The hypophyseal portal system makes it possible for an endocrine communication between the two structures. It is part of the hypothalamic-pituitary-adrenal axis. The anterior pituitary receives releasing and inhibitory hormones in the blood. Using these, the anterior pituitary is able to fulfill its function of regulating the other endocrine glands. It is one of only a few portal systems of circulation in the body; that is, it involves two capillary beds connected in series by venules.
A key to understanding the endocrine relationship between hypothalamus and anterior pituitary is to appreciate the vascular connections between these organs. As will be emphasized in later sections, secretion of hormones from the anterior pituitary is under strict control by hypothalamic hormones. These hypothalamic hormones reach the anterior pituitary through the following route: 1) a branch of the hypophyseal artery ramifies into a capillary bed in the lower hypothalamus, and hypothalmic hormones destined for the anterior pituitary are secreted into that capillary blood; 2) blood from those capillaries drains into hypothalamic-hypophyseal portal veins, with the hypothalamic-hypophyseal portal veins branching again into another series of capillaries within the anterior pituitary; 3) capillaries within the anterior pituitary, which carry hormones secreted by that gland, coalesce into veins that drain into the systemic venous blood.
Hypophyseal Portal System