What condition is characterized by a less than normal amount of oxygen in the tissues?

Hypoxia refers to a condition in which there is a deficiency of oxygen in the tissues. It is distinct from other related concepts, as it specifically addresses the oxygen levels available at the cellular level. When tissues receive inadequate oxygen, it can significantly affect cellular metabolism, leading to various physiological and pathological consequences.

Hypoxia can occur even when blood oxygen levels are normal; it's about the delivery and utilization of oxygen at the tissue level. This situation can arise from various underlying conditions, such as respiratory diseases, anemia, or other factors that impair oxygen transport or utilization.

In contrast, hypoxemia refers to low levels of oxygen in the blood itself, which may or may not lead to hypoxia if the tissues are unable to adequately receive or utilize that oxygen. Anemia involves a decrease in hemoglobin levels in the blood, which can result in reduced oxygen transport capabilities but does not directly define the oxygen availability in tissues. Ischemia, on the other hand, specifically denotes a restriction of blood flow to tissues, which can lead to a lack of oxygen but is more about blood supply than just oxygen deficit.

In summary, hypoxia precisely identifies the state of tissue oxygen deficiency, making it the correct choice in this context.