S2: CO2 Transport

Question 1

How does CO2 transport differ to O2?

Answer 1

1. CO2 has a higher H2O solubility than O2 does so a greater percentage of CO2 is transported simply dissolved in plasma (CO2 ≈ 7%, O2 ≈ 1%)
2. CO2 binds to Hb at different sites than O2 (R–NH2 residues at the end of peptide chains, forming carbamino-Hb, R-NHCOOH) and with decreased affinity. Thus, a lower % of CO2 is transported in this manner (≈ 23%) compared to O2.
3. CO2 reacts with water to form carbonic acid, which accounts for the majority (≈70%) of CO2 transported as HCO3-

In an CO2 dissociation curve, the HbCO2 is less saturated than the O2 one as CO2 transport is less reliant on saturating systems

Question 2

What are the 3 ways CO2 is transported in the blood?

Answer 2

Carbon dioxide that diffuses into the blood from tissues, is transported in 3 ways:

1. Dissolved in plasma as CO2 (7%).
2. Converted to HCO3- (70%) by carbonic anhydrase.
3. Bound to haemoglobin (23%).

Question 3

What is the Haldane Effect?

Answer 3

Venous blood carries more CO2 than arterial blood

Deoxygenated Hb has a higher affinity for CO2 and H+ than oxygenated Hb does so the more oxy-Hb present the less CO2 carried.

• Deoxygenated blood carries more CO2
• Oxygenation of blood causes CO2 to leave (both points = “the Haldane effect”).

Question 4

Explain how deoxygenated Hb acts as a better buffer than oxygenated Hb

Answer 4

Deoxy-Hb has higher affinity for CO2. It also binds H+.

CO2 produced by the tissues dissolves in plasma (PaCO2). Co2 dissolves in RBCs and becomes bound to Hb.
Oxygen causes the dissociation of CO2 from Hb and CO2 can then be removed by the lung.

Question 5

Describe CO2 transport from tissues to RBC

Answer 5

1. CO2 is produced by respiring cells and dissolves in the plasma and enters RBCs
2. Conversion of CO2 + H2O to H2CO3 catalysed by carbonic anyhydrase present within RBCs
3. The effective removal of CO2 into H2CO3 enables further CO2 to diffuse into the RBC so more can enter plasma
4. H2CO3 ionises into HCO3- + H+. The red cell membrane is impermeable to H+ therefore H+ cannot leave
5. Accumulation of H+ and cessation of step 2 within cell is prevented by deoxy-Hb acting as a buffer and binding H+. Movement of O2 into tissues from RBCs increases [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 6

Describe CO2 transport in lungs

Answer 6

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 enter the RBC in exchange for Cl-.

Question 7

What type of Hb binds the most H+ and CO2?

Answer 7

Deoxygenated Hb

Question 8

How does increased CO2 cause increased H+?

Answer 8

CO2 reacts with water to produce carbonic acid (catalysed by carbonic anhydrase, an enzyme concentrated within RBCs). Carbonic acid dissociates into hydrogen and bicarbonate ions.

CO2 + H2O H2CO3 H+ +HCO3-

Question 9

How does CO2 and H+ affect Hb binding affinity to O2?

Answer 9

This means that ↑CO2 = ↑H+ (increased acidity, ↓pH). This is because when CO2 and H + bind to the Hb molecule (at different sites to O2), they induce a conformation change in the Hb molecule which changes the structure of the O2 binding site, altering O2-Hb binding affinity.

• Increased CO2
• Decreased pH
• Increased 2,3-DPG
• Increased temperature

Question 10

How do lungs contribute to acid base balance?

Answer 10

They play a role in the homeostasis of CO2 levels

Question 11

Describe transport of non respiratory gases e.g. Nitrogen at normal pressures, high pressures and moving rapidly from high to low rpessures

Answer 11

• Air contains other gases (e.g. air = 78% N2) which dissolve in the blood (PaN2 > PaO2), but have little effect at normal biological pressures.
• At abnormally high pressures (extreme underwater diving), certain gases will produce an anaesthetic effect (e.g. nitrogen narcosis).
• Moving too rapidly from high to low pressure (e.g. when ascending from depth) will also cause gases to come out of solution rapidly and form bubbles (less dissolved in blood due to lower pressure) , causing joint and nervous system issues (decompression sickness, aka ‘the bends’)