What is the primary function of the circulatory system?

The primary function of the circulatory system is to transport materials around the body. This system is essential for the movement of blood, which carries oxygen and nutrients to the body's cells and removes carbon dioxide and waste products. The circulatory system includes the heart, blood vessels, and blood, all working together to ensure that vital substances are delivered efficiently throughout the body.

This transportation mechanism is crucial not only for cellular function and energy production but also for maintaining homeostasis, which refers to the stable internal conditions necessary for survival. For example, as blood circulates, it helps regulate levels of various elements and compounds, supports hormonal distribution, and plays a role in cell signaling.

While the other options are significant functions related to different systems in the body, they do not reflect the primary purpose of the circulatory system. Structural support is primarily provided by the skeletal system, immune defense against diseases involves the lymphatic and immune systems, and temperature regulation is mainly handled by the integumentary system in conjunction with the circulatory system. Thus, the transport of materials is the centerpiece of what the circulatory system fundamentally does.