What ensures that blood levels of a specific releasing hormone are locally high in order to stimulate the pituitary?

The correct answer is: hypothalamic-pituitary portal system.

Explanation:

The hypothalamic-pituitary portal system, also called the hypophyseal portal system, is a system of blood vessels that connect the hypothalamus with the anterior pituitary to transport hormones between these two glands.

The hypothalamus and the anterior pituitary are a hugely important part of the endocrine system, since many hormones are secreted by these glands. Not only that but the hormones released by the hypothalamus have a direct effect on the hormones released by the anterior pituitary gland, which affects different peripheric glands as well. For example, the hypothalamus releases the thyrotropin-releasing hormone, which stimulates the anterior pituitary to release the thyroid-stimulating hormone, which stimulates the thyroid to secrete thyroid hormone. This sort of information between the hypothalamus and the anterior pituitary is transmitted thanks to the hypothalamic-pituitary portal system, which has fenestrated capillaries that allow an even faster exchange between the glands. If the blood in this portal system has high levels of, for example, the thyrotropin-releasing hormone, the anterior pituitary will soon start releasing the thyroid-stimulating hormone.