Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Your horse’s care<br />
Control<br />
centre<br />
Hormones do a lot more than make your mare<br />
moody. Vet Claire Dixon MRCVS explains how the<br />
endocrine system works and what can go wrong<br />
Photos: Bauer Library, unless stated<br />
out<br />
Horses<br />
inside<br />
horses insIde out l endocrine system<br />
meet the<br />
expert<br />
Claire Dixon<br />
MRCVS is a clinician<br />
in equine medicine<br />
at the University of<br />
Glasgow, splitting her<br />
time between working in the<br />
hospital and teaching students.<br />
Prior to this, she worked at<br />
Tufts University (USA),<br />
initially as a resident in<br />
large animal internal<br />
medicine, followed by<br />
a period as an<br />
emergency clinician.<br />
While you probably<br />
associate hormones<br />
with mood swings<br />
and mareish<br />
behaviour, there are<br />
many other functions that fall under the<br />
regulation of the body’s most vital control<br />
centre — the endocrine system.<br />
The horse’s endocrine system is the<br />
collection of glands that produce hormones<br />
which regulate a number of vital functions.<br />
In order for the body to respond and adapt<br />
to changes, both internally and externally,<br />
the various functions and processes must be<br />
controlled in a co-ordinated fashion.<br />
The endocrine system is part of this, and<br />
involves hormones being produced by<br />
specific tissues that then travel through the<br />
blood to exert their influence in a variety of<br />
ways. Some of these hormones, such as<br />
adrenaline, have an instant effect; others<br />
operate more slowly. Often, there are pairs of<br />
hormones with complementary functions.<br />
Calcitonin, for example, decreases blood<br />
calcium levels, whereas parathyroid<br />
increases them. This combination allows<br />
a faster response and a greater degree of<br />
control and fine tuning over the body.<br />
The pituitary gland<br />
in the brain helps<br />
with a number of<br />
foaling functions<br />
Parathyroid<br />
glands<br />
Adrenal<br />
glands<br />
Thyroid<br />
gland<br />
Pituitary<br />
gland<br />
Hormones<br />
in a nutshell<br />
There are a large number of<br />
hormones, each exerting a very<br />
different effect on different tissues<br />
and organs. Hormones control<br />
everything from growth and<br />
development (for example, replacing<br />
old, worn out cells), lactation,<br />
metabolism, fluid balance, blood<br />
pressure, the “fight or flight”<br />
response and digestion of food.<br />
In fact, there is very little that<br />
isn’t related in some way to<br />
the endocrine system.<br />
Pancreas<br />
Ovaries<br />
(female)<br />
Testes<br />
(male)<br />
Hormones’ six homes<br />
The endocrine system encompasses most of<br />
the organs of the body, with hormones<br />
secreted from the stomach and intestines to<br />
regulate digestion, and even from fat (leptin,<br />
which inhibits appetite). While some<br />
hormones work only on a specific tissue,<br />
others exert their effect over the whole body.<br />
Most hormones are secreted from the<br />
following six structures (see diagram, left):<br />
Pituitary gland The pituitary gland sits<br />
at the base of the brain. It works closely<br />
with the hypothalamus, just next to it, to<br />
regulate a number of functions.<br />
It is divided into three parts, the pars<br />
distalis, pars intermedia and pars nervosa.<br />
The pars distalis contains a number of cells,<br />
which each produce a different hormone<br />
Antidiuretic<br />
hormone helps<br />
to maintain<br />
hydration<br />
with a different function:<br />
n Growth hormone, which promotes growth<br />
over the entire body.<br />
n Follicle stimulating hormone (FSH) and<br />
luteinising hormone (LH), which control<br />
the reproductive activities of the ovaries<br />
and testes.<br />
n Prolactin, involved in milk production.<br />
Adrenocorticotropin hormone (ACTH) is<br />
produced from the pars intermedia. ACTH is<br />
probably the best known of the equine<br />
hormones, as it is increased in horses with<br />
pituitary pars intermedia dysfunction (PPID<br />
— Cushing’s) and testing the level of ACTH in<br />
the blood is used for diagnosis. In normal<br />
horses, ACTH controls the adrenal gland.<br />
The pars nervosa produces the antidiuretic<br />
hormone, which controls the amount of<br />
urine production, helping to maintain<br />
hydration when water intake is decreased.<br />
It also produces the hormone oxytocin,<br />
which is involved in lactation and foaling.<br />
88 your horse August 2019 www.yourhorse.co.uk www.yourhorse.co.uk<br />
August 2019 your horse 89