Transport of Gases in Humans
The process of gas transport is an essential part of respiration. It ensures that oxygen (O₂) is transported from the lungs to all tissues, while carbon dioxide (CO₂) is carried back from the tissues to the lungs. This dual transport maintains life by supporting cellular respiration and removing waste gases from the body.
🌬️ Transport of Oxygen
The partial pressure of oxygen in the alveoli allows oxygen to diffuse into the pulmonary capillaries. Inside the blood, only a small amount of oxygen dissolves in plasma — about 3 mL of O₂ per liter. However, whole blood carries nearly 200 mL O₂ per liter, because most of the oxygen binds with haemoglobin (Hb) inside RBCs, forming oxyhaemoglobin (HbO₂).
Oxyhaemoglobin is bright red in color, while deoxyhaemoglobin is dark red. This color difference makes oxygenated blood appear red, while oxygen-poor blood looks bluish, especially in veins.
At sea level, the partial pressure of oxygen in alveoli is approximately 105 mm Hg, which is slightly lower than that in the atmosphere. About 97% of haemoglobin in RBCs combines with oxygen to form oxyhaemoglobin — a bright red molecule.
As blood passes through the tissues, some oxygen is released, converting oxyhaemoglobin to deoxyhaemoglobin, which is dark red. When venous blood leaves the tissues, its oxygen partial pressure is 40 mm Hg, and about 75% of haemoglobin remains saturated — meaning that 22% of oxygen has been released to the tissues. This 78% oxygen reserve ensures the body’s oxygen needs are met both at rest and during exercise.
This oxygen reserve also helps the body survive 4–5 minutes even if breathing stops or the heart ceases to pump.
💪 Factors Affecting Oxygen Transport
During exercise, muscles consume more oxygen, reducing venous oxygen pressure to 20 mm Hg. In this case, haemoglobin saturation drops from 75% to 35%, meaning 62% oxygen is unloaded to active muscles.
-
Bohr Effect:
When tissues produce more CO₂, it lowers blood pH, which decreases haemoglobin’s affinity for oxygen. As a result, haemoglobin releases more oxygen readily. -
Temperature Effect:
Increased temperature during exercise also reduces haemoglobin’s affinity for oxygen, further enhancing oxygen release to active muscles.
Thus, the combined effects of pH and temperature allow efficient oxygen delivery when the body needs it most.
🌫️ Transport of Carbon Dioxide
Blood capillaries deliver oxygen to tissues and collect carbon dioxide. Because tissue PCO₂ is higher than blood PCO₂, carbon dioxide diffuses from tissues into blood. The reverse occurs in the lungs, where alveolar PCO₂ is lower, allowing CO₂ to move from blood into alveoli and be exhaled.
Blood transports carbon dioxide in three main forms:
• As Bicarbonate Ions (HCO₃⁻)
About 72% of CO₂ is carried as bicarbonate ions. Inside RBCs, CO₂ reacts with water to form carbonic acid (H₂CO₃), catalyzed by the enzyme carbonic anhydrase.
-
The H⁺ ions combine with oxyhaemoglobin, causing oxygen release to tissues.
-
The bicarbonate ions (HCO₃⁻) move out of RBCs into plasma, while chloride ions (Cl⁻) move in to maintain electrical balance — this exchange is called the Chloride Shift or Hamburger Phenomenon.
This reaction also helps maintain the acid–base balance of blood, as bicarbonate acts as a major plasma buffer.
• As Carbaminohaemoglobin (HbCO₂)
Around 20% of CO₂ is transported as carbaminohaemoglobin. When the partial pressure of CO₂ is high, it combines with the globin (protein) part of haemoglobin, not the iron (haem) part.
Thus, CO₂ and O₂ do not compete for attachment to haemoglobin.
• As Dissolved CO₂ in Plasma
Nearly 8% of CO₂ dissolves directly in blood plasma. Although small, this fraction plays a vital role in maintaining CO₂ diffusion gradients.
When blood reaches the lungs, the lower PCO₂ in alveoli drives the breakdown of carbonic acid into water and CO₂, which then diffuses into alveoli and is expelled during exhalation.
☠️ Carbon Monoxide (CO) Poisoning
Incomplete combustion of fuels like wood, gasoline, propane, or natural gas produces carbon monoxide gas (CO). If inhaled in closed spaces (for example, from gas heaters left burning overnight), CO binds with haemoglobin with much higher affinity than oxygen — about 200 times stronger.
This binding forms carboxyhaemoglobin, which blocks oxygen transport, leading to tissue hypoxia (oxygen deprivation).
Symptoms include:
-
Headache
-
Dizziness
-
Weakness
-
Nausea
-
Confusion
-
Shortness of breath
-
Chest pain
-
Loss of consciousness
In severe cases, it can cause brain damage or death.
💨 MCQs – Transport of Gases in Humans
Q1. The majority of oxygen in blood is carried in which form?
-
a) Dissolved in plasma
-
b) Bound to haemoglobin as oxyhaemoglobin ✅
-
c) As bicarbonate ions
-
d) As carbonic acid
Answer: b) Bound to haemoglobin as oxyhaemoglobin
Explanation: Only ~3% of oxygen dissolves in plasma. Most oxygen (~97%) binds to haemoglobin in RBCs forming oxyhaemoglobin.
Q2. What is the approximate oxygen partial pressure in alveoli at sea level?
-
a) 100 mm Hg
-
b) 105 mm Hg ✅
-
c) 80 mm Hg
-
d) 40 mm Hg
Answer: b) 105 mm Hg
Explanation: The alveolar PO₂ is slightly lower than atmospheric PO₂, allowing efficient oxygen diffusion into pulmonary capillaries.
Q3. The oxygen reserve in blood ensures survival for how many minutes if breathing stops?
-
a) 1–2 min
-
b) 4–5 min ✅
-
c) 10 min
-
d) 15 min
Answer: b) 4–5 min
Explanation: A large reserve of oxygen bound to haemoglobin allows the body to survive briefly without respiration.
Q4. What effect does exercise have on oxygen unloading from haemoglobin?
-
a) Decreases oxygen unloading
-
b) Increases oxygen unloading ✅
-
c) No effect
-
d) Increases plasma oxygen only
Answer: b) Increases oxygen unloading
Explanation: Exercise reduces venous PO₂ and increases temperature, which decreases haemoglobin’s affinity for oxygen (Bohr effect), promoting oxygen release to tissues.
Q5. The Bohr effect refers to:
-
a) Increased CO₂ transport by plasma
-
b) Decreased haemoglobin affinity for oxygen due to low pH ✅
-
c) Increased haemoglobin synthesis
-
d) Oxygen dissolved in plasma
Answer: b) Decreased haemoglobin affinity for oxygen due to low pH
Explanation: CO₂ produced by tissues lowers blood pH, causing haemoglobin to release oxygen more readily.
Q6. Carbon dioxide is transported in blood mainly as:
-
a) Dissolved CO₂
-
b) Carbaminohaemoglobin
-
c) Bicarbonate ions ✅
-
d) Oxyhaemoglobin
Answer: c) Bicarbonate ions
Explanation: About 72% of CO₂ is transported as bicarbonate ions, formed by CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻.
Q7. The chloride shift is important because:
-
a) It maintains blood pH ✅
-
b) It transports oxygen
-
c) It dissolves CO₂ in plasma
-
d) It prevents haemoglobin binding
Answer: a) It maintains blood pH
Explanation: Exchange of Cl⁻ for HCO₃⁻ maintains ionic balance and contributes to acid–base regulation of blood.
Q8. Carbaminohaemoglobin forms when CO₂ binds to:
-
a) Haem iron
-
b) Globin protein of haemoglobin ✅
-
c) Plasma water
-
d) Oxygen
Answer: b) Globin protein of haemoglobin
Explanation: CO₂ binds to the protein (globin) portion, not iron, so it does not compete with oxygen.
Q9. Approximately what percentage of CO₂ is dissolved directly in plasma?
-
a) 5%
-
b) 8% ✅
-
c) 15%
-
d) 20%
Answer: b) 8%
Explanation: A small fraction of CO₂ dissolves directly in plasma, allowing a diffusion gradient for gas exchange.
Q10. Carbon monoxide poisoning occurs because CO:
-
a) Increases haemoglobin synthesis
-
b) Binds haemoglobin more strongly than oxygen ✅
-
c) Dissolves in plasma
-
d) Increases oxygen transport
Answer: b) Binds haemoglobin more strongly than oxygen
Explanation: CO binds haemoglobin ~200x more strongly than oxygen, forming carboxyhaemoglobin, preventing oxygen transport and causing hypoxia.
Q11. Which of the following is NOT a factor affecting oxygen transport in blood?
-
a) Blood pH
-
b) Temperature
-
c) Partial pressure of O₂
-
d) Blood glucose ✅
Answer: d) Blood glucose
Explanation: Oxygen transport is affected by PO₂, pH, temperature, and haemoglobin levels, not directly by glucose.
Q12. The total oxygen content in blood is mainly determined by:
-
a) Haemoglobin concentration ✅
-
b) Blood plasma volume
-
c) CO₂ content
-
d) Chloride ions
Answer: a) Haemoglobin concentration
Explanation: The more haemoglobin, the more oxygen can bind and be transported.
Q13. Which of these statements about oxygen transport is TRUE?
-
a) All oxygen is dissolved in plasma
-
b) Only 3% of oxygen dissolves in plasma ✅
-
c) Oxygen cannot bind haemoglobin
-
d) Oxygen is carried as bicarbonate
Answer: b) Only 3% of oxygen dissolves in plasma
Q14. What happens to oxyhaemoglobin in tissues during exercise?
-
a) It increases
-
b) It releases more oxygen ✅
-
c) It forms carbonic acid
-
d) It binds more CO₂
Answer: b) It releases more oxygen
Explanation: Low PO₂, low pH, and high temperature in muscles during exercise increase oxygen release.
Q15. CO₂ + H₂O → H₂CO₃ reaction is catalyzed by:
-
a) Haemoglobin
-
b) Carbonic anhydrase ✅
-
c) Oxygen
-
d) Bicarbonate
Answer: b) Carbonic anhydrase
🌬️ Short Questions – Transport of Gases in Humans
• Q1. What is the main form in which oxygen is transported in blood?
Answer: Most oxygen is transported as oxyhaemoglobin bound to haemoglobin in RBCs.
• Q2. Why does blood appear red when oxygenated and bluish when deoxygenated?
Answer: Oxyhaemoglobin is bright red, while deoxyhaemoglobin is dark red, giving veins a bluish tinge.
• Q3. What is the partial pressure of oxygen in alveoli at sea level?
Answer: Approximately 105 mm Hg.
• Q4. How much oxygen dissolves in plasma per litre of blood?
Answer: Only about 3 mL O₂ per litre.
• Q5. What percentage of haemoglobin is saturated with oxygen at rest in venous blood?
Answer: About 75%.
• Q6. Define the Bohr effect.
Answer: The Bohr effect is the decreased affinity of haemoglobin for oxygen at low pH, allowing easier oxygen release in tissues.
• Q7. How does exercise affect oxygen unloading from haemoglobin?
Answer: During exercise, oxygen unloading increases due to low PO₂, low pH, and increased temperature in muscles.
• Q8. What are the three main forms in which carbon dioxide is transported in blood?
Answer: 1) Bicarbonate ions (~72%), 2) Carbaminohaemoglobin (~20%), 3) Dissolved CO₂ (~8%).
• Q9. Explain the chloride shift (Hamburger phenomenon).
Answer: Bicarbonate ions (HCO₃⁻) move out of RBCs into plasma and chloride ions (Cl⁻) move in to maintain electrical balance and acid-base homeostasis.
• Q10. What is carbaminohaemoglobin?
Answer: CO₂ bound to the globin (protein) portion of haemoglobin, forming carbaminohaemoglobin.
• Q11. How much CO₂ is dissolved directly in plasma?
Answer: About 8%.
• Q12. What happens to carbonic acid in lungs?
Answer: Carbonic acid (H₂CO₃) breaks down into water and CO₂, which diffuses into alveoli and is exhaled.
• Q13. Why is there an oxygen reserve in blood?
Answer: To ensure enough oxygen for tissues during exercise or if breathing stops for 4–5 minutes.
• Q14. Why does carbon monoxide cause poisoning?
Answer: CO binds haemoglobin 200 times more strongly than oxygen, forming carboxyhaemoglobin and preventing oxygen transport.
• Q15. How does temperature affect oxygen transport?
Answer: Higher temperature reduces haemoglobin’s affinity for oxygen, increasing oxygen release in active muscles.
• Q16. Which enzyme catalyzes CO₂ + H₂O → H₂CO₃ reaction?
Answer: Carbonic anhydrase.
• Q17. What is the role of haemoglobin in oxygen transport?
Answer: Haemoglobin binds oxygen to form oxyhaemoglobin, increasing oxygen-carrying capacity of blood.
• Q18. Why don’t CO₂ and O₂ compete for binding on haemoglobin?
Answer: O₂ binds haem iron, while CO₂ binds the globin protein.
• Q19. State the percentage of oxygen unloaded from haemoglobin at rest and during exercise.
Answer: At rest ~22% is unloaded; during exercise ~62% is unloaded.
• Q20. Why is blood coloured blue in veins?
Answer: Because deoxyhaemoglobin is dark red, giving tissues and veins a bluish appearance.
🫁 Long Questions – Transport of Gases in Humans
• Q1. Explain the process of oxygen transport in human blood.
Answer:
Oxygen is transported from the lungs to tissues via blood. Only a small amount (~3 mL/L) of oxygen dissolves in plasma. The majority binds to haemoglobin in RBCs forming oxyhaemoglobin. At rest, 97% of haemoglobin is saturated in arterial blood, and 75% in venous blood. This creates an oxygen reserve (~22% released at rest) for tissues. During exercise, low PO₂, low pH, and higher temperature increase oxygen unloading (~62% released). This ensures muscles receive enough oxygen during high activity.
• Q2. Describe the Bohr effect and its significance in oxygen transport.
Answer:
The Bohr effect is the phenomenon where haemoglobin’s affinity for oxygen decreases at low pH (acidic conditions) and high CO₂ levels. In actively respiring tissues, CO₂ production lowers blood pH, which causes haemoglobin to release oxygen more readily. This ensures efficient oxygen delivery to tissues that need it most. Increased temperature during exercise also enhances this effect, aiding oxygen supply to muscles.
• Q3. Discuss the transport of carbon dioxide in the blood.
Answer:
CO₂ produced by tissues is transported to the lungs in three ways:
-
Bicarbonate ions (~72%): CO₂ enters RBCs, combines with water to form carbonic acid (H₂CO₃) via carbonic anhydrase, which dissociates into H⁺ and HCO₃⁻. H⁺ binds to haemoglobin, and HCO₃⁻ enters plasma (chloride shift).
-
Carbaminohaemoglobin (~20%): CO₂ binds to the globin protein of haemoglobin.
-
Dissolved CO₂ (~8%): Small amount dissolves directly in plasma.
In lungs, low alveolar CO₂ causes H₂CO₃ → CO₂ + H₂O, which diffuses into alveoli and is exhaled.
• Q4. Explain the chloride shift and its role in maintaining acid-base balance.
Answer:
The chloride shift (Hamburger phenomenon) occurs when HCO₃⁻ ions produced in RBCs are exchanged with Cl⁻ ions from plasma. This maintains electrical neutrality and helps regulate blood pH. It also facilitates CO₂ transport as bicarbonate without disturbing ionic balance, making it a crucial mechanism for acid-base homeostasis during respiration.
• Q5. What is carbon monoxide poisoning? Explain its mechanism and symptoms.
Answer:
Carbon monoxide (CO) is a toxic gas produced by incomplete combustion of fuels like wood, gasoline, and natural gas. CO binds to haemoglobin with ~200x greater affinity than oxygen, forming carboxyhaemoglobin. This reduces haemoglobin available for oxygen transport, causing tissue hypoxia. Symptoms include headache, dizziness, nausea, confusion, shortness of breath, chest pain, and in severe cases, loss of consciousness, brain damage, or death.
• Q6. Compare oxygen and carbon dioxide transport in blood.
Answer:
| Feature | Oxygen | Carbon Dioxide |
|---|---|---|
| Major transport form | Oxyhaemoglobin (~97%) | Bicarbonate ions (~72%) |
| Dissolved in plasma | ~3% | ~8% |
| Protein binding | Haem iron in haemoglobin | Globin protein (carbaminohaemoglobin) |
| Partial pressure in tissues | PO₂ ~40 mm Hg | PCO₂ ~46 mm Hg |
| Effect of pH/temperature | Bohr effect: low pH increases unloading | Acid-base buffer via HCO₃⁻ formation |
| Reserve | ~22% at rest | None; continuously converted to plasma/bicarbonate |
This shows O₂ binds mainly to haemoglobin for transport, while CO₂ uses multiple mechanisms, including bicarbonate formation, carbaminohaemoglobin, and plasma dissolution.
• Q7. Explain how oxygen transport is adapted during exercise.
Answer:
During exercise, muscles consume more oxygen, lowering venous PO₂ (~20 mm Hg). Haemoglobin unloads more oxygen due to:
-
Lower pH (Bohr effect)
-
Increased temperature in muscles
-
Higher CO₂ concentration
This increases oxygen release from ~22% at rest to ~62% during exercise. Oxygen reserve ensures sufficient supply even if breathing is temporarily interrupted.
Interactive Quiz: Transport of Gases in Humans – Test Your Knowledge
Test your understanding of oxygen and carbon dioxide transport in human blood with this interactive quiz. Challenge yourself with MCQs, track your score, and learn key concepts of human respiration!
Transport of Gases in Humans
FFSA Biology | 10 Questions | 20s per Question
0 Comments