Ameen Khan
 Homeostasis mean “same state” and refers to the
process of keeping the internal body in the same state
when the external environment is changed.
Homeostasis is mainly achieved through negative
feedback which is when the body automatically tries to
return the body to its normal state. The hormone
system and the autonomic nervous system play a big
role in homeostasis and are coordinated by the
hypothalamus.
 Humans are endotherms which means they have a
constant internal body temperature. (35-40 C)
 Body temperature is controlled by the
thermoregulatory center in the hypothalamus.
 The thermoregulatory center uses
 Receptors in hypothalamus which measure temperature
of the blood as it passes through the brain
 Skin receptors that monitor outside temperature
 The thermoregulatory system uses these receptors to
send signals to other parts of the body to maintains
temperature.
response
Response to low
temperature
Response to high
temperature
Smooth muscles in skin
Muscles contract and less
heat is carried to the
surface of the body.
Extremities turn blue.
(frostbite)
Muscles relax and heat is
carried to the surface of
the body. This is when
skin turns “red”.
Sweat glands
No sweat is produced
Glands secrete sweat onto
the surface of the skin.
The water takes heat with
it and evaporates into the
air.
Erector pilli muscle
Muscles contract and
raise the skin hair which
keeps a layer of warm air
on the skin.
(goosebumps)
Muscles relax and lowers
skin hair. This allows for
more evaporation.
Response to Low temp.
Response to high temp.
Skeletal muscles
Muscles contract and
relax repeatedly which
generates heat from
friction and from
metabolic reactions.
(shivering)
No shivering
Adrenal and Thyroid
Glands
Glands secrete adrenaline The glands stop secreting
and thyroxine which
adrenaline and thyroxine.
increases metabolic rate
in different tissues,
mainly the liver. This
generates heat.
The thermoregulatory center usually keeps temperature in the 35-40 C range but
sometimes it increases temperature in humans. In a fever, white blood cells
release chemicals which increase body temperature by 2-3 C to kill off bacteria
and other viruses.
 Glucose is a very important carbohydrate in animals,
and its concentration in the blood affects all the cell in
the body. Very high or low levels of glucose can lead to
death.
 Glucose concentration in the blood is controlled by
the pancreas which has glucose receptor cells which
monitor glucose levels and secrete hormones if
needed. The α-cells secrete the hormone glucagon,
while the β-cells secrete the hormone insulin.
When Glucose Level Is Too High (After a meal)
 Insulin from β-cells - makes cells use up extra glucose
through respiration and makes the liver convert
glucose to glycogen . Insulin decreases blood glucose
level.
When Glucose Level is Too Low
 Glucagon from α-cells: causes the liver to break down
glycogen to glucose, which diffuses into the blood.
Glucagon increases blood glucose level.
 Homeostasis is also maintained by the excretions of
organs to get rid of wastes in our body.
 Skin- sweat ( water, ions, urea)
 Lungs- carbon dioxide and water
 Kidneys- excrete urine which consists of urea and
many other substances that the blood doesn’t need.
 controlled by the hypothalamus which contains
osmosreceptor cells, which detect changes in the
water potential of the blood.
 When water levels are low, the hypothalamus makes
us feel thirsty, and it also secretes the hormone ADH.
 ADH targets kidney cells and causes water channels
in the cells to open so more water is reabsorbed from
urine.
 JGA is a special tissue that supplies blood to the
glomerulus (which is near the kidneys).
 When blood pressure becomes too low, JGA releases
an enzyme called renin.
 Renin converts angiotensinogen to angiotensin II in
the blood.
 Angiotensin II increases blood pressure by causing
arterioles to constrict, and it also signals nephrons to
reabsorb more NACl and water.
 Medulla controls respiration rate as well as the heart
rate.
 The amount of carbon dioxide in the blood is one of
the main things that determines whether the
breathing rate needs to be changed.
 If carbon dioxide in the blood stream increases, then
chemoreceptors send messages to the medulla .
 Medulla sends nerve impulses down to the rib
muscles and the diaphragm which causes them to
contract and relax more quickly .
 This increases the breathing rate to get more oxygen to
the blood stream and bring back equilibrium of
oxygen and carbon dioxide levels in the blood stream.
 Special receptors located within the muscles send
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impulses to the medulla.
The medulla secretes epinephrine and
norepinephrine.
The two chemicals reach the Sino-atrial node, which
acts like a pacemaker.
These chemicals arouse the Sino-atrial node, which
increases heart rate.
This also occurs in times of stress or danger.
 Muscles send additional impulses to the medulla.
 Medulla then secretes the hormone acetylcholine.
 Acetylcholine decreases the heart rate by slowing
down the electrical impulses from the Sino-atrial
node.
 http://www.biology-online.org/articles/introduction-
homeostasis.html
 http://www.biologymad.com/resources/A2%20Home
ostasis.pdf
 Biology book