Where is most hematopoietic tissue found in adults?

Hematopoietic tissue, which is responsible for the production of blood cells, is primarily located in the bone marrow in adults. The long bones, particularly the proximal ends of the humerus and femur, contain significant amounts of red bone marrow, which is the site of active hematopoiesis. While the spleen plays a crucial role in filtering blood and managing blood cells, it is not primarily where blood cell production takes place. Instead, it acts mainly as a reservoir and site for the immune response related to blood cells.

In adults, the liver is involved in various metabolic functions and hematological roles, especially during fetal development, but it does not serve as a major site of blood cell production in adulthood. The lungs are primarily involved in gas exchange and do not have a significant role in hematopoiesis.

Therefore, while the spleen does have some hematopoietic activity, particularly in certain pathological conditions, the long bones are the most critical site of hematopoietic tissue in adults, making that the correct answer.