Transport of Oxygen and CO₂ Flashcards
In what three main ways is carbon dioxide transported in the blood?
As hydrogencarbonate ions (HCO₃⁻) – ~70–85%
Bound to haemoglobin as carbaminohaemoglobin – ~10–20%
Dissolved directly in plasma – ~5–10%
How is CO₂ converted into hydrogencarbonate ions in red blood cells?
CO₂ diffuses into RBCs and combines with water to form carbonic acid (H₂CO₃).
This reaction is catalysed by the enzyme carbonic anhydrase.
Carbonic acid then dissociates into H⁺ and HCO₃⁻ (hydrogencarbonate ions).
What happens to the HCO₃⁻ ions formed in RBCs?
HCO₃⁻ ions diffuse out of RBCs into plasma.
To maintain electrical neutrality, Cl⁻ ions move in — this is called the chloride shift.
This ensures continued CO₂ transport and avoids pH imbalance.
What happens to the H⁺ ions produced in RBCs?
H⁺ ions are buffered by haemoglobin, forming haemoglobinic acid (HHb).
This prevents a dangerous drop in blood pH (acidosis).
Haemoglobin acts as a buffer, maintaining acid-base balance.
What is the role of carbonic anhydrase in CO₂ transport?
Catalyses the reversible reaction:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
Speeds up conversion of CO₂ into carbonic acid in red blood cells.
Essential for efficient CO₂ transport and maintaining pH balance.
What is the chloride shift, and why is it important?
The chloride shift is the movement of Cl⁻ ions into RBCs to replace HCO₃⁻ ions that leave.
It maintains electrical neutrality inside the RBC.
Without it, the RBC would become positively charged and osmosis and transport would be disrupted.
How is CO₂ transported as carbaminohaemoglobin?
CO₂ binds to the amino groups (–NH₂) on haemoglobin, forming carbaminohaemoglobin.
This is a reversible reaction.
This form helps remove CO₂ at tissues and release it at lungs.
How does CO₂ affect the oxygen dissociation curve (Bohr effect)?
CO₂ increases H⁺ concentration, lowering pH.
This reduces haemoglobin’s affinity for O₂.
More O₂ is released to respiring tissues — known as the Bohr shift.
Curve shifts right: more unloading of O₂ at a given partial pressure.
Why is it important that CO₂ is rapidly removed from respiring tissues?
High CO₂ levels lead to acidic conditions (carbonic acid formation).
This can denature enzymes, disturb cell function, and lower blood pH.
Efficient CO₂ removal is essential to maintain homeostasis.
What happens to CO₂ at the lungs?
The low pCO₂ at the alveoli causes:
HCO₃⁻ ions re-enter RBCs and recombine with H⁺ to form H₂CO₃.
Carbonic anhydrase converts H₂CO₃ back to CO₂ and H₂O.
CO₂ then diffuses out into alveoli to be exhaled.
Cl⁻ ions move back out (reverse chloride shift).
How is oxygen transported in the blood?
Mostly by binding to haemoglobin (Hb) in red blood cells to form oxyhaemoglobin (HbO₈)
Each haemoglobin molecule can bind up to 4 O₂ molecules
Binding is reversible
A small amount (~2%) is dissolved in plasma
What is meant by ‘affinity’ in the context of haemoglobin?
Affinity is haemoglobin’s attraction to oxygen
High affinity = oxygen binds easily
Low affinity = oxygen is released readily
What is the structure of haemoglobin?
A globular protein with quaternary structure
Made of 4 polypeptide chains, each with a haem group
Each haem group contains an iron (Fe²⁺) ion that can bind one O₂ molecule
Forms oxyhaemoglobin when oxygen is bound
Describe the shape of the oxygen dissociation curve and explain why it is sigmoidal.
The curve is S-shaped (sigmoidal) due to cooperative binding
First O₂ binds slowly due to low affinity
Binding causes conformational change, increasing affinity
Second and third O₂ bind more easily
Curve levels off as haemoglobin becomes saturated
What is cooperative binding?
When the first O₂ molecule binds to haemoglobin, it causes a conformational change
This increases the affinity for the next O₂ molecule
This continues until the fourth O₂ molecules which is the hardest to bind
Explains the steep middle section of the dissociation curve
What does a shift to the right in the oxygen dissociation curve mean?
Known as the Bohr shift
Haemoglobin’s affinity for O₂ decreases
Occurs in high CO₂, low pH, or high temperature environments
More O₂ is released to tissues
What is the Bohr effect?
In respiring tissues, CO₂ concentration increases, lowering pH
Low pH reduces haemoglobin’s oxygen affinity
More oxygen is released where it is needed
Causes the oxygen dissociation curve to shift right
How does haemoglobin adapt its function in different parts of the body?
In lungs (high O₂, low CO₂): high affinity → haemoglobin loads O₂
In tissues (low O₂, high CO₂): lower affinity → haemoglobin unloads O₂
Ensures efficient gas exchange
What is partial pressure of oxygen (pO₂)?
A measure of oxygen concentration
High pO₂ = more O₂ available
Haemoglobin’s affinity increases with rising pO₂
Explains loading in lungs, unloading in tissues
What factors affect haemoglobin’s affinity for oxygen?
pO₂ (partial pressure of O₂)
pCO₂ (partial pressure of CO₂)
Temperature
pH (Bohr effect)
Allosteric changes in haemoglobin structure
What is fetal haemoglobin and how is it different?
Found in fetuses; has a higher affinity for O₂ than adult Hb
Allows fetus to extract O₂ from mother’s blood across placenta
Curve is shifted to the left
Why must fetal haemoglobin have higher affinity for O₂?
Maternal blood in placenta has lower pO₂ (due to fetal use)
Fetal Hb must bind O₂ more readily to ensure sufficient oxygen delivery
Enables effective oxygen uptake despite low pO₂
How does myoglobin compare to haemoglobin?
Myoglobin is a muscle protein that stores oxygen
Has very high affinity for O₂
Releases O₂ only at very low pO₂ (e.g. during intense exercise)
Helps maintain aerobic respiration in muscles
What would happen to oxygen transport if haemoglobin affinity were too high everywhere?
Oxygen would bind in lungs but not be released in tissues
Tissues would become hypoxic
Affinity must vary with conditions (like pO₂ and pCO₂)
