Question

what does the respiratory system do to maintain a balance of oxygen and carbon dioxide in the blood?
secrets hormones to regulate metabolism.
adjusts the breathing rate.
controls the bodys temperature.
regulates the heart rate.
filters toxins from the blood.

Explanation:

The respiratory system's main function related to gas balance is through breathing. Adjusting the breathing rate directly affects the intake of oxygen ($O_2$) and expulsion of carbon dioxide ($CO_2$). Secretion of hormones (endocrine system), temperature control (integumentary system, etc.), heart - rate regulation (cardiovascular and nervous systems), and filtering toxins (excretory system like kidneys) are not primary respiratory system functions for $O_2 - CO_2$ balance.

Answer:

Adjusts the breathing rate.