Which type of hypoxia affects the cells directly, preventing them from using oxygen?

Histotoxic hypoxia occurs when the body's cells are unable to utilize oxygen effectively, despite the presence of sufficient oxygen in the bloodstream. This typically happens due to the action of certain toxins or substances that interfere with cellular metabolism. For example, carbon monoxide is a well-known culprit; it binds to hemoglobin more effectively than oxygen, reducing the blood's capacity to deliver oxygen to tissues. In the case of histotoxic hypoxia, even if the blood is adequately oxygenated, the cells fail to use it due to the impairment in their biochemical processes.

In contrast, hypoxic hypoxia arises from a deficiency of oxygen in the atmosphere or a reduced availability of oxygen to the lungs, while hypemic hypoxia is linked to an insufficient quantity of hemoglobin or issues with blood's oxygen-carrying capacity. Stagnant hypoxia involves inadequate blood flow, leading to an insufficient supply of oxygen to the tissues. Thus, histotoxic hypoxia stands out as the type that directly affects the cells’ ability to utilize the oxygen available.