Chapter 6 Endocrine system
• The endocrine system is part of the regulatory system of the body and works in conjunction with the nervous system. The nervous system produces an immediate response while the endocrine system produces a slower but longer-lasting effect.
• Each target organ responds only to a particular hormone or to a group of hormones and is unaffected by other hormones.
• Hormonal secretion is controlled by different mechanisms, e.g. levels of a chemical in the blood, or by a feedback loop.
Basic functions and hormones

Fig. 6.1 Locations of important endocrine glands in the cat.
(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 290.)
Secretion of a hormone occurs in response to a specific stimulus (Table 6.1). Stimuli may be:
• Nerve impulses, e.g. adrenaline (epinephrine) is released from the adrenal medulla in response to nerve impulses from the sympathetic nervous system.
• A stimulating or releasing hormone, e.g. TSH (thyrotrophic/thyroid-stimulating hormone) from the anterior pituitary gland activates the thyroid gland.
• Levels of certain chemicals in the blood, e.g. raised blood glucose levels stimulate the release of insulin from the pancreas.
• Feedback loops, which prevent oversecretion or reduce secretion once the effect has been achieved, e.g. oestrogen from the ovarian follicles prevents further secretion of follicle-stimulating hormone from the anterior pituitary gland, so preventing further follicular development (see Fig. 6.4).
• Gastrin – produced by the wall of the stomach. As food enters the stomach via the cardiac sphincter, gastrin stimulates the release of gastric juices from the gastric glands and digestion begins.
• Secretin – produced by the wall of the small intestine. As food enters the duodenum from the stomach, secretin stimulates the secretion of intestinal and pancreatic juices which continue the process of digestion.
• Chorionic gonadotrophin – produced during pregnancy by the ectodermal layer of the chorion surrounding the conceptus. It helps to maintain the corpus luteum in the ovary throughout gestation.
Endocrine glands
Pituitary gland
Anterior pituitary gland
Also known as the adenohypophysis, this produces:
• Adrenocorticotrophic hormone (ACTH) – stimulates the adrenal cortex. Oversecretion may cause the symptoms of Cushing’s disease.
• Prolactin – stimulates the development of the mammary glands and the secretion of milk during the latter half of pregnancy. The milk cannot be released unless oxytocin is produced at the end of the gestation period.
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