Endocrine system

Chapter 6 Endocrine system




KEY POINTS



• 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.

• Endocrine glands secrete hormones, which are carried by the blood to specific target organs.

• 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.

• Each hormone has a specific effect and many interrelate with other hormones to create a complex network that acts to maintain the body in a state of equilibrium.


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).


image

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.)


The glands are:


• Pituitary gland

• Thyroid gland

• Parathyroid glands

• Pancreas

• Ovaries

• Testes

• Adrenal glands.

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).


Table 6.1 Endocrine glands and their associated hormones

image

image

Fig. 6.4 Inter-relationships between the female reproductive hormones.


Not all hormones are secreted by endocrine glands – some are produced from tissue within another organ, including:


• 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.

• Erythropoietin or erythropoietic-stimulating factor – produced by the kidney in response to low levels of blood oxygen. It stimulates the bone marrow to produce erythrocytes or red blood cells.


Endocrine glands



Pituitary gland


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:


• Thyrotrophic/thyroid stimulating hormone (TSH) – stimulates the thyroid gland.

• Growth hormone or somatotrophin – controls the rate of growth in young animals by:
image Controlling the rate of growth at the epiphyses of the bones

image Being involved in protein production from amino acids

image Regulating the use of energy during periods of poor food supply. Glucose is conserved for use by the nervous system and fat is broken down to be used as a source of energy by the rest of the body

• 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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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Endocrine system
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