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The Endocrine
System
CHEMICAL COMMUNICATION
The Endocrine System
Endocrine system: secretes hormones in the
human body to maintain homeostasis
Major glands: pituitary, thyroid, parathyroid,
adrenal, pancreas, pineal, thymus, testes,
ovaries
Hormones
Hormone: a chemical messenger that carries a signal
from an endocrine gland to regulate the function of a
target cell
2 Types:
◦ Steroid hormones: made from cholesterol (are lipids), water
insoluble, diffuse easily into cell
◦ Non-steroid hormones: made from amino acids (are proteins),
bind to receptors on cell membrane
Hormone secretion is controlled by negative feedback
mechanisms
Pituitary Gland
Hormone release is controlled by the hypothalamus
2 Parts: anterior and posterior pituitary
Anterior Pituitary Hormones:
◦ Growth Hormone (GH): regulates cell division
◦ Prolactin (PRL): regulates milk production in females after
childbirth
◦ Follicle-stimulating hormone (FSH) and luteinizing hormone
(LH): act on testes and ovaries
Pituitary Gland, Cont’d
Posterior Pituitary hormones:
◦ Antidiuretic hormone (ADH): decreases urine production; too
little water in the bloodstream triggers release and less urine,
too much water stops release and more urine is produced
◦ Oxytocin (OT): contracts smooth muscle in walls of uterus;
stimulates uterine contractions in childbirth (Positive Feedback
Loop!)
Thyroid Gland
Action is controlled by hormones in pituitary gland
Important Hormones:
◦ Thyroxine: regulates metabolism of carbohydrates, lipids, and
proteins; Determines basal metabolic rate (BMR)- how many
calories you burn at rest
◦ Calcitonin: regulates concentration of blood calcium
Parathyroid Glands
4 located on thyroid gland
Secretes parathyroid hormone (PTH)
PTH regulates blood calcium and blood phosphate
concentration
Adrenal Glands
Action controlled by hormones in pituitary gland
Hormones:
◦ Epinephrine (adrenalin) and norepinephrine: increase
breathing rate, heart rate, blood pressure, blood glucose
level
◦ Aldosterone: regulates Na+ and K+ levels in body
◦ Cortisol: affects glucose metabolism
Pancreas
Insulin: lowers blood glucose levels, stimulates products
of glycogen (sugar storage)
Glucagon: stimulates breakdown of glycogen, elevates
blood glucose levels
Diabetes: body loses ability to regulate blood glucose
levels
◦ Type 1: immune system destroys cells in pancreas, no more
insulin produced
◦ Type 2: pancreas produces insulin, body cells don’t recognize it.
Other Glands
Pineal Gland
◦ Melatonin: sleep hormone, released in response to
darkness, enables body to distinguish day and night
Thymus Gland
◦ Thymosins: affect production of white blood cells
Feedback Systems
CONNECTING ELECTRICAL AND CHEMICAL
COMMUNICATION IN YOUR BODY
What is Feedback?
Feedback is the process in
which part of the output of a
system is returned to its input
in order to regulate its further
output.
Your body has POSITIVE and
NEGATIVE feedback systems to
maintain homeostasis.
Negative Feedback
Negative feedback occurs when the output of a system
acts to oppose changes to the input of the system.
A thermostat is an example of a negative feedback
system.
Room temp
increases
Room temp is
below the
setpoint
Set point
is reached
Goal: Maintain Stable Temperature!
Set point
is reached
Room temp
decreases
Room temp is
above the
setpoint
Negative Feedback in Biology
Negative feedback also regulates
many systems in organisms.
The endocrine system has many
examples!
This diagram shows a negative
feedback loop for stress
hormones.
Areas of negative feedback are
indicated with a minus sign, (-).
Endocrine System <3 Nervous System!
The brain continuously sends signals to
the endocrine glands to secrete and
release hormones and the glands, in
turn, send feedback to the nervous
system.
The hypothalamus in the brain is the
master switch that sends signals to the
pituitary gland which can release up to
eight hormones into the bloodstream.
The hormone travels to its target organ
and usually results in the release of
another hormone into the bloodstream.
Endocrine System
The hypothalamus then detects the rising hormone
levels from the target organ and decreases the
release of hormones from the pituitary which
results in a decrease in hormone release from the
target organ.
The process of maintaining normal body function
through negative feedback mechanisms is called
homeostasis.
Negative Feedback Example:
Glucose and Insulin
Pancreas
Glucose intake occurs during
digestion of food that is needed for
energy expenditure to perform
routine physical activities.
The pancreas is the key organ that
regulates the glucose levels in body
by secreting two hormones, insulin
and glucagon.
The liver also helps to store the
excess glucose in form of glycogen to
be utilized later.
Liver
Glucose and Insulin Negative Feedback Loop
Eating cake
Increases
Glucose
Levels
(-)
CYCLE 1
Lowers Blood
Glucose levels
Insulin stimulates the
cells to take up glucose
from the blood.
Stimulates β cells of
pancreas to secrete insulin
CYCLE 2
Low Blood Glucose
Levels
(-)
High blood glucose levels
and Cycle 1 continues
Stimulated Alpha Cells in
Pancreas
Glucagon is released
Glucagon stimulates liver cells to
release glucose into the blood
Glucose and Insulin Negative
Feedback Loop
Two primary Hormones
Insulin
Lowers Blood Glucose
Levels
Glucagon
Raises Blood Glucose
Levels
The opposite actions of these two hormones, insulin and
glucagon, helps to maintain normal blood sugar levels in the
body hence maintain homeostasis of the body.
Feedback Check-up....
◦ What is a feedback and an example of a negative AND
positive system?
◦ What is the endocrine system and why is it
important?
◦ Define homeostasis and how normal body function is
maintained with feedback mechanisms.
Kidneys and Water Regulation
The kidneys play a key role in maintaining water regulation.
Renal Cortex
Renal Medulla
Kidney and Water Regulation
The nephron is the most
important functional part
of the kidney.
It filters nutrients like
salts and amino acids in
the Bowman’s capsule
into ascending loop and
filters the urine.
Kidney and Water Regulation
Anti-Diuretic Hormone, ADH (also called vasopressin), is
secreted by the pituitary gland and acts on the nephron to
conserve water and regulate the tonicity of body fluids.
AntiDiuretic
Hormone
ADH acts on Nephron to
reabsorb water and decrease
blood osmolality (saltiness)
Your Turn!
Draw a Negative
Feedback loop
that demonstrates
how your kidneys,
blood, and brain
work together to
regulate the
amount of water
in your body.
Include these
terms:
ADH
Nephron
Distal Collecting
Tubules
Osmoreceptors
Hypothalamus
Pituitary Gland
Concentrated
Urine
Dilute Urine
Permeability
ADH regulated water conservation in
kidneys
Excess water
Less water in the
blood
Stimulates osmoreceptors in
hypothalamus to send signals to
the pituitary gland
Pituitary glands secretes
high levels of ADH
ADH makes the tubules more permeable
and more water is reabsorbed back into the
bloodstream (urine is concentrated).
in the blood
Stimulates osmoreceptors in
hypothalamus to send signals to
the pituitary gland
Pituitary glands secretes
low levels of ADH
Less ADH makes the tubules less
permeable and less water is reabsorbed
back into the bloodstream (urine is
dilute).
ADH regulated water conservation in
kidneys
Osmoregulators send negative feedback to the
hypothalamus about the concentration of water in
the bloodstream.
The hypothalamus then stimulates the pituitary
glands to secrete high or low concentrations of antidiuretic hormone.
ADH then makes the tubules more or less
permeable and hence, maintains water and
electrolyte homeostasis.
Positive Feedback Loops
A positive feedback loop occurs when the output of a
system acts to enhance the changes to the input of the
system.
One example of a biological positive feedback loop is
the onset of contractions in childbirth.
◦ When a contraction occurs, the hormone oxytocin is released
into the body, which stimulates further contractions.
◦ This results in contractions increasing in amplitude and
frequency.
Positive Feedback
Another example is blood clotting.
◦ The loop is initiated when injured tissue releases signal
chemicals that activate platelets in the blood.
◦ An activated platelet releases chemicals to activate more
platelets, causing a rapid cascade and the formation of a blood
clot.
Lactation involves positive feedback so that the more the
baby suckles, the more milk is produced.
Positive Feedback
In most cases, once the purpose of the feedback loop is
completed, counter-signals are released that suppress or
break the loop.
◦ Childbirth contractions stop when the baby is out of the
mother's body.
◦ Chemicals break down the blood clot.
◦ Lactation stops when the baby no longer nurses.