Favourable Conditions for Oxyhaemoglobin Formation at Alveoli
Biology · Breathing and Exchange of Gases · NEET
Oxyhaemoglobin forms at the alveoli under four favourable conditions: high pO2, low pCO2, lesser H+ concentration (higher pH), and lower temperature. The exact opposite set (low pO2, high pCO2, more H+, higher temperature) exists at the tissues and favours the release of oxygen. Memory hook: ALVEOLI = "Air Loads Oxygen" — everything is LOW except pO2, which is HIGH.
At the alveoli all four factors favour oxyhaemoglobin formation (only pO2 is high); at the tissues every factor flips to favour dissociation and delivery of O2.
Your doubts, answered
What are the four favourable conditions for oxyhaemoglobin formation at the alveoli?
NCERT lists exactly four: high pO2, low pCO2, lesser H+ concentration, and lower temperature. When all four are present, oxygen binds haemoglobin to form oxyhaemoglobin. Learn them as a fixed set, because NEET asks this word-for-word.
Should pO2 be high or low for oxyhaemoglobin to form?
pO2 must be HIGH. Binding of oxygen with haemoglobin is primarily related to the partial pressure of O2. In the alveoli pO2 is about 104 mm Hg (high), so O2 loads onto haemoglobin. This is the only one of the four factors that is HIGH — the other three are LOW.
Why does LOW temperature favour oxyhaemoglobin formation and not high temperature?
Lower temperature increases the affinity of haemoglobin for oxygen, so it holds oxygen more tightly and forms oxyhaemoglobin. Higher temperature (as in working tissues) lowers affinity and releases oxygen. A common wrong answer is 'higher temperature' — that condition belongs to the TISSUES, not the alveoli.
Does higher H+ concentration favour oxyhaemoglobin formation?
No. Higher H+ (lower pH, more acidic) favours DISSOCIATION of oxygen at the tissues, not formation. At the alveoli the H+ concentration is LESSER (higher pH). NEET 2020 asked this as the WRONG statement — 'Higher H+ favours oxyhaemoglobin' is false.
How are the conditions at the tissues different from the alveoli?
They are the exact mirror image. Tissues have low pO2, high pCO2, high H+ concentration and higher temperature — these favour DISSOCIATION of O2 from oxyhaemoglobin so oxygen is delivered to cells. Alveoli have the opposite, favouring FORMATION. Learn one set and flip every factor for the other.
How much oxygen does blood actually deliver to the tissues?
Every 100 mL of oxygenated blood delivers around 5 mL of O2 to the tissues under normal physiological conditions. About 97% of O2 is carried by RBCs (as oxyhaemoglobin) and only 3% is dissolved in plasma.
⚠️ The NEET trap ✗ Choosing 'higher temperature' or 'more H+' as favourable for oxyhaemoglobin formation because tissues seem active. ✓ At the alveoli the favourable set is high pO2, low pCO2, LESS H+, and LOWER temperature. Higher temperature and more H+ favour dissociation at the tissues, not formation. 🧠 Three factors are LOW at the alveoli (pCO2, H+, temperature); only pO2 is HIGH. If an option makes any of those three high, it is wrong.
Real NEET questions
NEET 2021
Select the favourable conditions required for the formation of oxyhaemoglobin at the alveoli.
A · High pO2, high pCO2, less H+, higher temperature
B · Low pO2, low pCO2, more H+, higher temperature
C · High pO2, low pCO2, less H+, lower temperature ✓
D · Low pO2, high pCO2, more H+, higher temperature
Solution: Oxyhaemoglobin forms at the alveoli where all four factors are favourable: high pO2, low pCO2, lesser H+ concentration and lower temperature. The opposite set favours dissociation at the tissues. (NCERT Ch 14, p.189)
NEET 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 H+ concentration is LESSER, not higher. A higher H+ concentration favours dissociation at the tissues, so statement (a) is false. Statements (b), (c) and (d) are all correct.
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.
What are the favourable conditions for oxyhaemoglobin formation at the alveoli?
High pO2, low pCO2, lesser H+ concentration, and lower temperature. These four together let oxygen bind haemoglobin to form oxyhaemoglobin.
Which single factor mainly controls oxygen binding with haemoglobin?
Partial pressure of oxygen (pO2). NCERT states that binding of O2 with haemoglobin is primarily related to pO2; pCO2, H+ concentration and temperature only interfere with it.
What conditions favour dissociation of oxygen at the tissues?
Low pO2, high pCO2, high H+ concentration and higher temperature. These release O2 from oxyhaemoglobin so it can be delivered to cells.
How much of oxygen is transported as oxyhaemoglobin?
About 97% of oxygen is carried by RBCs bound to haemoglobin as oxyhaemoglobin; the remaining 3% travels dissolved in plasma.
How many O2 molecules can one haemoglobin carry?
Each haemoglobin molecule can carry a maximum of four O2 molecules, binding in a reversible manner.
