CO2 transport

Question 1

Why does venous blood carry more CO2 than arterial blood?

Answer 1

Deoxy-Hb has a higher affinity for CO2 and H+ than oxy-HB
-Therefore increase in oxy-Hb leads to a decrease in CO2 carried

Question 2

Why is rapid O2 therapy in hypercapnic individuals with COPD dangerous?

Answer 2

If excess dissolved CO2 cannot be released (e.g.
outside lung), then oxygenation of blood enables
less CO2 to be transported.
CO2 accumulation = acidosis

Question 3

Steps in CO2 transport in tissues?

Answer 3

1. CO2 is produced by respiring cells and
   dissolves in the plasma + enters RBCs.
2. Conversion of CO2 + H2O to H2CO3 within
   RBCs (catalysed by carbonic anhydrase)
3. The effective removal of CO2 by (2)
   enables further CO2 to diffuse into the
   RBC (& more can then enter the plasma).
4. H2CO3 ionises to HCO3- + H+. The RBC cell
   membrane is impermeable to H+,
   therefore H+ cannot leave
5. Accumulation of H+ within cell, and ∴
   cessation of (2), is prevented by deoxy-
   Hb acting as a buffer and binding H+.
   Movement of O2 into tissues from RBCs
   ∴ ↑[deoxy-Hb] and enables more CO2
   to be transported.
6. The increased [HCO3-] creates a diffusion
   gradient for HCO3- to leave the cell. It is
   exchanged for Cl- to maintain electrical
   neutrality.

Question 4

Steps in CO2 transport in lungs

Answer 4

1. Low PACO2, creates a diffusion gradient
   for CO2 to diffuse out of the blood into
   the airspace
2. Increased PAO2 leads to O2-Hb binding.
   O2-Hb binds less H+ than deoxy-Hb,
   increasing free [H+]
3. Increased free [H+] leads to increased
   H2CO3 and ultimately CO2 which
   contributes to CO2 plasma saturation.
4. The changing equilibrium of carbonic
   acid reaction, also leads to decreased
   [HCO3-], as it binds the free H+. This
   creates a diffusion gradient that allows
   HCO3- ions to entry the RBC in exchange
   for Cl-.

Question 5

What does the relationship between PCO2 and [H2CO3] mean?

Answer 5

The relationship between PCO2 and [H2CO3] means
that CO2 transport is important in acid-base balance

Question 6

What does the accumulation of CO2 do to the relationship between PCO2 and [H2CO3]?

Answer 6

1. Accumulation of CO2 (hypoventilation) leads to an increase in [H2CO3]
2. This results in an increase [H+] which causes acidosis

Question 7

What does removal of CO2 do to the relationship between PCO2 and [H2CO3]?

Answer 7

1. Removal of CO2 (hyperventilation) leads to a decrease in [H2CO3]
2. This results in a decrease [H+] which causes alkalosis

Question 8

What organs maintain blood pH homeostasis, by regulating what?

Answer 8

The lungs & kidneys maintain blood pH homeostasis
by regulating PaCO2 & [HCO3-], respectively

Question 9

What does the kidney regulate in blood pH homeostasis and how is this?

Answer 9

Renal regulation of HCO3-
E.g. regulating reabsorption/ excretion in
glomerular filtrate (timeframe = hours to days)