YH 454 combined pdf

Your horse’s care
Control
centre
Hormones do a lot more than make your mare
moody. Vet Claire Dixon MRCVS explains how the
endocrine system works and what can go wrong
Photos: Bauer Library, unless stated
out
Horses
inside
horses insIde out l endocrine system
meet the
expert
Claire Dixon
MRCVS is a clinician
in equine medicine
at the University of
Glasgow, splitting her
time between working in the
hospital and teaching students.
Prior to this, she worked at
Tufts University (USA),
initially as a resident in
large animal internal
medicine, followed by
a period as an
emergency clinician.
While you probably
associate hormones
with mood swings
and mareish
behaviour, there are
many other functions that fall under the
regulation of the body’s most vital control
centre — the endocrine system.
The horse’s endocrine system is the
collection of glands that produce hormones
which regulate a number of vital functions.
In order for the body to respond and adapt
to changes, both internally and externally,
the various functions and processes must be
controlled in a co-ordinated fashion.
The endocrine system is part of this, and
involves hormones being produced by
specific tissues that then travel through the
blood to exert their influence in a variety of
ways. Some of these hormones, such as
adrenaline, have an instant effect; others
operate more slowly. Often, there are pairs of
hormones with complementary functions.
Calcitonin, for example, decreases blood
calcium levels, whereas parathyroid
increases them. This combination allows
a faster response and a greater degree of
control and fine tuning over the body.
The pituitary gland
in the brain helps
with a number of
foaling functions
Parathyroid
glands
Adrenal
glands
Thyroid
gland
Pituitary
gland
Hormones
in a nutshell
There are a large number of
hormones, each exerting a very
different effect on different tissues
and organs. Hormones control
everything from growth and
development (for example, replacing
old, worn out cells), lactation,
metabolism, fluid balance, blood
pressure, the “fight or flight”
response and digestion of food.
In fact, there is very little that
isn’t related in some way to
the endocrine system.
Pancreas
Ovaries
(female)
Testes
(male)
Hormones’ six homes
The endocrine system encompasses most of
the organs of the body, with hormones
secreted from the stomach and intestines to
regulate digestion, and even from fat (leptin,
which inhibits appetite). While some
hormones work only on a specific tissue,
others exert their effect over the whole body.
Most hormones are secreted from the
following six structures (see diagram, left):
Pituitary gland The pituitary gland sits
at the base of the brain. It works closely
with the hypothalamus, just next to it, to
regulate a number of functions.
It is divided into three parts, the pars
distalis, pars intermedia and pars nervosa.
The pars distalis contains a number of cells,
which each produce a different hormone
Antidiuretic
hormone helps
to maintain
hydration
with a different function:
n Growth hormone, which promotes growth
over the entire body.
n Follicle stimulating hormone (FSH) and
luteinising hormone (LH), which control
the reproductive activities of the ovaries
and testes.
n Prolactin, involved in milk production.
Adrenocorticotropin hormone (ACTH) is
produced from the pars intermedia. ACTH is
probably the best known of the equine
hormones, as it is increased in horses with
pituitary pars intermedia dysfunction (PPID
— Cushing’s) and testing the level of ACTH in
the blood is used for diagnosis. In normal
horses, ACTH controls the adrenal gland.
The pars nervosa produces the antidiuretic
hormone, which controls the amount of
urine production, helping to maintain
hydration when water intake is decreased.
It also produces the hormone oxytocin,
which is involved in lactation and foaling.
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