Chapter 6 Endocrine system
Basic functions and hormones
The endocrine system forms part of the regulatory system of the body and works in conjunction with the other regulatory mechanism, the nervous system. A series of endocrine glands secrete chemical messengers known as hormones, which are carried by the blood to their target organs. These may be some distance away from the gland. In contrast, exocrine glands produce secretions that are released via ducts and lie close to their effector organs, e.g. sweat glands, gastric glands.
Chemically, hormones may be steroids, proteins or amines (derived from amino acids). They regulate the activity of the target organ, which responds to that particular hormone. All other organs are unaffected. The response produced by hormones is slower and lasts longer, and complements the rapid and relatively short-lived responses produced by the nervous system.
The endocrine glands are distributed throughout the body and may secrete more than one hormone (Fig. 6.1).
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:
Not all hormones are secreted by endocrine glands – some are produced from tissue within another organ, including:
This is a small gland lying ventral to the hypothalamus in the forebrain (Fig. 6.1). It is also known as the hypophysis and is divided into two lobes, each of which acts as a separate gland. The pituitary gland is often referred to as the ‘master gland’ as its hormones control the secretions of many of the other endocrine glands.
Anterior pituitary gland
Also known as the adenohypophysis, this produces: