Which of the following is a primary function of the circulatory system?

The primary function of the circulatory system is transporting nutrients and oxygen to cells. This system, which includes the heart, blood, and blood vessels, plays a crucial role in ensuring that all tissues in the body receive the vital substances they need for energy production, growth, and maintenance. As blood circulates through the body, it picks up oxygen in the lungs and delivers it to cells, while also carrying essential nutrients absorbed from the digestive tract.

In contrast, other options, while important physiological functions, are not primary roles of the circulatory system. The production of hormone secretions is typically a function of the endocrine system. Regulating body pH levels, although influenced by the circulatory system through the buffering capacity of blood, is not its primary function. The removal of waste through urine is primarily managed by the urinary system, which filters blood and excretes waste products. Therefore, the transportation of nutrients and oxygen stands out as the definitive and primary role of the circulatory system.