Which statement best describes the circulatory and respiratory systems?

• A. Circulatory system transports blood; respiratory system exchanges gases
• B. Digestive and excretory systems
• C. Nervous and endocrine systems
• D. Skeletal and muscular systems

Answer: (A)

The main idea is how the circulatory and respiratory systems work together to move gases needed by the body. The circulatory system moves blood through the heart and vessels, delivering oxygen-rich blood from the lungs to every cell and returning oxygen-poor blood back to the lungs. The respiratory system brings air into the lungs and provides the site where oxygen and carbon dioxide are exchanged: oxygen diffuses into the blood, and carbon dioxide diffuses out to be exhaled. Together they ensure oxygen reaches tissues for energy and carbon dioxide, a waste product of metabolism, is removed. Other options describe functions of different systems (digestion and excretion; nervous and endocrine signaling; skeletal and muscular structure and movement) and don’t capture the link between blood transport and gas exchange.