This system, like the Nervous System, sends messages around the body. However, the endocrine system sends information from one source to many destinations causing long lasting responses.
The Endocrine system consists of a number of glands which secrete hormones. These hormones are organic chemicals which travel in the blood or lymph to cells in the body and affect a response. Each hormone has its own 'target cell' which it affects and so they can all be carried around the body together.
These hormones are effective in small concentrations and can be continuously secreted into the bloodstream. Their effects can be specific or widespread as they also control growth and metabolism!
Hormone secretion is regulated by nerve stimulation, by other hormones, negative feedback or chemical balance. Insulin production is influenced by blood glucose levels.
- Hypothalamus - this is a nerve control centre at the base of the brain. It controls the Pituitary Gland.
- Pituitary Gland - again at the base of the brain this gland consists of 2 lobes. The anterior lobe produces hormones which act on the thyroid and adrenal glands and the ovaries in a mare and the testes of a stallion.
- Thyroid - found on either side of the larynx this gland controls metabolism and growth. If underactive the result is a lack of energy and a tendency for the horse to be overweight. If overactive the horse will be the opposite! The hormone it produces is dependent on Iodine so a shortage of this will affect the effectiveness of the thyroid.
- Parathyroids - situated near the thyroid these are a group of 4 small glands which control calcium and phosphorus metabolism in the body. See my minerals blog.
- Adrenal glands - located close to the kidneys these produce 2 hormones; cortisol and adrenalin. Cortisol has a number of different effects including the control of inflammation. Adrenalin is the 'fight or flight' hormone and is a stimulant in response to stress - increasing the blood pressure and heart rate.
- Pancreas - found behind the stomach in a loop of the small intestine this secretes insulin and digestive juices. Insulin controls the level of blood sugar.
- Thymus - situated just under the breastbone between the lungs this gland is concerned with immunity. A source of blood lymphocytes (see my immune system blog) it is particularly large in foals.
- Other glands - in mares the uterus produces prostaglandin and the ovaries secrete oestrogen and progesterone. In stallions the testes produce testosterone.
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