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HOMEOSTASIS

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Lesson 1

Learning Objectives1. Define homeostasis as the maintenance of a constant

internal environment.2. Explain the concept of control by negative feedback.

Outline1. What is homeostasis?2. What are stimulus, receptor, corrective mechanism

and negative feedback?

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Homeostasis

Homeostasis is the maintenance of a

constant internal environment.

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Internal Environment

The cells in our body are bathed in fluids (blood, lymph and tissue fluid). These fluids make up the

internal environment.

This environment must be kept constant for healthy functioning of body cells.

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Homeostasis involves:• A change in the internal environment

(stimulus)• A receptor that can detect the stimulus.• A corrective mechanism – a system that return

the conditions to normal, and• Concept of control by negative feedback. (A

correction/change in the opposite direction, hence the term negative).

Homeostasis

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Homeostasis involves constant adjustments by various organs working together to achieve this

internal stability.

The control center of homeostasis is in the brain. The liver, kidneys, skin, lungs and hormones also

play important roles in homeostasis.

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Examples of Homeostasis in Man

Blood glucose level

Blood water potential

Carbon dioxide level

Body temperature

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Lungs Skin Liver Kidney

Variable factor CO2 and O2 Temperature Glucose Water, urea,

mineral salts

Mode of control

Regulated by respiratory centre of brain

Controlled by hypothalamus of brain

Regulated by insulin/ glucagon produced by pancreas

Regulated by ADH. Urea and excess mineral salts lost in urine.

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Regulation of Blood Glucose Level

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Discussion

Recall the topic 'excretion', and discuss the regulation of water potential in blood.

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Regulation of Blood Water Potential

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Lesson 2

Learning Objective1. Identify, on diagram of the skin, hairs, sweat

glands, temperature receptors, blood vessels and fatty tissue.

Outline1. Skin structure and function

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• The largest organ in the human body.• An important sense organ.• Barrier between the body and the external

environment• Organ that regulates our body temperature.

The Skin

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Structure of The Skin

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The Skin

Layers

Epidermis Dermis

Cornified layerGranular layer

Malpighian layer

Hairs

Sweat glandErector muscleBlood capillary

Sense receptorsSebaceous

gland

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Cornified Layer

• Outer layer.• Made up of dead keratinised cells.

Functions:• Makes the skin waterproof, hence reducing water

loss.• Protects against mechanical, thermal and

chemical injury.• Protects against invasion by germs.

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Granular Layer

• Layer of cells that begin to die due to the increasing distance from the blood supply of the dermis

Function:• Replaces dead cells in cornified

layer.

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Malpighian Layer

• Made up of living cells and pigmented with melanin.

Functions:• Undergoes cell division to produces new cells

to replace cells in the granular layer.• Melanin prevents entry of harmful ultraviolet

rays.

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Hairs

• Made up of dead cells and keratin.• Projects from the hair follicle.

Functions:• Traps air in order to form an insulating layer

when cold.• May act as a camouflage.

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Hair Erector Muscle

• Attached to the base of each hair follicle.

Functions:• Contracts to make hair stand upright.• Relaxes to make hair lies flat on the skin.

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Sebaceous Gland

• Opens into the hair follicle.

Functions:• Secretes sebum (oily substance into hair

follicle).• Lubricates hair and keeps skin smooth and soft.• Prevents dehydration and prevents bacteria

growth (antiseptic)

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Sweat Gland

• Surrounded by blood capillaries.• Connected to sweat pores by sweat ducts.

Functions:• Excretes sweat.• When sweat evaporates, it cools the body.

Sweat = Mainly water + salt + urea

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Sense Receptors

• Consist of nerve endings found in the epidermis and dermis.

Functions:• Detects touch/pain/pressure/temperature

changes in external environment.

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Blood Vessels

• Network of capillaries.

Functions:• Arterioles dilate (vasodilation) or constrict

(vasoconstriction).• Vasodilation increases blood flow to skin surface to

maximise heat loss to the surrounding by conduction, convection and radiation.

• Vasoconstriction decreases blood flow to skin surface to minimise heat loss to the surrounding by conduction, convection and radiation

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Adipose Tissue

• Layers of fat cells.

Functions:• Provides insulation against cold.• Stores energy in the form of fat.

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Discussion

Referring to the functions of the structures of the skin, what can you conclude about the general

function(s) of the skin?

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Skin Function• Regulates body temperature

– Sweat gland– Hairs and hair erector muscle– Adipose tissue– Temperature receptors

• Protects the body

– Epidermis– Sebaceous gland

• Touch-sensitive

– Touch receptors• Production of vitamin D when expose to sunlight.

– Vitamin D is necessary for the absorption of calcium to form strong bones and teeth.

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Lesson 3Learning Objective1. 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 skins surface and the coordinating role of the brain.

Outline1. Temperature regulation

Hot day Cold day

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Temperature Regulation The thermoreceptors (hot and cold sensors) in the

skin detect the change in temperature. Messages are sent to hypothalamus in the brain. Hypothalamus is the temperature control center.

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Temperature Regulation – Hot Day

StimulusBlood and skin temperatures rise

ReceptorThermoreceptors detect changes and send nerve impulses to the brain

Hypothalamus of brain is stimulated and sends nerve impulses to relevant body parts

Corrective MechanismIncrease in heat loss and decrease in heat production

Blood temperature returns to normal levels

Negativefeedback

Decreased metabolic rate reduces heat production

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Corrective Mechanisms:

1. Vasodilation: widening of arterioles in the skin allows more warm blood to flow through the capillaries near the surface of the skin resulting in more heat lost by radiation.

2. Sweat glands become active and cause sweating. Evaporation of sweat brings about heat loss and cools body.

3. Hair lies flat because the erector muscles relax. The layer of air trapped is very thin. Thus heat is easily lost from the body by conduction and convection.

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Temperature Regulation – Cold Day

Corrective MechanismDecrease in heat loss and increase in heat production

Increased metabolic rate increases heat production

Blood temperature returns to normal levels

Negativefeedback

StimulusBlood and skin temperatures fall

ReceptorTemperature receptors detect changes and send nerve impulses to the brain

Hypothalamus of brain is stimulated and sends nerve impulses to relevant body parts

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Corrective Mechanisms:

1.Vasoconstriction: Narrowing of arterioles in the skin reduces the amount of warm blood flowing near the surface of the skin so resulting in reduced heat loss by radiation.

2.Sweat glands are less active thus reducing sweating. Less evaporation resulting in reduced heat loss from body

3.Erector muscles contract causing hairs to stand on end. The layer of air trapped between the hairs become thicker. Since air is a poor conductor of heat, it insulates the body.