Which description best characterizes the circulatory system?

• A. A system of airways and air sacs
• B. Heart and a closed system of vessels that maintains continuous blood flow through capillaries
• C. A network of bones and joints
• D. A digestive tract forming a tube

Answer: (B)

The circulatory system is best described as a heart plus a closed network of vessels that maintains continuous blood flow through capillaries. The heart acts as a pump, pushing blood into arteries, through a branching network of vessels, and back through veins, so blood stays inside vessels rather than circulating in body cavities. Capillaries form tiny, extensive networks where gas, nutrient, and waste exchange occurs between blood and tissues, made possible by the continuous flow and the thin walls of these vessels. This closed system ensures one-way flow and allows regulation of pressure and distribution to the lungs for oxygen uptake and to tissues for nourishment. The other descriptions don’t fit: airways and air sacs are part of the respiratory system responsible for gas exchange outside of the circulatory network; bones and joints describe the skeletal system; and the digestive tract processes food. So the heart and a closed network of vessels maintaining continuous capillary flow best characterize the circulatory system.