Which organ in the body can store approximately two cups of blood for emergency release?

The spleen is the organ in the body that can store approximately two cups of blood for emergency release. This function allows the spleen to act as a reservoir for blood, which can be vital in situations where there is a significant loss of blood elsewhere in the body, such as during injury or hemorrhage. When the body is under stress or when there is a demand for more blood, the spleen can contract and release stored blood back into the circulatory system, helping to maintain blood pressure and supply vital organs with oxygenated blood.

In contrast, the liver primarily functions to metabolize nutrients and detoxify substances; while it plays a role in blood regulation, it does not serve as a direct blood storage organ in the same sense as the spleen. The kidneys are responsible for filtering blood and regulating electrolytes rather than storing blood, and the heart is a muscular organ that pumps blood throughout the body but does not store it in large quantities. Thus, the unique capacity of the spleen to store and mobilize blood during critical moments makes it the correct answer in this scenario.