CHAPTER 12 Homeostasis
CHAPTER 12Homeostasis
12.1 The Need for Homeostasis
12.2 Structure of the Human Skin
12.3 Temperature Regulation
Chapter 12
Homeostasis
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
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
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.
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
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
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
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
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
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
12.1 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?
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 MechanismPituitary 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
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 MechanismPituitary 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
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 MechanismIslets 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
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 MechanismIslets 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
12.1 The Need for Homeostasis
12.2 Structure of the Human Skin
12.3 Temperature Regulation
Chapter 12
Homeostasis
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
Epidermis(outermost layer)
Dermis
Subcutaneous(consisting of fatty tissue)
12.2 Structure of the Human Skin
Main parts of the skin
URL
• 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
• 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
• 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
• 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
• 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
12.1 The Need for Homeostasis
12.2 Structure of Mammalian Skin
12.3 Temperature Regulation
Chapter 12
Homeostasis
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.
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
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
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
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
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
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
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
Reduced metabolic rate
12.3
• Rate of metabolic activities slow down.
• Less heat is produced within body.
Temperature Regulation
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
12.3
Arterioles in skin constrict
1
Less blood flows to capillaries in skin
2
3
Less heat loss
4Vasoconstriction
• 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
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
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
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