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