What anatomical structure does the circle of Willis surround?

The circle of Willis is a vital arterial structure located at the base of the brain. It is formed by the anastomosis of the internal carotid arteries and the vertebrobasilar system, providing a collateral circulation around the brain in case of vascular occlusion.

The circle of Willis surrounds the stalk of the pituitary gland, which is situated just above the sella turcica. This anatomical relationship is significant because the pituitary gland, or hypothalamo-hypophyseal axis, is critical for endocrine regulation. Its position and the vascular supply provided by the circle of Willis highlight the importance of maintaining blood flow to this gland and surrounding structures.

The other anatomical options presented, such as the brainstem, cerebellum, and thalamus, are not directly encompassed by the circle of Willis. While these structures are certainly served by branches of the arteries contributing to the circle, they do not lie within the immediate region that the circle encircles, which is primarily associated with the pituitary gland. Understanding the precise anatomical relationships helps in diagnosing and managing conditions related to cerebrovascular health and endocrine function.