BiologyCambridge IGCSE

Homeostasis and Endocrine

Endocrine homeostasis refers to the balance and stability of the body’s internal environment, maintained by the endocrine system through the secretion of hormones. The endocrine system consists of glands that release hormones into the bloodstream, which then travel to various tissues and organs to regulate....
Homeostasis and Endocrine

Homeostasis is the maintenance of a constant internal environment. It involves control by negative feedback (a change that sets of a response that cancels out the change).


Several substances need to be maintained at a constant level:

Substance Too High? Too Low?
  • Carried out by the
Body temperature ↑…
one sweats and when the
sweat evaporates…
the body cools down.
If the body temperature ↓…
one shivers (uses glucose to
create heat in muscles) …
the body warms up.
  • Carried out by the
    pituitary gland
Water level ↑…
less ADH secreted so more
water is flushed out…
water level decreases.
Water level ↓…
More ADH secreted so more
water is reabsorbed…
water level increases.
O2 and CO2
  • Carried out by
    medulla oblongata
If there is any change to the optimum level of these gases,
the medulla oblongata detects it and sends signals to the
heart and lungs to work faster (too hight) /slower (too low).
  • Carried out by
Glucose ↑…
Insulin produced by
pancreas and will stimulate
the liver to convert excess
glucose to glycogen and it
will also stimulate the body
to use up the glucose…
glucose level returns.
Glucose ↓…
Glucagon produced by
pancreas and will stimulate
liver to convert glycogen to
glucose level returns.

Diabetes Mellitus (Hyperglycaemia)

  • Due to a mutation (random change in DNA) for which the gene that produces insulin is affected and insulin cannot be produced, the glucose level will always remain high. This is known as diabetes mellitus (type 1).
  • To deal with this, one must get regular insulin injections.
  • Some of the symptoms include:
    • Person is thin and weak and will lack energy
    • Excessive thirst, hunger and sugary urine production
    • Sweet smelling breath
    • Cataract formation

The Skin

The Skin

Mechanisms to Conserve Heat:

  • The skin reduces heat loss by reducing:
    • Convection – erector muscle contracts to pull hair up and trap air for insulation
    • Evaporation – sweat gland doesn’t secrete sweat
    • Radiation vasoconstriction of arterioles (as they have muscles) leading to surface capillaries, shunts blood away from skin
  • To generate heat – muscles generate more heat by shivering
  • Controlling external environment:
    • Addition of internal heating
    • Wearing a jacket

Mechanisms to Increase Heat Loss:

  • The skin increases heat loss by increasing:
    • Convection – erector muscles relax so hair lies flat and no air is trapped
    • Evaporation – sweat is produced by sweat glands and when it evaporates a cooling effect is created
    • Radiation vasodilation of arterioles leading to surface capillaries
  • Controlling the external environment:
    • AC/Fan can be switched on
    • Wearing thin clothes

Endocrine System

An endocrine gland (one without any ducts) secretes a substance into the bloodstream. This substance is known as a hormone which carries messages for target organs. The message instructs the organs to speed up or slow down a process. Hormones are always carried by a network of blood vessels.

A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Hormone Action

Hormone Action

Negative Feedback Control

Negative Feedback Control


  • Also known as the emergency or ‘fight or flight’ hormone, adrenaline is produced when one experiences an extreme emotion (such as nervousness, fear or anger)
  • It is secreted by adrenal glands
  • The purpose of adrenaline is to help get more energy (through respiration)
What Happens Due to Adrenaline?
  • Faster breathing and heart rate
  • Liver converts glycogen to glucose
  • Blood supply to some organs, which don’t require blood at that moment, is cut down (for example the digestive system [feeling of butterflies in the stomach] and the skin [the skin will pale]) so that more oxygen and glucose can go to muscles
  • Shivering muscles because of the high amount of energy produced
  • Dilated pupil to allow as much light as possible to enter the eye so that the brain can receive as much information as possible
  • Urinary bladder contracts

Differences Between Nerve Impulses and Hormone Action

Signal SpeedVery FastSlow
Reaction LengthShortVariable Lengths

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