Which of the following is an example of a regulated variable in homeostasis?

• A. Hair color
• B. Sleep schedule
• C. Blood glucose levels
• D. Blood type

Answer: (C)

In homeostasis, a variable that the body actively keeps within a narrow range using feedback is a regulated variable. Blood glucose levels fit this idea because the body continuously monitors and adjusts them to supply tissues with energy. After eating, glucose rises and the pancreas releases insulin, promoting uptake by liver and muscles and storage as glycogen to bring levels back down. Between meals, glucagon triggers glucose release to prevent drops too low. This tight regulation helps keep brain function and overall metabolism stable. Hair color is genetically fixed and doesn’t require ongoing internal feedback to stay the same. Sleep schedule is influenced by circadian rhythms and behavior rather than a single rapid feedback loop, and blood type is set at birth by genes, not regulated day-to-day.