Question

Because of the steep portion of the $O_2-Hb$ curve_______

• A. There is a good margin of safety in O₂-carrying capacity of the blood.
• B. A small drop in systemic capillary PPO₂ in a metabolically active tissue automatically makes large amounts of O₂ available, because O₂ is released from hemoglobin as a result of a big drop in percent hemoglobin saturation.
• C. O₂ loading is still almost normal even when alveolar PPO₂ falls up to 40%.
• D. More O₂ is physically dissolved in the systemic capillaries than in the pulmonary capillaries.

Answer 1

The correct option is B

A small drop in systemic capillary PPO₂ in a metabolically active tissue automatically makes large amounts of O₂ available, because O₂ is released from hemoglobin as a result of a big drop in percent hemoglobin saturation.

The steep portion of the curve between 0 and 60 mmHg is the blood range that exists at the systemic capillaries. In the systemic capillaries, the blood equilibrates with the surrounding tissue cells at an average of 40 mmHg.

A drop of 20 mmHg causes the %Hb saturation to drop from 75% to 30%, that is, about a 45% of the total oxy-hemoglobin will unload its $O_2$ to the nearby tissues. In this range, only a small drop in systemic capillary can automatically make large amounts of $O_2$ immediately available to meet the $O_2$ needs of more actively metabolizing tissues.