Which of the following is a variable commonly regulated by negative feedback?

• A. Blood pressure
• B. Hair length
• C. Blood type
• D. Blood vessel color

Answer: (A)

Negative feedback keeps a variable within a narrow range by reversing deviations from a set point. Blood pressure is a classic example: when arterial pressure rises, baroreceptors in the carotid arteries and aorta detect the change and send signals to the brainstem, which slows the heart and dilates vessels to lower pressure. When pressure falls, the opposite occurs—heart rate increases and vessels constrict—to raise pressure. This loop continuously works to keep blood pressure stable during movement, stress, or posture changes.

Hair length isn’t adjusted by a real-time feedback mechanism to a specific set point; it’s largely determined by genetics and hormonal cues over long periods. Blood type is fixed from birth based on genetic inheritance and doesn’t fluctuate with bodily conditions. Blood vessel color isn’t a parameter the body actively maintains through negative feedback.