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36 Cards. Created by Kate Bishop ().

Definitions of Homeostasis

  • The maintenance of a constant internal environment in response to the changing conditions of the external and internal environment
  • A self adjusting mechanism involving feedback where the response to a stimulus alters the internal conditions and may itself become a new stimulus

What does Homeostasis aim to achieve?

  • Aims to maintain cells, tissues and entire organisms with their biological tolerance
  • Does this by structural (physical) adaptations, behavioural adaptations, and physiological (functional) adaptations

What is a 'tolerance limit'?

The narrow range of conditions where cellular processes are able to function at a level consistent with the continuation of life

What are the internal conditions to maintain?

  • Carbon dioxide
  • Oxygen
  • Glucose
  • Wastes
  • Salts
  • Water
  • Temperature

Carbon Dioxide as an internal condition

  • Excess CO2 causes pH to reduce (Conditions become more acidic)
  • Changes in pH interfere with enzyme activity
  • CO2 dissolves freely in water

Oxygen as an internal condition

  • Levels need to be kept in range to maintain respiration rates
  • Does not dissolve in water as freely as CO2 (The amount dissolved reduces temperature increases)

Wastes as an internal condition

  • Urea (Example) a nitrogenous waste, is toxic in higher concentrations (Removed by excretion)
  • Other wastes include H2O, Ions and metabolites

Salts as an internal condition

  • Too little causes cells to become too hydrated
  • Too much causes cells to become dehydrated
  • Ions such as Na+ and K+ are essential for metabolic processes

Water as an internal condition

  • Too little may lead to some reactions occurring too quickly
  • Too much reduces the rate of reactions and thus the metabolism

Temperature as an internal condition

  • If the temperature of an organism falls below its tolerance range then the normal chemical reactions slow and stop (Hypothermia)
  • If the temperature of an organism rises above its tolerance range then it runs the risk of permanent damage to essential proteins in the body such as enzymes as proteins are denatured in extreme temperatures (Hyperthermia)

Behavioural adaptations for regulating body temperature

  1. Exposure control (Behaviours that aim to increase or decrease exposure to extremes in ambient temperature)

Structural adaptations for regulating body temperature

  1. Piloerection (Body hair stands on end to reduce heat loss by convection over the surface of the body)
  2. Adaptive increases or decreases in surface area (Organisms may have an increased or decreased SA, to allow for more efficient control and transfer of heat energy)

Physiological (Functional) adaptations for regulating body temperature

  1. Vasoconstriction and Vasodilation (The control of blood flow by reducing or increasing the diameter of blood vessels near the surface)
  2. Evaporative Cooling (Panting and sweating)
  3. Shivering (From the energy needed for muscle contraction, heat is produced which helps to increase the temperature of the body)
  4. Changes in metabolic rate (Will produce more or less heat as required to maintain body temperature within normal tolerance range)

Homeostasis in animals (water)

  • Water dissolves many chemicals (Flows readily and is a small molecule which moves readily through membranes)
  • All the chemical reactions that make up the metabolism of the organism occur in this medium (Allows substances to move and react)
  • Water is required for excretion, transport in animals and temperature control

Gain and loss of water

  • If water gain > water loss (Cells become turgid and excess water is eliminated - animal cells may burst as they have no cell wall)
  • If water gain

Homeostasis in animals (osmoregulation)

  • Aspects of metabolism (enzyme activity, membrane transport and nerve conduction) are dependent on particular concentrations of ions and metabolites
  • To achieve this balance, the salt and fluid content of the internal body fluids must be regulated (Which is osmoregulation)


Animals that regulate their salt and water changes


Cannot regulate their salt and water balance and their body fluid balance fluctuates with the environment

Fish in salt water

  • These generally have a body ion concentration which is lower than their environment (They are hypotonic)
  • There is a nett movement of water out of their body via osmosis and salts into the body via diffusion

Osmoregulation mechanism used by marine bony fish

  • Need to excrete the excess salts gained via their diet and nitrogenous wastes
  • Lose large amounts of water by osmosis across the gills Consequently
  • Urine is isotonic and produced in small amounts
  • Have few nephrons
  • Urine is produced by secretion of salts rather than filtration
  • Excess salts and ammonia are secrete by active transport across the gills