Internal and External Environments

Animals live in environments that constantly change.

The external environment includes conditions outside the body.

The internal environment includes conditions inside the body that surround cells and tissues.

Animals must keep many internal conditions within a narrow range, even when the external environment changes.

This process is called homeostasis.

The External Environment

The external environment is everything outside an animal's body.

It includes factors such as:

• air temperature
• humidity
• water availability
• food supply
• light
• predators
• pathogens
• oxygen availability
• salinity
• surrounding chemicals

These conditions can change quickly.

For example, an animal may move:

• from sunlight into shade
• from a cool area into a hot area
• from fresh water into saltier water
• from rest into intense exercise

The body must respond to these changes.

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The Internal Environment

The internal environment refers to the conditions inside the body, especially the fluid surrounding cells.

Important internal conditions include:

• body temperature
• blood glucose concentration
• water content
• salt concentration
• oxygen concentration
• carbon dioxide concentration
• pH

Cells function best when these conditions remain relatively stable.

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Why Internal Stability Matters

Cells depend on chemical reactions to stay alive.

These reactions are controlled by enzymes.

Enzymes work best under particular conditions.

If internal conditions change too much:

• enzymes may work more slowly
• proteins may lose their shape
• cells may lose or gain too much water
• chemical reactions may be disrupted
• tissues and organs may stop functioning properly

Therefore, maintaining a stable internal environment is essential for survival.

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Homeostasis

Homeostasis is the regulation of internal conditions so that they remain within suitable limits.

Homeostasis does not mean conditions stay perfectly constant.

Instead, they fluctuate slightly around an ideal range.

For example, human body temperature is usually kept close to:

37°C

It may rise or fall slightly, but the body responds to prevent large changes.

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Factors That Affect Internal Conditions

Many factors can disturb the body's internal environment.

These include:

• changes in external temperature
• exercise
• eating
• drinking
• sweating
• illness
• dehydration
• stress
• changes in oxygen availability

The body must detect these changes and respond.

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Example: Exercise

During exercise:

• muscles use more oxygen
• muscles produce more carbon dioxide
• body temperature rises
• glucose is used more quickly
• water may be lost through sweating

The body's organ systems respond to maintain internal balance.

For example:

• heart rate increases
• breathing rate increases
• sweating increases
• blood flow to the skin may increase

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Organ Systems and Internal Balance

Homeostasis depends on several organ systems working together.

Important systems include:

• nervous system
• endocrine system
• circulatory system
• respiratory system
• urinary system
• digestive system
• integumentary system

No single organ system maintains the internal environment by itself.

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The Nervous System

The nervous system detects changes and produces rapid responses.

It includes:

• brain
• spinal cord
• nerves
• sensory receptors

Receptors detect changes such as:

• temperature
• light
• pressure
• chemical concentrations

The nervous system sends electrical signals to effectors such as muscles and glands.

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The Endocrine System

The endocrine system uses hormones to regulate internal conditions.

Hormones are chemical messengers carried in the blood.

They help regulate:

• blood glucose
• growth
• metabolism
• water balance
• reproduction

Hormonal responses are usually slower than nervous responses but may last longer.

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The Circulatory System

The circulatory system transports substances around the body.

It carries:

• oxygen
• glucose
• hormones
• nutrients
• heat
• carbon dioxide
• metabolic wastes

This makes it essential for maintaining stable conditions around cells.

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The Respiratory System

The respiratory system helps regulate gases in the blood.

It brings oxygen into the body and removes carbon dioxide.

During exercise, breathing rate increases.

This helps:

• deliver more oxygen
• remove additional carbon dioxide
• maintain blood pH

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The Urinary System

The kidneys help regulate:

• water content
• salt concentration
• pH
• waste products

The kidneys filter the blood and adjust how much water and ions are lost in urine.

This is especially important when an animal:

• drinks large amounts of water
• becomes dehydrated
• consumes salty food

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The Skin and Temperature Control

The skin plays an important role in thermoregulation.

When the body becomes too hot:

• sweat production increases
• blood vessels near the skin surface may widen
• more heat is lost

When the body becomes too cold:

• skin blood vessels may narrow
• shivering may begin
• heat loss decreases

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Maintaining Body Temperature

Temperature affects the rate of chemical reactions.

If body temperature becomes too low:

• reactions may slow
• muscles may function poorly

If body temperature becomes too high:

• enzymes may lose their shape
• cells may become damaged

Animals therefore use physiological and behavioural responses to control temperature.

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Responses to Heat

When an animal becomes too hot, possible responses include:

• sweating
• panting
• moving into shade
• reducing activity
• increasing blood flow near the skin
• spreading out the body to lose heat

These responses help increase heat loss.

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Responses to Cold

When an animal becomes too cold, possible responses include:

• shivering
• reducing blood flow to the skin
• curling up
• seeking shelter
• increasing activity
• fluffing fur or feathers

These responses reduce heat loss or increase heat production.

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Behavioural Responses

Not all responses are controlled only by internal body systems.

Animals can change their behaviour.

Examples include:

• lizards moving into sunlight to warm up
• dogs seeking shade
• birds fluffing feathers
• mammals huddling together
• animals drinking when thirsty
• nocturnal animals becoming active at cooler times

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Physiological Responses

A physiological response is an internal body response.

Examples include:

• sweating
• shivering
• changing heart rate
• changing breathing rate
• changing hormone levels
• changing urine concentration

Physiological responses are controlled by the body's regulatory systems.

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Internal vs External Change

Suppose the external temperature rises.

This is an external change.

As a result, body temperature may begin to rise.

This is an internal change.

The body detects the internal change and responds.

For example:

external temperature rises → body temperature rises → sweating increases → body cools

The response helps restore internal stability.

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Negative Feedback

Most homeostatic systems operate using negative feedback.

Negative feedback means that a change causes a response that opposes the original change.

For example:

Body temperature rises.

The body responds by increasing heat loss.

Body temperature falls back toward normal.

Or:

Body temperature falls.

The body responds by producing and conserving heat.

Body temperature rises back toward normal.

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The Basic Feedback Pattern

A homeostatic response often follows this pattern:

stimulus → receptor → control centre → effector → response

For example:

body temperature rises → temperature receptors detect change → brain processes information → sweat glands activate → temperature falls

This returns conditions toward the normal range.

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Blood Glucose as an Internal Condition

Blood glucose must also be regulated.

After eating:

• blood glucose may rise
• the pancreas detects the change
• insulin is released
• cells take up more glucose
• the liver stores glucose as glycogen

Blood glucose then falls toward normal.

When blood glucose becomes too low:

• other hormonal responses help raise it again

This is another example of homeostasis.

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Water Balance

Animals must also regulate water levels.

If too much water is lost:

• blood becomes more concentrated
• cells may lose water
• dehydration can occur

The body may respond by:

• producing less urine
• producing more concentrated urine
• increasing thirst

If too much water is consumed:

• the kidneys can remove more water in urine

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Salt Balance

Cells also need stable concentrations of dissolved ions.

Too much or too little salt can affect osmosis.

This can cause cells to:

• gain too much water
• lose too much water

The kidneys help regulate salt concentration by changing how many ions are excreted.

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Changes in Oxygen Availability

Animals living at high altitude experience lower oxygen availability.

The body may respond by:

• increasing breathing rate
• increasing heart rate
• eventually producing more red blood cells

These responses help improve oxygen delivery to tissues.

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Short-Term and Long-Term Responses

Some responses happen quickly.

Examples:

• sweating
• shivering
• increased heart rate
• increased breathing rate

Other responses develop over longer periods.

Examples:

• acclimatization to high altitude
• seasonal changes in fur
• changes in body fat
• migration patterns

Both types of response help animals survive environmental change.

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Comparing Internal and External Environments

The external environment can change greatly, while the body works to keep internal conditions more stable.

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Worked Example: Hot Weather

A mammal is exposed to very hot weather.

External change:

air temperature increases

Possible internal change:

body temperature begins to rise

Responses:

• sweating increases
• blood vessels near the skin widen
• activity may decrease
• the animal may seek shade

Result:

Heat loss increases and body temperature moves back toward normal.

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Worked Example: Dehydration

An animal spends several hours without water.

External condition:

Low water availability.

Internal effect:

Water content decreases.

Responses may include:

• increased thirst
• reduced urine volume
• more concentrated urine
• reduced activity

These responses help conserve water.

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Worked Example: Exercise

During intense exercise, muscle cells use more oxygen.

They also release more carbon dioxide and heat.

The body responds by:

• increasing breathing rate
• increasing heart rate
• increasing blood flow
• increasing sweating

Several organ systems work together to maintain stable internal conditions.

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Structure and Survival

The ability to regulate internal conditions allows animals to survive in changing environments.

For example, mammals can often remain active across a range of external temperatures because they regulate their body temperature internally.

Without homeostasis, cells would be directly exposed to every external change.

Stable internal conditions therefore provide a more suitable environment for cells.

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Common Misconceptions

The internal environment means everything inside the skin.

More precisely, the term usually refers to the conditions surrounding and affecting cells within the body.

Homeostasis keeps conditions perfectly constant.

Incorrect. Conditions fluctuate within a narrow acceptable range.

Only the nervous system controls homeostasis.

Incorrect. The nervous, endocrine, circulatory, respiratory, urinary, and other systems all contribute.

Animals respond only after dangerous conditions occur.

Many regulatory responses begin as soon as changes are detected.

Behaviour is separate from homeostasis.

Behavioural responses such as seeking shade or drinking water can play an important role in maintaining internal balance.

The external environment does not affect the internal environment.

External changes often create challenges that the body must compensate for.

Did You Know?

A desert animal may experience very large changes in environmental temperature between day and night while keeping its internal conditions within a much narrower range.

This ability to separate the conditions inside the body from those outside is one of the major advantages of complex regulatory systems.

Key Terms

External environment – Conditions outside an organism's body.

Internal environment – Conditions inside the body that affect cells and tissues.

Homeostasis – Regulation of internal conditions within suitable limits.

Negative feedback – A regulatory process in which a response opposes the original change.

Stimulus – A detectable change in conditions.

Receptor – A structure that detects a stimulus.

Control centre – A part of a regulatory system that processes information and coordinates a response.

Effector – A muscle or gland that produces a response.

Physiological response – An internal body response to a change.

Behavioural response – An action taken by an organism in response to environmental conditions.

Thermoregulation – Regulation of body temperature.

Water balance – Regulation of water content within the body.

Key Takeaways

• The external environment includes conditions outside an animal's body.
• The internal environment includes conditions that surround and affect cells.
• Animals must keep many internal conditions within suitable ranges.
• This regulation is called homeostasis.
• Important internal conditions include temperature, water content, glucose, gases, salts, and pH.
• External changes can disturb internal conditions.
• Organ systems work together to maintain internal balance.
• The nervous and endocrine systems coordinate many responses.
• The circulatory system transports substances and heat.
• The respiratory system helps regulate oxygen and carbon dioxide.
• The kidneys regulate water, salts, and wastes.
• Animals can respond both physiologically and behaviourally.
• Most homeostatic systems use negative feedback.
• Stable internal conditions allow enzymes, cells, tissues, and organs to function properly.
• Maintaining a stable internal environment is essential for survival.