8.4 - Transport of oxygen and carbon dioxide in the blood Flashcards
What are the main functions of erythrocytes in gas transport?
- Transport oxygen from the lungs to body cells.
- Remove carbon dioxide from cells and transport it to the lungs for gaseous exchange.
How does the biconcave shape of erythrocytes support their function?
- Provides a larger surface area for oxygen diffusion.
- The thin shape allows efficient gas exchange.
- Enables flexibility, allowing them to pass through narrow capillaries.
Why do mature erythrocytes lack a nucleus, and how does this affect their function?
- Maximizes space for hemoglobin, allowing more oxygen to be carried.
- Increases efficiency of gas transport.
- However, limits their lifespan to about 120 days, as they cannot repair themselves.
Where are erythrocytes produced, and how long do they survive?
Produced continuously in red bone marrow.
Survive for approximately 120 days before being broken down in the liver and spleen.
What is hemoglobin, and why is it important in oxygen transport?
A globular conjugated protein responsible for carrying oxygen.
Each hemoglobin molecule contains four polypeptide chains, each with an iron-containing haem group.
Oxygen binds to the haem group, forming oxyhemoglobin.
Describe the structure of hemoglobin and how it allows oxygen transport. (4)
- Made of four polypeptide chains.
- Each chain contains a haem group with iron (Fe²⁺) that binds to oxygen.
- Each hemoglobin molecule can carry four oxygen molecules (O₂).
- There are approximately 300 million hemoglobin molecules per red blood cell.
What is the chemical equation for the reversible reaction between haemoglobin and oxygen?
Hb + 4O2 ⇌ Hb(O2)4
(Haemoglobin + Oxygen ⇌ Oxyhaemoglobin)
How does oxygen bind to hemoglobin in the lungs? (4)
- Oxygen levels in erythrocytes are low when they enter lung capillaries.
- A steep concentration gradient allows oxygen to diffuse into red blood cells.
- Oxygen binds to haem groups, forming oxyhemoglobin.
- Hemoglobin undergoes positive cooperativity, increasing oxygen-binding efficiency.
What is positive cooperativity, and how does it affect oxygen binding?
When one oxygen molecule binds, the hemoglobin molecule changes shape, making it easier for the next oxygen molecules to bind.
This ensures rapid oxygen uptake in the lungs.
How is oxygen released from hemoglobin in body tissues?
The oxygen concentration in tissues is lower than in erythrocytes.
Oxygen moves out of red blood cells by diffusion, down a concentration gradient.
Once one oxygen molecule is released, hemoglobin changes shape, making it easier for more oxygen to be released.
What does the oxygen dissociation curve show?
The relationship between partial pressure of oxygen (pO₂) and hemoglobin saturation.
Illustrates how readily hemoglobin binds to and releases oxygen.
Why is the oxygen dissociation curve S-shaped (sigmoidal)? (4)
Due to positive cooperativity:
- Once one oxygen molecule binds, hemoglobin’s shape changes, making it easier for more oxygen to bind.
- This causes a steep increase in oxygen binding at moderate pO₂ levels.
- At high pO₂, the curve flattens as all haem groups are saturated.
How does hemoglobin behave at high partial pressure of oxygen (e.g., in the lungs)?
Hemoglobin is rapidly loaded with oxygen.
Reaches near 100% saturation.
What happens at low partial pressure of oxygen (e.g., in respiring tissues)?
Hemoglobin releases oxygen due to the steep concentration gradient.
Small decreases in pO₂ cause large oxygen releases due to positive cooperativity.
What percentage of oxygen is typically released from hemoglobin at rest, and why is this important?
Only about 25% of oxygen is released at rest.
The remaining 75% acts as a reserve, ready to be released when oxygen demand increases (e.g., during exercise).
How does pH affect oxygen unloading in tissues?
A lower pH in tissues (due to respiration) reduces hemoglobin’s affinity for oxygen.
This causes more oxygen to be released where it is needed.
How does the presence of carbon dioxide affect oxygen unloading?
Carbon dioxide lowers blood pH, making hemoglobin release oxygen more easily.
This enhances oxygen delivery to actively respiring tissues.
What is the Bohr effect, and how does it influence oxygen transport?
The Bohr effect describes how increasing partial pressure of CO₂ causes hemoglobin to release oxygen more easily.
This occurs because CO₂ lowers blood pH, reducing hemoglobin’s affinity for oxygen.
Why is the Bohr effect important for oxygen transport?
In active tissues, where CO₂ levels are high, hemoglobin releases more oxygen for respiration.
In the lungs, where CO₂ levels are lower, hemoglobin binds to oxygen more readily, ensuring efficient oxygen uptake.
Why must fetal hemoglobin have a higher affinity for oxygen than adult hemoglobin?
- In the placenta, fetal blood flows close to but separate from maternal blood.
- If fetal hemoglobin had the same oxygen affinity as adult hemoglobin, little or no oxygen would be transferred to the fetus.
- Higher oxygen affinity ensures the fetus extracts oxygen efficiently from maternal blood.
How does fetal hemoglobin differ from adult hemoglobin in oxygen dissociation?
The oxygen dissociation curve of fetal hemoglobin is shifted to the left compared to adult hemoglobin.
This means fetal hemoglobin binds oxygen more strongly at any given partial pressure of oxygen (pO₂).
Ensures efficient oxygen transfer from mother to fetus.
What are the three ways carbon dioxide is transported in the blood?
- 5% dissolves in plasma.
- 10–20% binds to hemoglobin to form carbaminohaemoglobin.
- 75–85% is converted into hydrogen carbonate (HCO₃⁻) ions in red blood cells.
What is the main form in which carbon dioxide is transported in the blood?
The majority (75–85%) of CO₂ is transported as hydrogen carbonate (HCO₃⁻) ions.
How is carbon dioxide converted into hydrogen carbonate ions (HCO₃⁻)?
CO₂ reacts with water to form carbonic acid (H₂CO₃).
Carbonic acid dissociates into H⁺ ions and HCO₃⁻ ions.
This reaction is catalyzed by carbonic anhydrase in red blood cells.
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
What is the role of carbonic anhydrase in CO₂ transport?
Speeds up the reversible reaction between CO₂ and H₂O, forming carbonic acid.
Ensures rapid conversion of CO₂ into HCO₃⁻ and H⁺ ions inside red blood cells.
What is the chloride shift, and why is it necessary?
HCO₃⁻ ions diffuse out of red blood cells into plasma.
To maintain electrical neutrality, Cl⁻ ions diffuse into red blood cells from plasma.
This prevents pH imbalances and stabilizes charge distribution in the cell.
What happens to CO₂ transport in the lungs? (5)
- CO₂ concentration is low, causing carbonic anhydrase to reverse the reaction.
- HCO₃⁻ ions re-enter red blood cells and react with H⁺ to form carbonic acid.
- Carbonic acid breaks down into CO₂ and H₂O.
- CO₂ diffuses into alveoli and is exhaled.
- Cl⁻ ions diffuse back into plasma (reverse chloride shift).
How does hemoglobin help buffer blood pH during CO₂ transport?
- H⁺ ions, released from carbonic acid, could lower blood pH.
- Hemoglobin binds H⁺ ions in a reversible reaction to form haemoglobinic acid.
- This prevents dangerous pH changes, maintaining blood homeostasis.
