Page 47 - 86 human physiology part-2

Chapter 14

14 THE ENDOCRINE SYSTEM

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Introduction To The Endocrine System

T he endocrine system is a control system of ductless glands that secrete chemical messengers called

hormones that circulate within the body via the bloodstream to affect distant cells within specific
organs. Endocrine glands secrete their products immediately into the blood or interstitial fluid, without
storage of the chemical. Hormones act as "messengers," and are carried by the bloodstream to different
cells in the body, which interpret these messages and act on them.

It seems like a far fetched notion or idea that a small chemical can enter the bloodstream and cause
an action at a distant location in the body. Yet this occurs in our bodies everyday of our lives. The
ability to maintain homeostasis and respond to stimuli is largely due to hormones secreted within the
body. Without hormones, you could not grow, maintain a constant temperature, produce offspring, or
perform the basic actions and functions that are essential for life.

The endocrine system provides an electrochemical connection from the hypothalamus of the brain
to all the organs that control the body metabolism, growth and development, and reproduction.

There are two types of hormones secreted in the endocrine system: (1) steroidal and (2)
nonsteroidal, or protein based hormones.

The endocrine system regulates its hormones through negative feedback, except in very specific
cases like childbirth. Increases in hormone activity decrease the production of that hormone. The
immune system and other factors contribute as control factors also, altogether maintaining constant
levels of hormones.

Types of Glands

Exocrine Glands are those which release their cellular secretions through a duct which empties to
the outside or into the lumen (empty internal space) of an organ. These include certain sweat glands,
salivary and pancreatic glands, and mammary glands. They are not considered a part of the endocrine
system.

Endocrine Glands are those glands which have no duct and release their secretions directly into
the intercellular fluid or into the blood. The collection of endocrine glands make up the endocrine
system.

The main endocrine glands are the pituitary (anterior and posterior lobes), thyroid, parathyroids,
adrenal (cortex and medulla), pancreas and gonads.

The pituitary gland is attached to the hypothalamus of the lower forebrain.

The thyroid gland consists of two lateral masses, connected by a crossbridge, that are attached to

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