The correct answer is option 3, the spleen. The spleen is an organ located in the upper left side of the abdomen. It plays a vital role in the circulatory system as it helps store and release red blood cells as needed. While the spleen does not produce blood, it acts as a reservoir for extra blood that can be released in times of shortage.

When there is an increased demand for oxygen in the body, such as during physical activity or in cases of bleeding, the spleen contracts and pushes stored red blood cells into circulation. This process is known as splenic contraction and helps to increase the overall quantity of blood available for the body to use.

It is important to note that the other options mentioned in the question, such as the adrenal gland, pancreas, and thyroid gland, are not involved in storing or releasing extra blood in times of shortage.