In red bone marrow, newly formed blood cells enter circulation. Which type of capillary would you expect to be most abundant in this tissue?

• A. Continuous capillaries
• B. Fenestrated capillaries
• C. Sinusoids
• D. Veins

Answer: (C)

The main idea is that the capillary type in hematopoietic tissue must allow blood cells to exit into the circulation. In red bone marrow, developing blood cells need to move from the marrow into the bloodstream, so the vessels here are designed to be highly permissive. Sinusoids are discontinuous capillaries with a large, irregular lumen and gaps between endothelial cells, and their basement membranes are incomplete or absent in places. This structure gives them the ability to let whole blood cells pass through into the circulation, making them ideally suited for red marrow. Continuous capillaries have tight junctions and a complete basement membrane, which restricts cell passage; fenestrated capillaries have pores that speed exchange of small solutes but still don’t readily accommodate migrating cells; veins are not capillaries and don’t describe the vessel type in marrow.