Under normal resting conditions, every 100 mL of oxygenated blood delivers about 5 mL of oxygen (O2) to the tissues. In the same way, every 100 mL of deoxygenated blood delivers about 4 mL of carbon dioxide (CO2) to the alveoli. Memory hook: "5 O out, 4 C in" — 5 mL O2 leaves blood at tissues, 4 mL CO2 enters blood at tissues.
At the tissues, 100 mL of oxygenated blood delivers about 5 mL of O2 to cells, and the returning 100 mL of deoxygenated blood carries about 4 mL of CO2 to the alveoli.
Your doubts, answered
How much oxygen does 100 mL of oxygenated blood deliver to the tissues?
About 5 mL. This is the NCERT number you must remember. Blood picks up oxygen at the alveoli, and when it reaches the tissues, roughly 5 mL of O2 out of every 100 mL of blood is released for the cells to use.
Why is the CO2 number 4 mL and not 5 mL?
At the tissues, every 100 mL of deoxygenated blood picks up and carries about 4 mL of CO2 back to the alveoli. So the two numbers are different: 5 mL for O2 delivered to tissues, 4 mL for CO2 delivered to alveoli. NEET often swaps these two to trick you.
Does 100 mL of blood carry only 5 mL of oxygen in total?
No. Fully oxygenated blood carries about 20 mL of O2 per 100 mL. But it does not release all of it. It delivers only about 5 mL to the tissues at rest; the rest stays in the blood as a reserve. So 5 mL is the amount delivered, not the amount carried.
What is the memory trick for these two numbers?
Use "5 O out, 4 C in." At the tissues, 5 mL of O2 goes out of the blood into cells, and 4 mL of CO2 comes into the blood from cells. O2 = 5, CO2 = 4. The larger gas number belongs to oxygen.
Where exactly does this delivery happen?
At the systemic tissues (body cells), where the partial pressure of oxygen (pO2) is low. The low pO2 in tissues makes oxyhaemoglobin release O2. This unloading is helped by the Bohr effect (higher CO2, higher H+, lower pH, higher temperature).
⚠️ The NEET trap ✗ 100 mL of oxygenated blood delivers 4 mL of O2 to the tissues. ✓ 100 mL of oxygenated blood delivers about 5 mL of O2 to the tissues; the 4 mL figure is the CO2 that 100 mL of deoxygenated blood carries to the alveoli. 🧠 NEET puts both 4 and 5 in the options. Lock it as O2 = 5, CO2 = 4.
Real NEET questions
2022
Under normal physical conditions in human beings every 100 ml oxygenated blood can deliver ____ ml of O2 to the tissues.
A · 2 ml
B · 5 ml ✓
C · 4 ml
D · 10 ml
Solution: Under normal physiological conditions, every 100 mL of oxygenated blood delivers about 5 mL of O2 to the tissues. The distractor 4 mL is the amount of CO2 that 100 mL of deoxygenated blood delivers to the alveoli, so do not confuse the two.
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.
How much oxygen does 100 mL of blood deliver to tissues?
About 5 mL of O2 per 100 mL of oxygenated blood under normal resting conditions.
How much CO2 does 100 mL of deoxygenated blood deliver to the alveoli?
About 4 mL of CO2 per 100 mL of deoxygenated blood.
Is 5 mL the oxygen carried or the oxygen delivered?
It is the oxygen delivered to tissues. The total O2 carried by 100 mL of blood is about 20 mL, but only around 5 mL is released to the tissues at rest.
Why does more oxygen unload at the tissues?
Because tissues have low pO2, high pCO2, high H+ and higher temperature. These conditions (the Bohr effect) make oxyhaemoglobin release O2.
