Learning

Endocrine System
  1. Hormone Glands
  2. Target Cells
  3. Types of Hormones
  4. Feedback Loops

endocrine system

CAPTION: Male and Female Endocrine System (click image for animation) CREDIT: scienceblogs.com (image), Tulane/Xavier Center for Bioenvironmental Research (animation)



The endocrine system is a complex network of chemical signals and messages that control many immediate and life-long bodily responses and functions. Growing taller, developing male or female characteristics and reacting to fear are all partially directed by endocrine hormones. All animals with backbones — from fish to mammals — have an endocrine system that works hand-in-hand with the nervous system to: maintain the body's internal steady state (nutrition, metabolism, excretion, water and salt balance); react to stimuli from outside the body; regulate growth, development and reproduction; produce, use and store energy.




The endocrine system's three parts — glands, hormones and target cells — relay information and instructions throughout the body. Sometimes the whole process works within seconds, say, in response to fear. Other times it reacts more slowly, telling body parts when and how much to grow and developing characteristics that distinguish male from female. It happens like this:

  1. Glands and nerve cells signal endocrine glands about temperature changes, hunger, fear, growth needs or other stimuli.


  2. In response, endocrine glands release hormones to carry instructions to specific cells. These chemical messengers travel from head to toe or just to the cell next door looking for and locking onto special binding proteins, known as receptors, that are located in and on the target cells.


  3. Once bound, the receptor reads the hormone's message and carries out its instructions by starting one of two distinct cellular processes. The receptor can:

    1. turn on genes to make new proteins, which causes long-term effects such as:
      • growth — growth hormones control height and bone structure, too little causes dwarfism, too much causes giantism (acromegaly).
      • sexual and reproductive maturity — sex steroid hormones (estrogens, progestins, androgens) help develop, regulate and maintain male and female sex characteristics (breast size, bone density, muscle development, sperm production), cycles (uterine growth, pregnancy) and behavior.


    2. Alter the activity of existing cellular proteins, which produce rapid responses such as:
      • a faster heart beat — releasing stored adrenalin in response to being frightened or nervous causes the heart to race.
      • varied blood sugar levels — the hormone insulin regulates blood sugar by modifying glucose uptake by many tissues (low insulin levels leads to diabetes).