A Level Biology Practice Exam 2025 – All-in-One Study Guide to Excel in Your A Levels!

The circulatory system plays a crucial role in transporting nutrients and oxygen throughout an animal's body. It consists of the heart, blood vessels, and blood, which work together to ensure that essential substances reach every cell. Oxygen, absorbed in the lungs, is carried by red blood cells to cells where it is used for cellular respiration, a process that generates the energy needed for various cellular functions. Additionally, this system carries nutrients from the digestive tract to cells, providing them with the necessary building blocks for growth, repair, and energy.

While the other functions mentioned—facilitating cellular respiration directly, supporting the immune system, and regulating body temperature—are important, they are more indirect roles played by the circulatory system. For instance, while it assists the immune system by transporting white blood cells and antibodies, its primary responsibility lies in the direct transport of oxygen and nutrients. Hence, the statement that the circulatory system primarily transports nutrients and oxygen best captures its fundamental purpose.