B5: Homeostasis and Response

• Homeostasis


  Maintaining a stable internal environment
• The conditions inside your body need to be kept steady even when the external environment changes
• Homeostasis is about the regulation of the conditions inside your body (and cells) to maintain a stable internal state in response to changes in both internal and external conditions
• Automatic control systems in your body

  • Regulate your internal conditions, e.g. blood glucose level, body temperature, water and ion concentrations
• Receptors, coordination centres, effectors

  Components that work together to maintain a steady internal condition
• Negative feedback

  1. Receptor detects a stimulus

  2. Coordination centre receives and processes the information, then organises a response

  3. Effector produces a response which counteracts the change and restores the optimum level
• The effectors will just carry on producing the responses for as long as they're stimulated by the coordination centre
• The Nervous System

  Detects and reacts to stimuli
• Parts of the Nervous System

  • Central Nervous System (CNS)
  • Sensory Neurones
  • Motor Neurones
  • Effectors
• Receptors and Effectors

  • Receptors detect stimuli
  • Effectors respond to nervous impulses and bring about a change
• How the Central Nervous System coordinates a response

  1. Receptors detect a stimulus

  2. Sensory neurones carry information to the CNS

  3. CNS decides what to do

  4. Motor neurones carry information from the CNS to the effectors (muscles)
• Synapse


  Connection between neurones where nerve signals are transmitted
• Reflex

  Rapid, automatic response to a stimulus to prevent injury
• Reflex arc

  1. Receptors detect stimulus

  2. Sensory neurones carry impulses to the CNS

  3. Relay neurones in the CNS transmit impulses

  4. Motor neurones carry impulses to effectors (muscles)

  5. Muscle contracts
• Reflexes are quicker than normal responses because they don't require thinking
• Reaction time

  How quickly you respond to a stimulus
• Measuring reaction time using a ruler

  1. Person holds ruler vertically

  2. Ruler is dropped without warning

  3. Person tries to catch ruler as quickly as possible

  4. Distance ruler falls measures reaction time
• Caffeine can affect reaction time
• Measuring reaction time using a computer

  1. Person clicks mouse or presses key when they see a stimulus on the screen

  2. Computer records reaction time precisely in milliseconds
• The Brain


  • Part of the central nervous system
  • Responsible for complex behaviours and functions
• Parts of the Brain

  • Cerebrum
  • Medulla
  • Cerebellum
• Scientists use various methods to study the brain, including observing effects of brain damage, electrical stimulation, and brain imaging techniques like MRI
• Knowledge of how the brain works has led to treatments for nervous system disorders, but the brain is complex and delicate so investigation and treatment carries risks
• The Eye

  Focuses light onto the retina to enable vision
• Parts of the Eye

  • Cornea
  • Pupil
  • Lens
  • Retina
  • Optic Nerve
• Iris reflex - adjusting pupil size for bright light


  1. Bright light detected

  2. Pupil constricts to reduce light entering eye

  3. Dim light detected

  4. Pupil dilates to allow more light in
• Focusing on near and distant objects

  1. For near objects: Ciliary muscles contract, lens becomes more curved

  2. For distant objects: Ciliary muscles relax, lens becomes less curved
• As people age, the lens loses flexibility and cannot focus as easily, leading to vision problems like short or long-sightedness
• Long-sightedness

  The lens is the wrong shape and doesn't bend the light enough or the eyeball is too short, so the images of near objects are brought into focus behind the retina
• Short-sightedness

  The lens is the wrong shape and refracts the light too much or the eyeball is too long, so the images of distant objects are brought into focus in front of the retina
• Treatments for vision defects

  • Wearing glasses
  • Contact lenses
  • Laser eye surgery
  • Replacement lens surgery
• Contact lenses

  • Thin lenses that sit on the surface of the eye and are shaped to compensate for the fault in focusing
  • Popular because they are lightweight and almost invisible
  • More convenient than glasses for activities like sports
  • Two main types are hard lenses and soft lenses, with soft lenses generally more comfortable but carrying a higher risk of eye infections than hard lenses
• Laser eye surgery

  • A laser can be used to vaporise tissue, changing the shape of the cornea (and so changing how strongly it refracts light into the eye)
  • Slimming down the cornea makes it less powerful and can improve short-sightedness
  • Changing the shape so that it's more powerful will improve long-sightedness
  • The surgeon can precisely control how much tissue the laser takes off, completely correcting the vision
  • However, there is a risk of complications such as infection or the eye reacting in a way that makes vision worse than before
• Replacement lens surgery

  • The natural lens of the eye is removed and an artificial lens, made of clear plastic, is inserted in its place
  • As it involves work inside the eye, replacing a lens carries higher risks than laser eye surgery, including possible damage to the retina (which could lead to loss of sight)
• The body has to balance the amount of heat produced through metabolism with the amount of heat lost to the environment
• Thermoregulation

  1. Temperature receptors in the skin send information about temperature to the thermoregulatory centre

  2. The thermoregulatory centre processes the information and triggers effectors to produce a response

  3. Effectors work antagonistically, with some producing heat and others cooling the body, to maintain a precise body temperature
• Effectors for cooling the body when too hot

  • Sweat glands produce sweat, which evaporates and transfers energy to the environment
  • Blood vessels supplying the skin dilate (vasodilation) to allow more blood flow to the skin surface, transferring heat
• Effectors for warming the body when too cold

  • Hairs stand up to trap an insulating layer of air
  • No sweat is produced
  • Blood vessels supplying the skin capillaries constrict (vasoconstriction) to close off the skin's blood supply
  • Shivering, where muscles contract automatically, generates heat through respiration
• Hormones

  Chemical messengers sent in the blood that have relatively long-lasting effects, produced in and secreted by various glands
• Examples of endocrine glands

  • Pituitary gland
  • Ovaries (females only)
  • Testes (males only)
  • Thyroid
  • Adrenal gland
  • Pancreas