Transport of Oxygen and Carbon-dioxide During External and Internal Respiration and MCQ

The mechanism of respiration involves 2 processes-

  • exchange of oxygen and carbon-dioxide
  • transport of oxygen and carbon-dioxide

We have already studied, exchange of gases I previous article. in this article we will discuss transport of gases.

We know that blood is the medium through which the gases are transported between lungs and body tissues. When O2 and CO2 enters blood, certain chemical reactions occurs that facilitates the transport


Since O2 is not easily dissolved in blood plasma; only 1.5% of O2 is dissolved in blood plasma and the remaining oxygen binds with Hb of RBCs. The heme portion of hemoglobin contains 4 iron molecules which binds with 4 oxygen molecules in a easily reversible reaction to form oxyhemoglobin.

The factors that determine that how much oxygen is bind to the hemoglobin are as follows-

  1.  Partial pressure of oxygen:- this is one of the most important factors that determines how much oxygen will bind to the hemoglobin; the higher the partial pressure of O2, the more oxygen combines with Hb. When the remaining Hb is completely converted into oxyhemoglobin, this condition is known as fully saturated, but when the Hb consist f both hemoglobin and oxyhemoglobin, it is known as partially saturated. In lungs, the Po2(partial pressure of oxygen) is more so more amount of O2 binds to hemoglobin; but in tissue cells, The Po2 is lower and hence oxygen comes out of hb.
  2. Acidity(Ph):- as the acidity(Ph) increases, the affinity of oxygen towards the hemoglobin decreases and the oxygen more easily comes out of the Hb molecule.
  3. partial pressure of CO2:- CO2 can also bind to the hemoglobin molecule as oxygen binds. when Pco2 rises, hemoglobin releases oxygen more easily like in tissue cells. Ph and Pco2 and relatable factors because low Ph is due to high partial pressure of oxygen.
  4.  Temperature:- when the temp increases, the amount of oxygen released from Hb also increases.
  5. BPG:- a substance present in RBC is called as 2,3-bisphosphoglycerate(BPG) decreases the affinity of oxygen for hemoglobin and hence helps to unload oxygen in tissue cells.

Carbon-dioxide transport

During a resting condition, approximately every 100ml of deoxygenated blood contain 53 ml of gaseous CO2. the CO2 is transported in three different forms-

  1.  dissolved CO2:-about 7% of CO2 is dissolved in blood plasma. when it reaches the lungs, it diffuses into alveolar air and Is exhaled
  2.  carbamino compounds:- about 23% of CO2 combines with the amino groups of amino acids and proteins in the blood and forms carbamino compounds; and the most found protein in the blood is hemoglobin, so in this form CO2 is bound to hemoglobin and then it is transferred.
  3.  Bicarbonate ions:- about 70% CO2 is transported  in the blood as carbonate ions(HCO3-). blood picks up CO2,HCO3- stays inside the RBCs and some of the carbonate ions move into the blood plasma. To maintain the electrical balance, chloride ions moves from plasma to the RBCs and this is known as chloride shift.

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Multiple choice questions(MCQs)

1. What acts as a carrier for the transport of gases from lungs to the tissue cells and vice-versa?
a. carbonate ion B. blood
C. hemoglobin d. partial pressure

2. which factor is considered as the most important factor to describe the affinity of oxygen towards hemoglobin?
A. partial pressure
of oxygen B. temp
C. Ph D. BPG

3. what do you mean by chloride shift?
A. movement of Cl ions from plasma to the RBCs
b. movement of HCO3- from plasma to the RBC
c. movement of Cl ion from RBC to the plasma
D. movement of HCO3- from RBC to the plasma

4. Match the following-
a) CO2 in dissolved form            1. No effect
B) in carbonate ions                    2. 23% of CO2 in this form
C) as carbamino group               3. 70% of CO2 in this form
D) partial pressure of CO2        4. 7% of CO2

5. What happens to the oxyhemoglobin bond when the partial pressure of CO2 rises?
A. bond breaks easily in tissue B. bond is strong in tissue cells
C. no effect D. effect is not constant

6. In which form, the CO2 is transported?
A. dissolved form B. carbonate ions
C. carbamino compounds D. all of the above

7. Which of the following statement is true?
A. 1.5% of O2 is dissolved in blood plasma
B. the Po2 is higher in tissue cells, hence O2 comes out of Hb.
C. temp has no effect on oxygen transport
D. 70% of CO2 is transported as carbamino compounds

8. what happens when the concentration of BPG is higher in RBCs?
A. decreases the affinity B. increases affinity
C. no effect D. constant

9. What factors determine the CO2 transport?
A. temp b. Pco2
C. acidity D. none of the above

10. what is the effect of high temp on the oxygen affinity?
A. decreases affinity B. increases affinity
C. constant D. no constant effect


1. blood
2. partial pressure of oxygen
3. movement f Cl from plasma to RBC
4. a) – 4 b) – 3 c) – 2 d) – 1
5. bond break easily in tissue
6. all of the above
7. 1.5% of oxygen is dissolved in blood plasma
8. decreases the affinity in tissue cells
9. none of the above
10. decreases the affinity


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REFERENCE:-Gerard J. Tortora -Principles of anatomy and physiology; edition twelfth ; page no.-: 900-904.

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