Homeostasis and response

• what is homeostasis?

  The ability of an organism to maintain a stable internal environment despite external changes.
• negative feedback loop:

  
• the nervous system:

  • CNS( central nervous system)
  • Sensory neurons
  • motor neurons
  • effectors
• CNS


  in vertebrates (animals with backbones) this consists of the brain and spinal cords only

  in mammals the CNS is connected to the body via sensory neurons and motor neurons
• sensory neurons 


  the neurons that carry information as electrical impulses from the receptors to the CNS
• motor neurons


  the neurons that carry electrical impulses from the CNS to effectors
• effectors


  all of your muscles and glands which respond to nervous impulses
• receptors:


  the cells that detect stimuli

  there are many different types such as taste receptors (on the tounge) and sound receptors (in the ears)

  receptors can form part of larger, complex organs e.g. the retina of the eye is covered in light receptor cells
• effectors:


  effectors respond to nervous impulses and bring about a change

  muscles and glands are known as effectors- they respond in different ways. muscles contract in response to a nervous impulse whereas glands secrete hormones
• the CNS coordinates the response

  • the CNS is a coordination centre- it receives information from the receptors and coordinates a response
  • the response is carried out by effectors
• synapses connect neurones


  • the connection between 2 neurones is called a synapse
  • the nerve signal is transferred by chemicals which diffuse across the gap
  • these chemicals then set off a new electrical signal in the next neurone
• what are reflexes?


  rapid, automatic responses to certain stimuli that don't involve the conscious part of the brain- they can reduce the chances of being injured
• cerebral cortex 


  • the outer area
  • responsible for things like consciousness, intelligence, memory and language
• cerebelllum

  • responsible for muscle co-ordination
• medulla


  • controls unconcious activities (i.e breathing and heartbeat)
• the eye

  
• the cornea


  Refracts light - bends it as it enters the eye
• the iris 


  controls how much light enters the pupil
• the lens

  Further refracts light to focus it onto the retina
• the retina 


  contains light receptors
• the optic nerve


  Carries impulses between the eye and the brain
• the sclera


  Tough white outer layer of the eye. It helps protect the eye from injury
• iris reflex- adjusting for bright light 


  • when light receptors in the eye detect a very bright light a reflex is triggered to make to pupil smaller
  • the circular muscles in the iris contract and the radial muscles relax
  • the opposite process ocsurs in dim lighting
• to look at near objects:

  • the ciliary muscles contract, which slackens the suspensory ligaments
  • the lens becomes more curved
  • increases the amount by which it refracts light
• to look at distant objects

  • the ciliary muscles relax, which allows the suspensory ligaments to pull tight
  • this makes the lense go thin (less curved)
  • so refracts light by a smaller amount
• responses when you are too hot:

  • sweat is produced by sweat glands and evapourates from the skin - this transfers energy to the environment
  • vasodilation- the blood vessels supplying the skin dilate so more blood flows close to the skin. This helps transfer energy from the skin to the environment
• responses when you are too cold:

  • hairs stand up to trap an insulating layer of air
  • no sweat is produced
  • vasoconstriction - blood vessels supplying skin cappilaries constrict to close of skins blood supply
  • Shivering - requires respiration which transfers some energy to warm the body
• what are hormones?


  Chemical messengers produced by glands in the endocrine system that regulate various bodily functions.
• pituartary gland


  • produces many hormones that regulate body conditions
  • the master gland
• thyroid

  • produces thyroxine which is involved in regulating things like rate of metabolism, heart rate and temperature
• ovaries


  • produce oestrogen
  • involved in menstrual cycle
• testes


  • produce testosterone
  • controls puberty and sperm production in males
• adrenal gland 


  • produces adrenaline
  • used to prepare the body for a fight or flight response
• the pancreas


  • produces insuline
  • used to regulate blood glucose level
• hormones vs nerves


  • hormones- slower action, act for a long time, act in a more general way
  • nerves- very fast action, act for a very short time, act on a very precise area
• when blood levels are too high insulin is added
• when blood glucose levels are too low glucagon is added
• substances removed in urine:

  • urea
  • ions
  • water
• urea:

  • protiens cant be stored by the body- so any excess amnio acids are converted into fats and carbohydrates, which can be stored (this involves the liver and a process calles deamination)
  • ammonia is produced as a waste product from this process
  • ammonia is toxic and so is converted to urea in the liver
  • urea is then transported to the kidneys where its filtered out of the blood and excreted from the body in urine
• blood glucose level too high- insulin is added

  • insulin is secreted by pancreas into bloodstream
  • insulin causes liver to turn glucose into glycogen
  • glucose moves from blood into liver and muscle cells
  • blood glucose is reduced