Clarifying the Oxygen-Carrying Capacity of Hemoglobin and Blood

Recently, a student came across a statement from the NCERT (National Council of Educational Research and Training) which sparked a discussion: 'Every 100ml of blood can carry 5ml of oxygen'. Is this statement accurate? Let's delve deeper into the concepts related to the oxygen-carrying capacity of hemoglobin (Hb) and clarify the discrepancy.

Oxygen Carrying Capacity of Hemoglobin

Hemoglobin has the ability to carry approximately 1.34 mL of oxygen per gram of hemoglobin. Given that 100ml of blood contains about 15 grams of hemoglobin, we can calculate the oxygen carried as follows:

Let's perform the calculation step by step:

Total Hemoglobin in 100ml of blood 15 grams

Oxygen carried by Hb 15 grams * 1.34 mL/g 20.1 mL of O2

Total Oxygen Carrying Capacity of Blood

The NCERT statement that 100 mL of blood can carry about 5 mL of oxygen likely refers to the total oxygen content in arterial blood. This total oxygen content includes both the oxygen bound to hemoglobin and the small amount dissolved in plasma. In healthy individuals, the majority of oxygen is indeed carried by hemoglobin, but the 5 mL figure might represent an average or a simplified value for ease of understanding.

Discrepancy and Clarification

The discrepancy arises because the 5 mL figure is likely a rough estimate representing the amount of oxygen available for tissues under normal conditions, while your calculation focuses solely on the theoretical maximum based on hemoglobin content.

Therefore, while the NCERT figure may not be strictly incorrect, it is intended to be a simplified representation. The actual oxygen-carrying capacity of blood is indeed higher due to hemoglobin content, but the 5 mL figure reflects physiological conditions or educational averages used for practical purposes.

No Mistake in NCERT

The statement from the NCERT is not incorrect; it merely differentiates between the oxygen that is carried by the blood and the oxygen that is delivered to the tissues. It's important to focus on the words 'carry' and 'deliver' in the statement:

'Every 100ml of pure blood carries 20ml of oxygen.' 'But delivers only 5ml of oxygen to tissues during normal conditions.'

The carry capacity refers to the total oxygen bound to hemoglobin and dissolved in the blood, while the delivered amount refers to the oxygen that is actually utilized by the tissues. This distinction is crucial for accurate understanding.

Conclusion

In summary, the NCERT statement is a simplified representation of the complex physiological processes involved in oxygen delivery. The oxygen-carrying capacity of hemoglobin is indeed higher, but the 5 mL figure reflects the practical reality of oxygen delivery under normal conditions.