Which organ is NOT directly involved in the making of blood?

The gallbladder is not directly involved in the making of blood. Its primary function is to store and concentrate bile, which is produced by the liver and released into the digestive system to aid in the digestion and absorption of fats. Blood formation, or hematopoiesis, primarily occurs in the bone marrow, where red and white blood cells are produced, as well as in the spleen and liver, which also have roles in filtering blood and recycling components.

In contrast, the spleen plays a significant role in the production and storage of blood cells, particularly in filtering old red blood cells and contributing to immune responses. The kidneys are involved in the regulation of various blood components, including the production of erythropoietin, a hormone that stimulates the production of red blood cells in the bone marrow. The liver, while primarily known for its metabolic functions, is also involved in the synthesis of various proteins that are crucial to blood function, such as clotting factors and albumin.

Thus, while the spleen, kidneys, and liver have direct roles in aspects of blood production and metabolism, the gallbladder does not participate in these processes, making it the correct answer to the question.