anatomy of the hypothalamic pituitary axis

The anatomy of the hypothalamic pituitary axis is essential to its function. The neurons from the hypothalamus which synthesize hormones to be released by the posterior pituitary gland synapse directly onto the veins in the posterior pituitary, so they can release the hormones there to be sent out to the rest of the body. The hypothalamic-anterior pituitary section has a more unique structure. There is a portal system, 2 capillary beds connected by a veinous system, that joins them. One capillary bed is in the hypothalamus and the other is in the anterior pituitary. This contrasts normal veinous systems, where an artery goes in and leaves as a vein. Instead, the two capillary systems are joined. A set of neurons in the hypothalamus produces releasing factors and synapses in the capillary bed that is also in the hypothalamus, which is connected to the capillary bed in the anterior pituitary where hormones such as ACTH, FSH, and prolactin are synthesized. This connection allows it to trigger their release. The hormones in the portal system would degrade and be at concentrations too low/dilute in the general circulation, but when contained in the smaller portal system where they can flow, they are at a sufficient concentration.