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Thermoregulation

Chapter 8Homeostasis of body

temperature and body fluids

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Humans

In humans, body temperature is relatively constant

The usual body temperature is 36.8C In the body, heat gained = heat lost This process is called thermoregulation,

a term used to describe the processes which maintain the balance between heat production and heat loss

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© 2010 McGraw-Hill Australia

Why Thermoregulate?

Chemical reactions occurring in cells are very heat-sensitive

Enzymes that control cellular activity are heat-sensitive

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Rising Body Temperature

Rising body temperature prompts heat loss in the body through:blood flow to the skinmetabolic ratebehavioural responsessweating

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Blood Flow to Skin Vasodilation:

in response to rising temperature, the body increases blood flow to the skin

is controlled by the autonomic nervous system

results in an increase in blood flow to the skin, allowing heat loss via radiation, conduction, convection and evaporation

prompts cooling of the blood that is flowing through the skin

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Metabolic Rate

The metabolic rate changes through a reduction in the secretion of thyroxine

This results in a decrease in metabolic rate

The decrease in metabolic rate causes less heat to be produced in the body

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Behavioural Responses

We change our behavioural responses by:

staying still (decreasing activity) staying in the shade air conditioning wearing less clothing

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Sweat Sweat is controlled by the sympathetic

nervous system It is secreted from sweat glands to skin Sweat contains sodium chloride, urea,

lactic acid and potassium ions Sweat cools by evaporating from the body Sweat doesn’t work in a humid

environment Environmental temperature > body temp

for sweating to be effective

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Rising Body Temperature

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Falling Body Temperature

Falling body temperature prompts:

changes in blood flow to the skin

changes in metabolic rate shivering behavioural responses

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Blood Flow to Skin In response to falling temperatures, the

body restricts blood flow to the skin. This process is called vasoconstriction

Vasoconstriction: is controlled by the sympathetic nervous

system decreases blood flow to the skin from

internal organs, which• decreases the transfer of heat from the internal

body organs to the skin• allows less heat to be lost from the body surface • cools the skin

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Falling Body Temperature – Skin

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The Hypothalamus and the Adrenal Medulla

The hypothalamus stimulates the adrenal medulla via sympathetic nerves

The medulla secretes adrenaline and noradrenaline into the blood

This increases cellular metabolism, increasing heat production

This process helps maintain internal body temperature

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The Hypothalamus and the Anterior Pituitary

The hypothalamus also stimulates the anterior lobe of the pituitary

The anterior pituitary secretes thyroid stimulating hormone (TSH)

TSH acts on the thyroid gland The thyroid gland releases thyroxine

into the blood This increases metabolic rate,

increasing body temperature

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Shivering

Shivering is actually rhythmic muscle tremors occurring at a rate of around ten to twenty per second

The hypothalamus stimulates parts of the brain that increase skeletal muscle tone

This increases body heat production

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Behavioural Responses

We change our behavioural responses by:

huddling curling into a ball putting on more clothes using a heating device

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© 2010 McGraw-Hill Australia

Complete this negative feedback loop for rising body temperature.

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Complete this negative feedback loop for falling body temperature.