Homeostatis and negative feedback

• Metabolism


  The sum of all cellular reactions to maintain life
• When conditions are optimal, the cell's metabolism is at its most efficient
• Homeostasis


  The process of maintaining a steady optimal state
• The external environment is constantly changing
• Stimuli


  Information received from the external and internal environment
• Stimuli normally result in some sort of response from a particular tissue, organ or number of organs
• Nervous system and endocrine system

  Act together to coordinate responses to stimuli
• Stimuli from the environment

  • Heat from a fire
  • Stimulating special cells in the skin
• Response to environmental stimuli

  1. Nerve impulses transmitted to the brain

  2. Brain interprets the impulses

  3. Muscles of the arm pull the hand away
• Stimuli from inside the body

  • Increased cellular respiration in muscle cells
  • Increased CO2 levels in blood
  • Decreased oxygen levels
• Response to internal stimuli

  1. Increased breathing rate

  2. Gets rid of extra CO2

  3. Supplies more oxygen
• Responses to stimuli are responsible for maintaining the internal environment at a constant optimal level
• Homeostasis is the process of keeping conditions inside the organism relatively constant or stable.

   

  Homeostasis consists of two stages:

  ·         detecting changes from the stable state

  ·         counteracting changes from the stable state.

  
• vModel - the oven thermostat

  1. Something to detect the temperature (detects change)

  2. A control which is set to the required temperature

  3. A switching mechanism to switch the heating off or on (counteracts the change)

  4. Communication between the parts - electrical wiring
• As the heating of the oven increases the temperature

  It reaches and goes beyond the set temperature
• This is detected


  Causes the control mechanism to turn off the heating (counteracting the change)
• This causes the temperature to drop below the set temperature

  Turning the heating back on
• This is very similar to mechanisms in living things that work to maintain a stable internal environment
• This is just what happens inside living things
• Detectors


  Specialised sensory cells that detect both the external environment and the internal environment
• Types of detectors


  • Photoreceptor
  • Thermoreceptor
  • Mechanoreceptor
  • Chemoreceptor
• Photoreceptor


  • Detects light
  • Found in the retina at the back of the eye, has rod and cone cells
• Thermoreceptor


  • Detects heat and cold
  • Found in the skin
• Mechanoreceptor


  • Detects sound, touch, pressure, gravity
  • Found in blood vessels to detect blood pressure
• Chemoreceptor


  • Detects oxygen, carbon dioxide, water, pH, inorganic ions, nitrogenous wastes, glucose
  • Alpha and beta cells in the pancreas detect blood glucose levels
• Control and coordination

  Control and coordination

  As mentioned earlier homeostasis via the stimuli-response pathway is mediated by both the endocrine (hormonal) and nervous systems, each using different signalling mechanisms to receive and transmit information throughout the body.

  Either nerves or hormones carry messages from the receptor to the control centre (usually the brain) and different nerves or hormones carry messages to the effectors to trigger a response.
• Endotherms/endothermic species


  Birds and mammals that control their body temperatures at constant levels over the range of environmental temperatures in the places in which they live
• Body temperatures of endothermic animals


  • 32°C in monotreme mammals (platypus and echidnas)
  • 36°C in marsupials
  • 37°C in placental mammals
  • 4-5°C higher than placental mammals in birds
• How endothermic animals maintain a stable temperature

  1. Gain heat from the environment

  2. Most heat used to control body temperature is generated by their own metabolism

  3. Except in cold environments, produce more heat than needed for temperature regulation, excess heat needs to be lost to the environment
• Heat generation in mammals and birds

  Largely controlled by the endocrine system
• Minute to minute regulation of temperature

  Controlled by the nervous system, involving feedback systems including sweating, shivering and dilation/constriction of blood vessels
• It is the interaction of both the endocrine and nervous systems that controls body temperature
• Mammals


  Have two main ways of producing heat to keep the body temperature higher than the surroundings
• Ways mammals produce heat

  • Increasing the metabolic rate of cells throughout the body
  • Shivering
• Increasing the body's metabolic rate

  1. Thyroid gland produces thyroid hormones

  2. Thyroid hormones act on liver and muscles

  3. Increase level of metabolism

  4. Metabolic processes produce heat

  5. Hypothalamus detects temperature reduction

  6. Hypothalamus stimulates pituitary gland

  7. Pituitary gland secretes thyroid stimulating hormone (TSH)

  8. TSH stimulates thyroid gland to produce hormones

  9. Thyroid hormones increase metabolism

  10. Generate extra heat to control body temperature in cold
• In hotter conditions


  Less metabolic heat is lost to the environment
• Increase in blood temperature

  • Hypothalamus secretes less TSH
  • Less thyroid hormone secretion
  • Less heat generation from metabolism
• From winter to summer, the organism is able to turn up and turn down heat production so that a stable body temperature is maintained
• Shivering


  1. Muscle fibres rapidly contract and relax

  2. Generating lots of heat

  3. Used to maintain body temperature
• When environmental temperatures change quite quickly

  There is a more rapid response