Download The Endocrine System

The Endocrine System
Hormones
Recall that neurons have the ability to
communicate
 However, this is merely cell-to-cell
 What if a signal needed to be sent to
more than one cell?
 This is the role of the endocrine system

◦ Releases chemicals into bloodstream to rest
of body
These chemicals are hormones
 Target cells have the correct receptors
 Effects tend to be long lasting, but can
take extended periods to effect target
cells
 These are released by glands
 Don’t get confused with exocrine glands!

◦ Put secretions directly into organs that use
them

Endocrine glands release secretions into
blood stream
Major Glands
10 in total
Hypothalamus
 Hormones both used by, and stored in,
pituitary gland
Pituitary Gland
 Often called master gland
 Controls other glands
Pineal Gland
 Involved in circadian rhythm
Thyroid
 Regulates metabolism with thyroxine
Parathyroid glands
 4 in total
 Parathyroid hormone controls calcium
levels
Thymus
 Releases hormones to stimulate immune
system
Pancreas
 Produces insulin and glucagon
 Have opposite effects on blood sugar
levels
Adrenal glands
 Releases epinephrine
 Also site of cortisol release
Ovaries and Testes
 Produce sex-specific hormones
 Responsible for secondary sex
characteristics.

Maintaining Homeostasis

The body can respond to internal
conditions
◦ Insulin released when blood glucose levels are
high
However, what is released can have a
effect on the pathway that created it
 These are feedback mechanisms
 Have both positive and negative

Negative (more common) feedbacks
involve inhibition of pathways by end
products
 Positive (less common) feedbacks
stimulate pathways by end products

Metabolism Control
Increased thyroxine levels spike
metabolism
 But, how is it released?
 Lowered thyroxine stimulates
hypothalamus to release thyrotropinreleasing hormone (TRH)
 Travels to pituitary, releases thyroidstimulating hormone (TSH)
 Now thyroid releases thyroxine


The negative feedback comes in play
when high levels of thyroxine inhibits
release of both TRH and TSH
Insulin and Glucagon
Sometimes hormones act in opposing
mechanisms
 Blood glucose level needs to be kept at
90 mg/100mL
 When it drops, alpha cells of pancreas
release glucagon
 Absorbed by liver, glycogen broken down
into glucose
 Levels go up, alpha cells shut off

When blood glucose levels are high, beta
cells release insulin
 Body cells absorb glucose, liver
synthesizes glycogen
 Levels drop, beta cells shut off
 Problems in these systems lead to
diabetes and hypoglycemia

Homework!

Find and describe an example of a
positive feedback mechanism