Which physiological state is characterized by a lack of oxygen in the body's tissues?

Hypoxia is the physiological state characterized by a deficiency of oxygen in the body's tissues. This condition can result from various factors such as inadequate oxygen supply, respiratory or circulatory problems, or environmental factors such as high altitudes. When tissues are deprived of oxygen, they cannot perform their normal functions, leading to cellular damage and potentially severe organ dysfunction.

In contrast, hypercapnia refers to an excess of carbon dioxide in the blood, which can occur due to inadequate ventilation leading to respiratory failure. Acidosis is a state in which there is an excess of hydrogen ions in the body, resulting in a drop in blood pH, often linked to metabolic or respiratory issues. Ischemia refers specifically to a reduction in blood flow to a tissue, leading to insufficient oxygen and nutrient delivery; while this can lead to hypoxia, it does not encompass other causes of oxygen deficiency. Therefore, hypoxia specifically identifies the lack of oxygen at the tissue level, making it the correct answer.