Biology · Gaseous Exchange

Hemoglobin is the respiratory pigment in red blood cells responsible for carrying oxygen. Each gram of hemoglobin can bind approximately 1.34 ml of oxygen, and since the blood normally contains around 15 g of hemoglobin per 100 ml, the total oxygen-carrying capacity comes out to about 20 ml of oxygen per 100 ml of blood. While some references report a slightly lower figure like 19.6 ml/100 ml, the widely accepted rounded physiological maximum is 20 ml/100 ml blood. Higher values like 25 or 30 ml are unrealistic, as hemoglobin concentration and binding ability impose a strict limit. Therefore, the correct answer is 20 ml/100 ml blood.

This value is very close to the correct one. In fact, the oxygen-carrying capacity of hemoglobin is sometimes given as ~19.6 ml per 100 ml of blood. However, the rounded and most commonly accepted standard value is 20 ml/100 ml blood, so this option is considered slightly underestimated.

Each gram of hemoglobin can carry about 1.34 ml of oxygen. With about 15 g of hemoglobin per 100 ml of blood, the maximum capacity works out to 20.1 ml O₂/100 ml blood. Therefore, the standard physiological value for hemoglobin’s maximum oxygen-carrying capacity is 20 ml/100 ml blood, which is the correct choice.

This is higher than the actual capacity. Normal hemoglobin concentration cannot bind this much oxygen under physiological conditions. This value would overestimate hemoglobin’s oxygen-binding capacity.

This is far above the realistic oxygen-binding capacity of hemoglobin. Such a figure is physiologically impossible under normal human conditions and serves only as a distractor.