These capillaries may be lined with phagocytes that can extend their process into the plasma to catch 'prey.' What type of capillary is described?

• A. Continuous capillaries
• B. Fenestrated capillaries
• C. Sinusoids
• D. Postcapillary venules

Answer: (C)

The description points to sinusoid capillaries. Sinusoids have a highly discontinuous endothelium with large gaps and a leaky basement membrane, which allows immune cells to access the bloodstream. They are lined by phagocytic cells (like Kupffer cells in the liver) that can extend processes into the plasma to sample and remove pathogens or debris. This immune-surveillance feature is what distinguishes them from other capillary types. Continuous capillaries have a smooth, intact lining with tight junctions and don’t typically permit such macrophage extension into the lumen. Fenestrated capillaries have endothelial pores but remain more closed overall and aren’t the classic site for macrophages extending into the flow. Postcapillary venules are venous vessels, not capillaries, and don’t match the described structure.