Homeostasis and Temperature Control

Section 1 of 6Homeostasis (HT only)

Meaning

• Meaning Homeostasis is the maintenance of a constant internal environment.
• Conditions such as temperature, blood pressure, water concentration and glucose concentration must stay within set limits.

Why it matters

• Why it matters Body cells can only carry out their reactions properly within a narrow range of conditions,
  • and the organism can only live if its cells function.

Temperature and enzymes

• Temperature and enzymes The body holds its temperature near 37 °C, where enzymes work best.
• If body temperature rises too far, enzymes denature and reactions such as respiration are catalysed less effectively.

Section 2 of 6How the control system works (HT only)

1Detect

• Detect The brain has receptors sensitive to the temperature of the blood,
  • and the skin has temperature receptors that send impulses along sensory neurones.

2Coordinate

• Coordinate The brain processes the information and compares it with the optimum of 37 °C.

3Respond

• Respond Nerve impulses travel to effectors in the skin.

4Correct

• Correct Body temperature moves back towards the set point, and the cycle continues as negative feedback.

Section 3 of 6Responses to heat and cold (HT only)

Change	Response in the skin	Effect
Body too hot	Sweat glands release sweat	Evaporation of sweat transfers heat from the skin
Body too hot	Vasodilation: arterioles near the surface widen	More blood flows near the surface, so more heat is radiated
Body too cold	Shivering by skeletal muscles	Respiration in muscles releases heat
Body too cold	Vasoconstriction: arterioles narrow	Less blood flows near the surface, so less heat is lost
Body too cold	Hair erector muscles raise hairs	A layer of trapped air insulates the skin

Section 4 of 6Parts of the skin involved (HT only)

Skin and temperature

Epidermis

• Epidermis The outer layer forms a barrier and protects the tissues beneath.

Dermis

• Dermis This layer contains sweat glands, blood vessels, hair follicles and receptors.

Fatty tissue

• Fatty tissue The layer beneath the dermis insulates the body and reduces heat loss through the skin.

Blood vessels

• Blood vessels Arterioles change width to control how much blood passes near the surface;
  • capillaries themselves cannot move.

Section 5 of 6Key terms

Term	Meaning
Homeostasis	The maintenance of a constant internal environment.
Negative feedback	A response that reverses a change and returns a condition to its set level.
Vasodilation	Widening of arterioles near the skin surface so more blood flows through them.
Vasoconstriction	Narrowing of arterioles near the skin surface so less blood flows through them.
Effector	A muscle or gland that produces a response.
Denature	Change in the shape of an enzyme that stops it working.

Section 6 of 6Exam technique

1

Saying that blood vessels move closer to or further from the skin surface — they stay in place; the arterioles widen or narrow.

2

Writing that sweating cools the body simply by being released; the cooling comes from evaporation.

3

Mixing up vasodilation (hot) with vasoconstriction (cold).

4

Saying that the body temperature is controlled by the skin alone — the brain coordinates the response.