2 dissolved per litre of blood
Bound to terminal amine groups of proteins: 5% of total
7% of the plasma is made up of proteins
RBC are 30% Hb
The amine group of these proteins binds with CO2 🡪 forms carbamino compound
NB. CO2 is bound to α + β chains of globin in RBC
Deoxy-Hb binds more CO2 than oxy-Hb 🡪 Haldane effect see later
As bicarbonate ions: 90% of total
CO2 reacts with water to form carbonic acid (catalysed by carbonic anhydrase)
Carbonic acid (H2CO3) freely dissociates to form bicarbonate ion (HCO3-) + H+
The amount that is dissolved in blood depends on the partial pressure and solubility coefficient for CO2.
Solubility coefficient (α) for CO2 = 5.20 ml of CO2 L-1 kPa-1 (much greater than for oxygen)
According to Henry’s Law:
At 5 kPa (arterial blood): there are 26 ml of CO2 dissolved per litre of blood
NB arterial blood still has quite a lot of CO2 🡪 helps regulate pH
At 6 kPa (venous blood) there are 31 ml of CO2 dissolved per litre of blood
7% of the plasma is made up of proteins
RBC are 30% Hb
The amine group of these proteins binds with CO2 🡪 forms carbamino compound
NB. CO2 is bound to α + β chains of globin in RBC
Deoxy-Hb binds more CO2 than oxy-Hb 🡪 Haldane effect see later
CO2 reacts with water to form carbonic acid (catalysed by carbonic anhydrase)
Carbonic acid (H2CO3) freely dissociates to form bicarbonate ion (HCO3-) + H+
Movement of CO2 between the compartments:
There is a higher partial pressure of CO2 inside cells.
Therefore, CO2 diffuses down partial pressure gradient into plasma (some retained in plasma as dissolved CO2).
CO2 then diffuses into RBCs
Tissues [CO2 is physically dissolved]
CO2 is formed in cells as product of metabolism
It is dissolved in the solution of the cells
As a result, there is a higher partial pressure of CO2 inside cells.
Therefore, CO2 diffuses down it partial pressure gradient into plasma.
Metabolism in cells requires oxygen:
Therefore, oxygen is diffusing in the opposite direction to CO2 down its own partial pressure gradient
It moves from Oxy-Hb in RBC 🡪 plasma 🡪 then diffuses into cells of the tissue, dissolves and then enters the mitochondria.
Blood plasma [CO2 is in the form of bicarbonate ions and bound to terminal amine groups]
Dissolved in the plasma (very small amount)
As bicarbonate ions
CO2 reacts with water to form carbonic acid:
CO2 + H2O → H2CO3
This is relatively slow + NOT catalysed by enzyme
Carbonic acid readily dissociates easily into bicarbonate ion (HCO3-) + H+:
H2CO3 → HCO3- + H+
Overall: CO2 + H2O → H2CO3 → HCO3- + H+
Bound to terminal amine groups
CO2 can react with terminal amine group R-NH2 of plasma protein to form carbamino compound R-NHCOO-
R-NHCOO- has a negative charge. Therefore, it buffers the H+ present
This prevents the ↓ in pH that otherwise would occur
RBCs [as bicarbonate ions + bound to terminal amine groups]
Dissolved in the cytoplasm (very small amount)
As bicarbonate ions
CO2 reacts with water to form carbonic acid:
CO2 + H2O → H2CO3
But this is catalysed by carbonic anhydrase (↑ rate)
Carbonic acid readily dissociates into bicarbonate ion (HCO3-) + H+:
H2CO3 → HCO3- + H+
The bicarbonate ions (HCO3-) produced in RBC ↑ concentration to the point where HCO3- moves along its chemical gradient out of RBC into plasma
Therefore most of HCO3- (produced as a product of CO2 formation in cells) is located in the plasma
This movement of HCO3- out of RBCs leaves behind an electrical gradient (have lost negatively charged ions)
This gradient is recovered by chloride shift
Chloride shift: Cl- move from the plasma into RBCs
Maintains electrical neutrality across RBC membrane
Bound to terminal amine groups
CO2 can react with terminal amine group R-NH2 on Hb to form carbamino compound R-NHCOO-
R-NHCOO- buffers H+ in RBC
Hb therefore has a role in carrying CO2 + O2, as well as buffering H+ ↑ that would occur otherwise
This whole scenario would come to a natural equilibration if H+ ion was not removed (buffered) from the above reaction occurring in RBC
This buffering of H+ allows reaction to be pulled to the right
Therefore, CO2 in tissues will continue to be pulled across to the right into RBCs
If H+ remained high (not buffered), the rate of bicarbonate reaction would ↓ + rate of CO2 diffusion across from tissues to RBCs would ↓
CO2 is formed in cells as product of metabolism
It is dissolved in the solution of the cells
As a result, there is a higher partial pressure of CO2 inside cells.
Therefore, CO2 diffuses down it partial pressure gradient into plasma.
Metabolism in cells requires oxygen:
Therefore, oxygen is diffusing in the opposite direction to CO2 down its own partial pressure gradient
It moves from Oxy-Hb in RBC 🡪 plasma 🡪 then diffuses into cells of the tissue, dissolves and then enters the mitochondria.
