Final answer:
The enlarged capillaries within the red pulp of the spleen are called splenic sinusoids, which have a discontinuous basal lamina and allow easy entry and exit of blood cells. The correct answer is D. splenic sinusoids.
Step-by-step explanation:
The correct answer is D. splenic sinusoids. Within the red pulp of the spleen, the enlarged capillaries are called splenic sinusoids. These sinusoids have a discontinuous basal lamina that allows blood cells to enter and exit easily. Sinusoids are specialized post-capillary vessels that have extensive intercellular gaps and incomplete basement membranes, allowing for the passage of large molecules and even cells. They are found in organs such as the liver, spleen, bone marrow, and lymph nodes, and play a crucial role in various physiological functions.