About 97% of oxygen is carried in the blood bound to haemoglobin as oxyhaemoglobin; only about 3% is dissolved in plasma. Oxygen binds haemoglobin in a reversible way, and each haemoglobin molecule can hold a maximum of four O2 molecules. Memory hook: "4 seats per Hb, and high pO2 fills all four."
At the alveoli, high pO2 drives oxygen to bind haemoglobin (up to 4 O2) forming oxyhaemoglobin, about 97% of blood oxygen; the binding is reversible, so at the tissues (low pO2) O2 is released.
Your doubts, answered
Is the binding of oxygen to haemoglobin reversible?
Yes. NCERT clearly states O2 binds haemoglobin in a reversible manner to form oxyhaemoglobin. This is why the same haemoglobin can pick up O2 at the lungs and release it at the tissues. If binding were permanent, oxygen could never be delivered.
How many oxygen molecules can one haemoglobin molecule carry?
A maximum of four O2 molecules. Haemoglobin is an iron-containing red pigment in the RBCs, and each molecule has four binding sites, so it carries up to four oxygen molecules when fully saturated.
Which single factor mainly decides oxygen binding to haemoglobin?
Partial pressure of oxygen (pO2) is the primary factor. NCERT says binding of oxygen with haemoglobin is primarily related to pO2. Other factors, pCO2, hydrogen ion (H+) concentration and temperature, can interfere with this binding but pO2 is the main one.
Where in the body is oxyhaemoglobin actually formed?
At the alveoli (lung surface). There the pO2 is high, so oxygen loads onto haemoglobin and forms oxyhaemoglobin. At the tissues the pO2 is low, so oxyhaemoglobin dissociates and releases oxygen.
Does oxygen dissolve in plasma or bind to haemoglobin?
Mostly it binds to haemoglobin. About 97% of O2 is transported as oxyhaemoglobin inside RBCs; only around 3% is carried dissolved in plasma. So the bulk of oxygen transport depends on haemoglobin, not on plasma.
⚠️ The NEET trap ✗ A higher H+ concentration in the alveoli helps form more oxyhaemoglobin. ✓ Oxyhaemoglobin forms when H+ concentration is LOWER (along with high pO2, low pCO2, lower temperature). Higher H+ favours dissociation, not formation. 🧠 High pO2 loads O2; high H+ unloads it. Do not swap them.
Real NEET questions
2020
Identify the wrong statement with reference to transport of oxygen.
A · Higher H+ conc. in alveoli favours the formation of oxyhaemoglobin ✓
B · Low pCO2 in alveoli favours the formation of oxyhaemoglobin
C · Binding of oxygen with haemoglobin is mainly related to partial pressure of O2
D · Partial pressure of CO2 can interfere with O2 binding with haemoglobin
Solution: The question asks for the WRONG statement. In the alveoli the conditions are high pO2, low pCO2, lesser H+ concentration and lower temperature, and these favour oxyhaemoglobin formation. So a higher H+ concentration would NOT favour formation, making statement (a) false. Options (b), (c) and (d) are all correct: low pCO2 favours formation, binding is mainly related to pO2, and pCO2 can interfere with binding.
Solved Breathing and Exchange of Gases NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Oxyhaemoglobin is the compound formed when oxygen binds reversibly to haemoglobin in the RBCs. It is the main form in which oxygen travels in the blood from the lungs to the tissues.
What percentage of oxygen is carried by haemoglobin?
About 97% of oxygen is transported bound to haemoglobin as oxyhaemoglobin, and only about 3% is carried dissolved in the plasma.
Why is oxygen binding to haemoglobin reversible?
Because oxygen must be loaded at the lungs and released at the tissues. Reversible binding lets haemoglobin form oxyhaemoglobin at high pO2 (alveoli) and release O2 at low pO2 (tissues).
How much oxygen does 100 mL of blood deliver to tissues?
Under normal conditions, every 100 mL of oxygenated blood delivers about 5 mL of O2 to the tissues.
Why is this concept important for NEET?
NEET repeatedly tests the number 4 (O2 per haemoglobin), the primary role of pO2, and the favourable conditions for oxyhaemoglobin formation. These are direct one-line memory facts that appear almost every year.
