Download endocrine system

Endocrine System,
Hormones, and
Homeostasis
THE ENDOCRINE SYSTEM
• Two body systems are responsible for sending and receiving
sensory information and coordinating body responses.
• These are the nervous system and the endocrine system.
Together, they are sometimes referred to as the neuro-endocrine
system.
• The endocrine system regulates body activities by releasing
hormones (chemical messengers) into the bloodstream, where
they are carried throughout the entire body.
• Hormonal responses may be almost instantaneous, or may occur
days later.
• There is a wide variety of hormonal effects.
EXOCRINE & ENDOCRINE GLANDS
• The body contains two kinds of glands:
• Exocrine glands secrete their products into body
ducts, which carry the products into body cavities, the
lumen of an organ, or the outer surface of the body.
• Sudoriferous glands, sebaceous glands, mucous glands, and
digestive glands.
• Endocrine glands secrete their products (hormones)
into the extracellular space around the secretory
cells. The secretions diffuse into capillaries and are
carried throughout the body by the circulatory system.
ENDOCRINE SYSTEM
• The endocrine system is composed of the body’s
endocrine glands. These include:
• The pituitary, thyroid, parathyroid, adrenal, and
pineal glands
• There are also many organs that have cells which
secrete hormones, but are not exclusively
endocrine organs. These include:
• The hypothalamus, thymus, pancreas, ovaries, testes,
kidneys, stomach, liver, small intestine, skin, heart,
and placenta.
HORMONES
• Hormones can have very powerful effects, even when present in
very low concentrations.
• There are approximately 50 different hormones produced in the
human body. Most of these only affect a few types of cells.
• The specific cells which are affected by a hormone are called
target cells.
• Hormones influence their target cells by binding to proteins or
glycoproteins in the cell membrane called receptors. Only the
target cells for a certain hormone have receptors that will
recognize and bind to that hormone.
HORMONES
• Hormones that pass into the blood and act on distant target cells
are called circulating hormones or endocrines.
• Circulating hormones may linger in the blood for minutes to hours,
exerting their effects for a prolonged period of time.
• Eventually, circulating hormones are inactivated by the liver and
excreted by the kidneys.
• Hormones that act locally without first entering the bloodstream
are called local hormones.
• Local hormones that act on neighboring cells are called paracrines.
• Local hormones that act on the same cell that secreted them are called
autocrines.
• Local hormones are usually inactivated very quickly.
I.
Homeostasis
A. The process of maintaining a stable internal
environment (for the body) at a constant level
or between narrow limits
1. Examples include blood pH, oxygen and carbon
dioxide concentrations, blood glucose, body temp,
and water balance
2. By maintaining homeostasis all bodily functions can
take place properly
3. All body enzymes work within the specific limits
maintained by homeostasis (i.e. certain blood
enzymes only work at a particular pH)
4. Homeostasis involves monitoring levels of variables
and correcting changes in levels by negative
feedback mechanisms
a. These negative feedback loops are most evident in the
endocrine and nervous systems
B.One example of a nervous system
feedback loop would be control of
blood pressure
C. A hormonal example would be the control of water
balance
II. Control of body temperature is another excellent
example of homeostasis
a. Body heat is created by metabolic activities in
cells, esp. those of the liver, muscles, and brain
b. During digestion and physical activity heat
production is increased
c. Shivering is the result of muscles contracting
rapidly in order to create heat
d. Heat is lost from an organism through conduction
(transferring directly from skin to surrounding air) and
evaporation (via sweat)
e. Heat is carried in the blood
f. Areas where blood is close to the skin
surface cool faster (which is why some
people put their wrists under cold water or
put a cold cloth on their neck
and forehead)
conserve
g. Vasoconstriction helps the body to
heat and vasodialation helps the body
to lose heat
h. The hypothalamus helps to regulate heat
production and loss via feedback loops
3. Control of Blood Glucose Concentration
a. Although other sugars exist in the
blood, it is glucose that is most
important and is monitored in order to
maintain homeostasis
b.Normally, blood glucose levels stay
between 4 and 8 mmols/l
c. Levels rise after meals and are
usually lowest in the morning before
breakfast
d. If levels drop too low, hypoglycemia
develops
- symptoms include lethargy,
impaired mental function and
irritability
e. If levels are too high,
hyperglycemia develops
- symptoms include
diabetes and its associated
problems (eye, kidney, and
nerve damage)
f. Glucose levels are monitored by
cells in the pancreas
g. If blood glucose drops to a
dangerous level, some of those
cells (called alpha cells) produce
glucagon, a hormone which
activates liver cells to convert
glycogen to glucose
(glycogenolysis) and thus raise
blood sugar
h. When levels of blood sugar rise (either
due to a meal or as a result of glycogen
conversion) insulin is released from the
beta cells of the islet of langerhans
(part of the pancreas)
- insulin causes the liver to convert
glucose to glycogen for storage
i. Diabetes mellitus type I is caused by
insufficient or non-existent production of
insulin
j. Diabetes mellitus type II is caused by a
decreased response to insulin the tissues
of the body