As will be discussed 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 hypothalamic-hypophyseal portal vessels.
The hypothalamic-hypophyseal portal vessels (portal
veins are defined as veins between two capillary beds) start from
a capillary bed that is an offshoot of the the hypophyseal artery.
Hypothalmic hormones destined for the anterior pituitary are secreted
into this capillary blood.
The hypothalamic-hypophyseal portal veins branch again
into another series of capillaries within the anterior pituitary.
Other capillaries within the anterior pituitary,
carring the hormones secreted by that gland, coalesce into veins
that drain into the systemic venous blood. Those veins also collect
capillary blood from the posterior pituitary gland.