5.1 and 2 Maintaining Homeostasis

• Homeostasis - the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes
• Homeostasis include the control of:

  • Blood glucose concentration
  • Body temperature
  • Water levels
• Control systems

  • Receptors to detect stimuli
  • Coordination centres to receive and process information from receptors
  • Effectors to bring about responses which restore optimum levels
• Reflex Arc

  1. Stimulus detected by receptor
  2. Electrical impulses pass from receptor along sensory neurone to CNS
  3. A chemical is released diffuses from sensory to relay neurone across a synapse to trigger an electrical impulse
  4. The electrical impulse now passes across the relay neurone and a chemical is released to trigger an electrical impulse in a motor neurone
  5. Electrical impulse passes down the motor neurone to an effector
  6. The effector carries out a response
• stimulus -> receptor -> coordinator -> effector -> response
• Reflex actions are automatic and rapid as they do not involve the conscious part of the brain, which helps us to protect from danger.
• The nervous system enables humans to react to their surroundings and

  to coordinate their behaviour.
• The Brain

  • Cerebral cortex - language, memory, consciousness
  • Cerebellum - balance, movements
  • Medulla - heart rate, breathing rate
• Challenges of studying the brain

  • Tricky to access as it is protected by skull
  • Structures are extremely complex
  • Extremely delicate and easy to damage
• The Brain

  A) Cerebral cortex

  B) Cerebellum

  C) Medulla

  
• 3 ways to investigate the brain

  Looking at patients with brain damage

  • Links the damaged part in the brain with the function

  Electrically stimulate different parts of the brain

  • Look at effects on person's behaviour to narrow down specific regions to functions

  MRI Scanning

  • Look at which parts of the brain are most active during different activities
• The eye detecting light

  1. Light rays pass through the cornea (starts the focussing of the light rays)
  2. Light rays pass through the pupil in the centre of the iris
  3. Light rays pass through the lens (focus light rays onto the back of the eye)
  4. Light rays are focused onto the retina (contain receptor cells for light intensity and colour)
  5. The receptor cells send electrical impulses down the optic nerve to the brain.
• The eye

  • Cornea - starts focussing of the light rays
  • Lens - focuses light rays onto the back of the eye
  • Retina - contains receptor cells for light intensity and colour
  • Sclera - tough outer structure protecting the eye
  • Ciliary muscles, suspensory ligaments, lens - focus on distant or near objects
  • Pupil, iris - allows light rays to enter eye
• The eye in different lights

  1. Drop / increase in light intensity is sensed by light receptors in the retina
  2. Electrical impulses are sent to the brain
  3. The brain sends electrical impulses to specific muscles in the iris
  4. Muscles in iris contract or relax to increase or decrease light entering eye
• The Eye

  A) Pupil

  B) Iris

  C) Lens

  D) Ciliary muscles

  E) Suspensory ligaments

  F) Sclera

  G) Cornea

  H) Retina

  I) Optic nerve

  
• Iris reflex in dim light

  • Radial muscles contract
  • Circular muscles relax
  • Pupils get larger
  • More light enters
• Iris reflex in bright light

  • Radial muscles relax
  • Circular muscles contract
  • Pupils get smaller
  • Less light enters
• Accommodation - the ability to change the shape of the lens to focus on near or distant objects
• Focusing on distant objects (need to be focused by a small amount)

  • Ciliary muscles relax
  • Suspensory ligaments pulled tight
  • Lens is now pulled thin
  • Light rays are only slightly refracted
• Focusing on near objects (need to be focused by a large amount)

  • Ciliary muscles contract
  • Ciliary muscles now have a smaller diameter
  • Suspensory ligaments loosen
  • Lens thickens
  • Lens is more convergent / lens refracts light more
  • Light rays are refracted more strongly and are focused on retina
• Hyperopia

  • Long sighted
  • Short eyeball or thin (weak, inelastic) lens
  • Focal point is after the retina
• Treatments for eye defects

  • Convex lens for hyperopia - refracts light more so light rays focus on retina
  • Concave lens for myopia - diverges light so diverged light rays are brought to a focus further back on to the retina
• Myopia

  • Short sighted
  • Long eyeball or strong lens
  • Focal point is before the retina
• Contact lenses

  • Pros - lightweight, unnoticeable
  • Cons - risk of infection
• Laser eye surgery

  • Pros - shape of the cornea can be changed to refract light more or less, no need to wear glasses or use lenses
  • Cons - risk of damage to eye, post surgery procedures
• Replacement lens surgery

  • Pros - effective
  • Cons - risk of damage to eye during surgery
• The body temperature is monitored and controlled by the thermoregulatory centre in the brain which contain receptors sensitive to the temperature of the blood
• Temperature too high

  • Sweat glands release sweat onto skin surface - sweat evaporates and takes energy from the body to cool it down
  • Vasodilation - blood vessels get wider and have more blood flowing through capillaries so heat can transfer out of the blood
• Temperature too low

  • Vasoconstriction - less blood flows through capillaries so less heat lost through body
  • Shivering / muscle contraction - increased rate of respiration releases heat to warm the body
  • Stop sweating
• Endocrine System

  • Composed of glands which secrete hormones directly into the bloodstream
  • The blood carries the hormone to a target organ where it produces an effect
  • Effects are slower than nervous system, but act for longer
• Roles of glands

  • Pancreas - controls concentration of glucose in blood
  • Ovaries and testes - involved in puberty and reproduction
  • Thyroid - growth and regulation of body's basal metabolic rate (how rapidly the body's reactions take place)
  • Adrenal - produces adrenaline in times of fear or stress
  • Pituitary (master) - act on other glands to cause other hormones to be released
• Glands

  A) Pituitary

  B) Thyroid

  C) Pancreas

  D) Adrenal

  E) Testes

  F) Ovaries

  
• Water, ions and urea are lost by

  • Lungs during exhalation
  • Skin when sweating
  • Kidneys in the urine
• Urine

  1. Blood containing urea enters kidneys through artery
  2. The kidney removes this urea as well as excess ions and water, which leave the kidney and is stored in the bladder
  3. Blood leaves kidney through a vein, and now contains no urea
• Filtration in the kidney

  1. The blood passes through capillaries
  2. Small molecules (e.g. glucose, ions, water, urea) are filtered out of the blood
  3. Selective reabsorption - all of the glucose, some ions and some water are reabsorbed back into the blood
  4. Urea, excess ions and excess water are stored as urine
• Deamination - the liver breaking down excess amino acids into ammonia (a very toxic chemical, which is converted into urea)
• Too little water

  1. Pituitary gland releases ADH
  2. ADH travels to kidneys and causes kidney tubules to be more permeable to water.
  3. More water is reabsorbed from tubules back into the blood
  4. Little but concentrated urine is produced and amount of water in blood returns back to normal.
• Too much water

  1. Pituitary gland releases less ADH
  2. The kidneys reabsorb less water into the blood
  3. Lots of dilute urine is produced
  4. Concentration of water in blood returns back to normal
• Kidney Dialysis

  1. Blood moves over a semi-permeable membrane which allows urea, ions and water through, but not larger molecules like protein or blood cells
  2. The dialysis fluid contains the normal concentrations of water and ions but not urea
  3. Urea, some water and some ions diffuse from the blood into the dialysis fluid
  4. Concentrations of water and ions in the patient's blood will return to normal
  5. The dialysis fluid is constantly refreshed to ensure a large concentration gradient for urea and to prevent build up of waste
• Kidney Dialysis

  Pros

  • Removes waste from blood
  • No shortage of machines

  Cons

  • Can cause infection due to repeated puncturing
  • Expensive in the long term
  • Blood clots
  • Frequent treatments and controlled diet