Homeostasis 09

Homeostasis

Chapter 8

• Some people have oily skin while some have dry skin and some have combination skin. Why is this so?

Learning Objectives

• define homeostasis as the maintenance of a constant internal environment

• explain the basic principles of homeostasis in terms of stimulus resulting from a change in the internal environment, a corrective mechanism and a negative feedback

• Homeostasis is the maintenance of a constant internal environment.

Definition

• internal environment is made up of blood and tissue fluid.

What factors within the internal environment must be kept constant?• The concentration of glucose• The concentration of ions, e.g. sodium and potassium• The concentration of carbon dioxide• The osmotic pressure, determined by the relative

concentrations of water and solutes (osmoregulation)• Temperature (thermoregulation)• The pH (acid-base balance)• Nitrogenous waste products and other toxic substances,

which are either eliminated or at least kept to a minimum

• Homeostasis keeps the body environment under control and keeps the conditions right for cells to live and function.

• For eg. Enzymes in our body can only work at a narrow range of pH and temperature. Changes in these conditions can lead to enzyme denaturation or inactivation that can subsequently affect the workings of our body.

Importance of Homeostasis

For any homeostatic control to occur there must be:

• A stimulus which is a change in the internal environment.

• A receptor which can detect the stimulus.

• An automatic or self-regulatory corrective mechanism to finally bring about

• A negative feedback.

• Homeostasis involves an important principle called negative feedback which is the reverse effect of a stimulus.

• An increase in temperature will induce a feedback to decrease the temperature.

• A decrease in temperature will induce a feedback to increase the temperature.

Negative Feedback

Examples of homeostasis in Man:

1. Regulation of blood glucose concentration

– Body cells need glucose for tissue respiration to provide energy. The concentration in blood plasma remains relatively constant.

– Glucose level may rise after a meal. It falls during exercise or starvation.

– How is the concentration of glucose in blood plasma regulated?

2. Regulation of blood water potential.– Recall the functions of kidney to regulate water

potential.

3. Regulation of carbon dioxide level.– Changes in carbon dioxide concentration of

blood automatically affects the rate of breathing.

– Exercising increase the concentration of carbon dioxide in blood hence the also the rate of breathing.

Learning Objectives

• define homeostasis as the maintenance of a constant internal environment

• explain the basic principles of homeostasis in terms of stimulus resulting from a change in the internal environment, a corrective mechanism and a negative feedback

• identify on a diagram of the skin: hairs, sweat glands, temperature receptors, blood vessels and fatty tissue

• describe the maintenance of a constant body temperature in humans in terms of insulation and the role of: temperature receptors in the skin, sweating, shivering, blood vessels near the skin surface and the co-ordinating role of the brain

4. Regulation of blood temperature

Functions of the Mammalian Skin

• Regulates body temperature • Protects the body against damages

• Prevents water loss

• Excretory organ

• Sensory organ

• Produces vitamin D

Structure of the Mammalian Skin

Structure of the Mammalian Skin:

• Largest organ of the body

• Made up of 3 parts: epidermis, dermis and hypodermis.

• The hypodermis is a few layers of adipose cells (adipose tissue) where fat is stored.

• The fat serves as an insulation layer as well as a food storage organ.

The Epidermis

- Outer cornified layer

- Granular layer

- Innermost Malpighian layer

Parts of the Mammalian Skin• Epidermis

– Outer cornified layer• Dead & dry cells• Contains keratin

which makes the skin water resistant and impenetratable

• Protects body from mechanical injury

• Prevents water loss through evaporation

– Granular layer• Living cells synthesizing

keratin• Flatter and ceases

division• Give rise towards

cornified layer– Innermost Malpighian layer

• Pigmented living cells• Undergoes cell division• Pigment protects against

harmful UV rays

Parts of the Mammalian Skin

• Dermis– Blood capillaries

• Arterioles that supply blood to the capillaries are controlled by vasomotor nerves

• Vasodilation: arterioles dilate; more blood sent to skin; heat lost

• Vasoconstriction: arterioles constrict; less blood sent to skin; heat retained

• Constriction and dilation helps to increase or decrease amount of blood brought to the surface hence amount of heat lost to environment

Parts of the Mammalian Skin

• Dermis– Hair

• Formed in hair follicle (tube-like structure)• Hair erector muscle attached to hair follicle• When contract, it cause the hair to stand forming

“goosebumps”• Base of follicle is the hair papilla where hair

develops; cells in papilla are nourished by surrounding blood capillaries

– Sweat gland• Coiled tubular gland producing sweat • Surrounded by blood capillaries• Sweat evaporates from skin for thermoregulation

Parts of the Mammalian Skin• Dermis

– Sebaceous gland• At least 2 per follicle• Secrete sebum into hair follicle• Sebum

– lubricates hair – maintain softness of skin– prevents dehydration– Antiseptic

– Sense receptors• Nerve endings (found in epidermis too)• Detect changes in external environment e.g. temp,

touch

Parts of the Mammalian Skin• Fatty tissues (adipose tissues)

– Insulating layer

Thermoregulation

HypothalamusHeat

Loss Centre

Heat Gain

Centre

Body Temp

Blood Temp

Temperature Receptors in

Skin

Ext. Temp Ext. Temp Body Temp

Blood Temp

Vasodilation

Sweat Pdtn

Rapid Breathin

g

Negative Feedback

Normal

Vaso-constriction

Sweat Pdtn

Shivering

Heat is lost from the body to prevent overheating by:

– Through the skin by convection, radiation and a little conduction.

– Evaporation of sweat from the surface of the skin.

– In the faeces and urine.– Expired air from lungs.

Advantages of constant body temperature:

– Can remain active throughout the day and year unchanged with temperature variation.

– Enzymes work best at a constant optimum temperature.

– Need not hibernate hence able to feed throughout the seasons.

– Exploit and colonize areas with different climate conditions.

• Be able to explain what happens when a mammal (Man) experiences an increase and decrease in environmental temperature.

• Be able to give special adaptations of what other kinds of mammals do to maintain a constant body temperature: (page 203)– horse, bear, rabbit– dog– whale– elephant– mice– polar bears