CHAPTER 13: ENDOCRINE SYSTEM

I. INTRODUCTION

Endocrine System - consists of glandular organs and acts together with the nervous and other forms of body communication in controlling body activities and maintaining homeostasis

Release and receipt of chemical messengers

Modes of Communication:

- paracrine factors – affect within a single tissue

- autocrine secretions – affect only the secreting

cell itself

- hormones – affect cells within different tissues

- nervous impulses

II. IMPORTANT DEFINITIONS:

A. ENDOCRINE GLAND = a gland that secretes hormones directly into the bloodstream; a ductless gland.

B. Exocrine gland = a gland that secretes substances into ducts which then leave the body (i.e. sweat/sebaceous glands) or into a internal space or lumen (i.e. digestive glands). Exocrine glands are not part of the endocrine system!

C. HORMONE = a very powerful substance secreted by an endocrine gland into the bloodstream, that affects the function of another cell or "target cell".

III. HORMONES

A. General Characteristics:

1. needed in very small amounts (potent);

2. produce long-lasting effects in the cells they target;

3. regulate metabolic processes (maintain homeostasis);

4. are regulated by negative-feedback mechanisms;

5. may be steroid (produced from cholesterol = fat-soluble) or non-steroid (water-soluble). See Fig 13.3, page 506.

a. A steroid hormone passes easily through the target cell membrane;

b. A non-steroid hormone requires a receptor on the target cell membrane to allow the hormone to enter the target cell.

Endocrine vs. Nervous Control

ENDOCRINE

- Secretes hormones that are carried by body fluids to the tissue they affect

- Causes changes in target tissues

- Effects exerted relatively slowly (minutes, hours, days)

- Effects are prolonged

- Response localized or general

NERVOUS

- Transmits electrical impulses along nerve fibers

- Causes muscles to contract or glands to secrete

- Effects exerted relatively rapidly (seconds or fractions of)

- Effects generally brief

- Responses generally specific and localized

FUNCTIONS OF THE ENDOCRINE SYSTEM

• Regulate metabolic processes within the body

– Aid in transport of substances through cell membranes

– Play vital role in cell growth

– Regulate water and electrolyte balance

– Control reproduction

– Involved in energy conversion

– Affect behavior

Hormones

- are specific (act only on target cells)

- target cells posses receptor sites

- EVERY cell is exposed to hormones in circulation,

but only target cells “read” certain hormones – rest

“junk mail” (each hormone has a unique molecular

structure, unique action

- are excreted from glands into extracellular spaces

surrounding the gland cells, absorbed by blood and

transported by circulation throughout the body

- are organic substances: steroids, proteins, or

amines

Control of Hormonal Secretion

-the concentration of each hormone in body fluid remains relatively stable (the rate at which a hormone is secreted is balanced by the rate of its usgae or destruction)

1. The hormone concentration in body fluids is regulated by a NEGATIVE FEEDBACK SYSTEM

2. Hormone secretion is also under NERVOUS CONTROL.

3. Some glands are controlled directly by the

HYPOTHALAMUS – the link between the endocrine and

nervous systems

Excess hormones in circulation are generally inactivated by liver or kidneys

Hyposecretion – too little hormone production

Hypersecretion – too much hormone production