2. 12.1 The Need for Homeostasis
12.2 Structure of the Human Skin
12.3 Temperature Regulation
Chapter
12
Homeostasis
3. Learning Outcomes
After this section, you should be able to:
• define homeostasis and explain its importance in living
organisms;
• explain the concept of negative feedback;
• describe the maintenance of blood glucose levels; and
• describe the process of osmoregulation.
12.1 The Need for Homeostasis
4. Maintaining a constant internal environment
Homeostasis is the maintenance of internal conditions
of an organism at all times.
12.1 The Need for Homeostasis
Large external
fluctuations
External environment
Homeostatic
mechanism
Internal environment
Small internal
fluctuations
body cells
5. Importance of homeostasis
12.1 The Need for Homeostasis
A stable internal environment allows an organism to
be independent of changes in the external
environment.
6. Internal conditions that should be kept constant include:
• Temperature
• pH
• Water potential
• Concentration of metabolic wastes
• Blood glucose concentration
• Carbon dioxide concentration
12.1 The Need for Homeostasis
7. Temperature and pH level
• Enzymes require an optimum temperature and
pH to function properly.
• Below the optimum temperature, enzymes are
inactivated. Beyond the optimum temperature,
enzymes are denatured.
• Drastic changes in pH level in the blood affect the
activity of enzymes and the rate of cellular
reactions.
12.1 The Need for Homeostasis
8. Water potential
• The composition of tissue fluid has to be
maintained to ensure that the water potential of cells
is kept constant.
• Changes in the water potential affect the body cells.
• Body cells will either shrink or burst in solutions with
water potentials different from that in the cytoplasm.
12.1 The Need for Homeostasis
9. Blood glucose level
• Glucose in food is required for tissue respiration
which releases energy for cells to carry out their
activities.
• Glucose levels increase after a meal and decrease
following physical exertion.
• If blood glucose concentration changes drastically,
it can be dangerous.
12.1 The Need for Homeostasis
10. Principles of homeostasis
• A stimulus is a change from normal conditions in the
internal environment.
• Receptors detect stimulus.
• A corrective mechanism brings about the reverse
effect of the stimulus.
• Negative feedback ‘tells’ the receptors that the
condition has been restored back to normal state.
12.1 The Need for Homeostasis
11. What happens when a condition rises
above normal?
12.1 The Need for Homeostasis
Normal condition or set point
Stimulus
(increases above norm)
Receptor
(detects the stimulus)
Self-regulatory
corrective mechanism
Condition
decreases
Negative feedback
12. 12.1 The Need for Homeostasis
Stimulus
(decreases below norm)
Receptor
(detects the stimulus)
Self-regulatory
corrective mechanism
Negative feedback
Normal condition or set point
Condition
increases
What happens when a condition decreases
below normal?
13. brain
12.1 The Need for Homeostasis
Receptor
Hypothalamus in brain
detects stimulus
Stimulus
Water potential of blood
increases above the
norm
Water potential of
blood returns to
normal
Less water is
reabsorbed by
kidney tubules
Corrective Mechanism
Pituitary gland releases less
anti-diuretic hormone (ADH)
More water is
excreted and urine
produced is more
dilute
Water potential of
blood decreases
Negative feedback
Regulating body water potential
14. brain
12.1 The Need for Homeostasis
Receptor
Hypothalamus in brain
detects stimulus
Stimulus
Water potential of blood
decreases below the
norm
Water potential of
blood returns to
normal
More water is
reabsorbed by
kidney tubules
Corrective Mechanism
Pituitary gland releases
more anti-diuretic hormone
(ADH)
Less water is
excreted and urine
produced is more
concentrated
Water potential of
blood increases
Negative feedback
Regulating body water potential
15. excess glucose glycogen
liver
pancreas
Regulating blood glucose concentration
12.1 The Need for Homeostasis
Receptor
Islets of Langerhans in
pancreas stimulated
Stimulus
Concentration of blood
glucose increases
above the norm
Concentration of
blood glucose
returns to normal
Permeability of cell
surface membrane
to glucose
increases
Corrective Mechanism
Islets of Langerhans
secretes more insulin,
which is transported by
blood to liver and muscles
Liver and muscles
convert excess
glucose to glycogen.
Glycogen is stored in
liver and muscles.
Concentration of
blood glucose
decreases and insulin
production falls.
Negative feedback
16. glycogen glucose
liver
pancreas
12.1 The Need for Homeostasis
Receptor
Islets of Langerhans in
pancreas stimulated
Stimulus
Concentration of blood
glucose decreases
below the norm
Concentration of
blood glucose
returns to normal
Corrective Mechanism
Islets of Langerhans secretes
more glucagon, which is
transported by blood to liver
and muscles.
Glucose is
released into the
bloodstream.
Concentration of
blood glucose
decreases and insulin
production falls.
Negative feedback
Glycogen stored in
liver is converted
to glucose.
Regulating blood glucose concentration
17. 12.1 The Need for Homeostasis
12.2 Structure of the Human Skin
12.3 Temperature Regulation
Chapter
12
Homeostasis
18. Learning Outcome
After this section, you should be able to:
• identify the different structures in the human skin
and understand their functions.
12.2 Structure of the Human Skin
20. • The dermis has
numerous blood
capillaries
supplying blood to
skin.
• When blood
vessels dilate,
more blood flows
to the skin
surface
(vasodilation).
12.2 Structure of the Human Skin
arterioles
capillaries
21. • When blood vessels
constrict, less blood
flows to the skin
surface
(vasoconstriction).
• Vasodilation and
vasoconstriction play
a role in the
regulation of body
temperature.
12.2 Structure of the Human Skin
22. • Hairs are embedded
within the dermis.
• Malpighian layer sinks
into dermis to form a
hair follicle.
12.2 Structure of the Human Skin
hair
hair follicle
23. • Hair papilla is found
at the base of the hair
follicle and consists of
blood capillaries and
nerves.
• Hair erector
muscles contract and
cause hairs to stand,
resulting in
appearance of goose
pimples.
12.2 Structure of the Human Skin
hair erector muscle
hair papilla
24. • Sweat gland is a
coiled tube formed by
the downgrowth of the
epidermis.
• It is richly surrounded
by blood capillaries.
• It secretes sweat
which flows through
the sweat duct and
sweat pore to the skin
surface.
12.2 Structure of the Human Skin
sweat gland
sweat pore
sweat duct
blood capillaries
25. 12.1 The Need for Homeostasis
12.2 Structure of Mammalian Skin
12.3 Temperature Regulation
Chapter
12
Homeostasis
26. 12.3 Temperature Regulation
Learning Outcomes
After this section, you should be able to:
• describe the role of the skin in the maintenance of
a constant body temperature in humans.
27. Heat gain by the body
12.3
• Heat produced is a result of metabolic activities in
the body.
• From the Sun and warm air on hot days.
• Intake of hot food and drinks.
• During physical exertion or exercise.
Temperature Regulation
28. Heat loss by the body
12.3
• From the skin surface via convection, radiation and
conduction.
• Evaporation of sweat.
• Expired air from lungs.
• Faeces and urine.
Temperature Regulation
29. How the body temperature is regulated
12.3
Normal body
temperature (37°C)
Body temperature rises
(heat gain greater than
heat loss)
Body temperature falls
(heat loss greater than
heat gain)
• Reduce heat production
• Increase heat loss to
surroundings
• Increase heat production
• Decrease heat loss to
surroundings
Temperature Regulation
30. 12.3
brain
Receptor
Temperature receptors in
skin and hypothalamus
of brain stimulated
Stimulus
Blood and skin
temperatures increase
Blood and skin
temperatures return
to normal
Corrective Mechanism
• Vasodilation of arterioles
near skin’s surface
• Increased production of
sweat
• Increased rate of breathing
• Metabolic rate decreases
Increased heat
loss and reduced
heat production
Body
temperature
decreases
Negative feedback
What happens on a hot day?
skin
Temperature Regulation
31. 12.3
arteriole
capillaries
skin surface
Arterioles
in skin
dilate
1
More blood flows
to capillaries in
skin
2
3
Greater heat loss
4
Vasodilation
1. Dilation of arterioles
2. More blood flows to
the capillaries
3. When shunt vessels
constrict, more blood
flows to capillaries.
4. More heat is lost
from skin by
radiation, convection
and conduction.
Shunt vessel
constricts
shunt
vessel
Temperature Regulation
32. Increased production of sweat
12.3
• Sweat glands become more active.
• More sweat is produced.
• As sweat evaporates from the surface of the skin,
heat is lost from the body via latent heat of
vaporisation.
more sweat
produced
more active
sweat gland
Temperature Regulation
33. Relaxation of hair erector muscles
12.3
• Hair erector muscles relax when body temperature
increases.
• Hairs lie flat, allowing air to circulate over skin. This
removes heat.
hairs lie flat
Temperature Regulation
34. Reduced metabolic rate
12.3
• Rate of metabolic activities slow down.
• Less heat is produced within body.
Temperature Regulation
35. 12.3
brain
Receptor
Temperature receptors in
skin and hypothalamus
of brain stimulated
Stimulus
Blood and skin
temperatures decrease
Blood and skin
temperatures return
to normal
Corrective Mechanism
• Vasoconstriction of arterioles
near skin’s surface
• Decreased production of
sweat
• Metabolic rate increases
• Shivering
Decreased heat
loss and
increased heat
production
Body
temperature
increases
Negative feedback
What happens on a cold day?
skin
Temperature Regulation
36. 12.3
Arterioles
in skin
constrict
1
Less blood flows
to capillaries in
skin
2
3
Less heat loss
4
Vasoconstriction
• Arterioles in skin
constrict.
• Less blood flows to
capillaries in the skin
• When shunt vessels
dilate, less blood flows
to capillaries.
• Less heat is lost from
skin by radiation,
convection and
conduction.
Shunt vessel
dilates
Temperature Regulation
arteriole
capillaries
skin surface
shunt
vessel
37. Decreased production of sweat
12.3
• Sweat glands become less active.
• Less sweat is produced.
• Less latent heat is removed when sweat
evaporates.
less sweat
produced
less active
sweat gland
Temperature Regulation
38. Contraction of hair erector muscles
12.3
• Hair erector muscles contract when body temperature
decreases.
• Hairs “stand up”, trapping an insulating layer of warm
air over skin.
hairs “stand
up”
Temperature Regulation
39. Increased metabolic rate
12.3
• Rate of metabolic activities increase.
• More heat is produced within body.
Shivering
• Occurs when more heat production is needed to
prevent drop in temperature.
• Reflex contraction of the body muscles generates
heat which increases body temperature to normal.
Temperature Regulation
Notes de l'éditeur
Notes:
The external environment is the environment in which an organism lives.
In a mammal, the internal environment consists of the blood and tissue fluid.
Note:
Cells are made of biological molecules such as proteins, which make them very sensitive to slight fluctuations in temperature and pH.
Note:
Point out that an animal cell will swell or even burst when it is placed in a solution of high water potential. When it is placed in a solution of low water potential, it will shrink in size.
Notes:
Water potential of blood can increase due to an increased intake of water.
The pituitary gland protrudes from the hypothalamus which is found at the base of the brain.
Notes:
Water potential of blood can decrease due to loss of water through profuse sweating.
Note:
Blood glucose concentration increases some time after a meal.
The increased permeability of the cell surface membranes of the liver and muscle cells is caused by insulin.
This enables glucose to be taken up more quickly by the liver and muscles.
Note:
Blood glucose concentration decreases during vigorous muscular contraction or during starvation.
Note:
Click on the Video-URL button to be directed to a website where an animation on the structure of the human skin can be found.
This animation introduces the main characteristics of the different layers of the skin. (The animation lasts for about 55 seconds.)
Note:
Vasodilation causes a person to blush or flush after vigorous exercise.
Note:
Vasoconstriction causes a person to become pale.
Note:
A hair follicle is a hollow tube
Note:
Ask students to think about why they experience pain when a hair is pulled out but no pain is experienced when their hair is cut by a hair stylist. Follow up by explaining that when the cells covering the hair papilla are pushed upwards, they die and harden, forming hair. Thus, the hair shaft consists of dead cells. This is why no pain is experienced when hair is being cut. However, when hair is pulled out, it tugs onto the hair papilla where nerves are found, thus causing pain.
Notes:
Point out that the skin is considered an excretory organ because along with sweat, a small amount of urea is removed through sweating.
When sweat evaporates from the body surface, it removes heat from the body and thus produces a cooling effect on the body.
Note:
Heat loss at the skin surface occurs mainly by convection and radiation.
Notes:
When the temperature receptors in the skin sense a change in the temperature, nerve impulses are sent to the brain. This stimulates the hypothalamus to send nerve impulses to relevant parts of the body.
Note:
Latent heat is the energy that is absorbed or released when a substance undergoes a change of state.
Note:
The relaxation of the hair erector muscles does not have much effect in increasing heat loss in humans.
Note:
We can counter the heat by putting on clothing made of materials that absorb sweat better, moving to a cool place or drinking cold water.
Note:
The contraction of the hair erector muscles does not have much effect in increasing heat production in humans.
Note:
We can counter the cold by putting on extra clothing, moving to a warmer place or drinking warm water.
