Endocrine System
anterior lobe. Here, releasing hormones from the hypothala-
mus primarily control secretion. These releasing hormones
are carried in the blood via a capillary bed associated with
the hypothalamus. The vessels merge to form the
po-F z
portal veins
that pass downward along
the pituitary stalk and give rise to a capillary bed in the ante-
rior lobe. Thus, substances released into the blood from the
hypothalamus are carried directly to the anterior lobe. The
hypothalamus, therefore, is an endocrine gland, yet it also
controls other endocrine glands. This is also true of the ante-
rior pituitary.
neurons called neurosecretory cells secrete two important
hormones, antidiuretic hormone (ADH) and oxytocin (OT),
into the bloodstream from their nerve endings in the poste-
rior lobe. The cell bodies of these neurosecretory cells are in
the hypothalamus.
During fetal development, a narrow region between the anterior
and posterior lobes of the pituitary gland, the
intermediate lobe
intermedia), produces melanocyte-stimulating hormone (MSH). This
hormone regulates the formation of melanin—the pigment in the
skin and in parts of the eyes and brain. This intermediate lobe seems
to disappear during fetal development, but its secretory cells persist
and become part of the two remaining lobes.
The brain controls most of the pituitary gland’s activities
(f g. 13.12)
. The pituitary gland’s posterior lobe releases hor-
mones into the bloodstream in response to nerve impulses
from the hypothalamus. A different mechanism controls the
Anterior cerebral
Optic nerve
Pituitary stalk
Anterior lobe
of pituitary
Sphenoid bone
Optic chiasma
Posterior lobe
of pituitary
Sella turcica
Third ventricle
Trochlear nerve
Basilar artery
The arrangement of two capillaries in series is unusual and is called
portal system.
It exists in only three places in the body: the hepatic
portal vein connects intestinal capillaries to special liver capillaries
called sinusoids, the eF
erent arteriole of kidney nephrons connects
two sets of capillaries, and the hypophyseal portal vein gives rise to a
capillary net in the anterior lobe of the pituitary gland.
The pituitary gland is attached to the hypothalamus and lies in the sella turcica of the sphenoid bone.
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