Page 255 - Biology_F5

Biology for Advanced Level Secondary Schools

controls the secretions of other endocrine pituitary hormones stimulate the release
glands. of target gland hormones. As their levels
increase, they inhibit the secretion of
The pituitary gland, which is located hypothalamus and pituitary hormones.
at the base of the brain, is directly When their levels in the blood fall below a
connected to the brain region called certain level, hypothalamus and pituitary
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hypothalamus. This physical link between inhibition stop and start secreting their
the hypothalamus and pituitary is the basis chemicals again. This is also referred to as
for the connection or link between the a negative feedback mechanism.
central nervous system and the endocrine
system. The pituitary has two distinct The posterior pituitary gland is an
segments, namely the anterior pituitary extension of the brain. It stores and
and the posterior pituitary. The anterior releases Antidiuretic Hormone (ADH)
pituitary gland is connected by blood or vasopressin and oxytocin hormone
vessels called the portal system, which which are produced by neurosecretory
has one capillary bed in the hypothalamus cell bodies, lying in the hypothalamus.
and another in the anterior pituitary. Also They pass down the nerve fibres. Nerve
it has nerve terminals that release two impulses are relayed to the cell bodies
groups of chemical substances known as of these neurosecretory cells from other
releasing and inhibiting factors into the regions of the brain. They are transmitted
blood capillaries at the hypothalamus
end of portal system. These chemical down the axons, where hormones are
substances pass to the pituitary end and stored in vesicles (Figure 4.37). The
cause the release of six trophic hormones whole process involves both nervous and
(hormones that stimulate other endocrine endocrine systems. This is referred to
hormones to release hormones) which are as a neuroendocrine response, resulting
stored in the anterior pituitary gland. into a pattern of behaviour known as a
neuroendocrine reflex.
In the year 1930, a biologist documented
the consequences of removing the entire ADH is released in response to a fall in the
pituitary gland from laboratory rats. As water content of blood plasma and leads
a result of pituitary removal, the animals to an increase in the permeability to water
stopped growing, failed to maintain of distal convoluted tubule in the nephron
normal body temperature, and suffered of the kidney. On the other hand, oxytocin
atrophy (shrinkage) of their genitals, causes the contraction of the uterus during
thyroid glands, and adrenal cortex. Not birth and ejection of milk from nipples.
surprisingly, their life span shortened
dramatically. These experiments suggested Contraction of the uterus resulting from
that, in addition to secreting growth the effect of oxytocin is a positive feedback
hormones, the pituitary gland secretes mechanism. The more the concentrations
hormones that regulate the production of oxytocin, the stronger the contraction
of a wide variety of other hormones. All of the uterus becomes.

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