USA Biology Olympiad (USABO) Practice Exam

If carbon particles are injected into the blood of mice, where are they most likely to be found?

1. In the Aorta

2. In Plasma

3. In Monocytes of the Spleen

4. In Kidney Cells

The correct answer is: In Monocytes of the Spleen

When carbon particles are injected into the bloodstream, they are typically engulfed by phagocytic cells, such as monocytes and macrophages. The spleen is a key organ in the immune system and plays a significant role in filtering blood and removing foreign particles, including pathogens and debris. Monocytes in the spleen can migrate from the bloodstream and are actively involved in phagocytosis, meaning they will effectively absorb and engulf the injected carbon particles for removal from circulation. While carbon particles might initially travel through the aorta and plasma after injection, they would not remain there long-term. The kidneys are involved in filtering blood and excreting waste, but they are not the primary site for the uptake of injected carbon particles. Hence, the presence of carbon particles is most likely in the monocytes of the spleen as these cells perform the necessary immune response functions.