Which organ is NOT a part of the circulatory system?

The kidney is not considered a part of the circulatory system because its primary function is to filter blood and produce urine, which is essential for maintaining homeostasis and regulating blood pressure, electrolyte balance, and fluid balance. The circulatory system, on the other hand, is specifically responsible for the transportation of blood, nutrients, gases, and waste products throughout the body.

The heart is a muscular organ that pumps blood throughout the body, while blood vessels, including arteries, veins, and capillaries, are the conduits through which blood flows. The spleen plays a role in the immune system and blood filtration but is also associated with the circulatory system because of its role in blood maintenance and storage. Thus, the kidney is categorized separately due to its distinct functions in excretion and regulation rather than direct involvement in the circulation of blood.