B5

• Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes
• Examples of homeostasis in the human body

  • Blood glucose concentration
  • Body temperature
  • Water levels
• Components of control systems

  • Cells called receptors, which detect stimuli
  • Coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors
  • Effectors, muscles or glands, which bring about responses which restore optimum levels
• The nervous system enables humans to react to their surroundings and to coordinate their behaviour
• Reflex arc


  1. Sensory neurone

  2. Synapse

  3. Relay neurone

  4. Motor neurone
• Reflex actions are automatic and rapid; they do not involve the conscious part of the brain
• The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions
• Neuroscientists have been able to map the regions of the brain to particular functions by studying patients with brain damage, electrically stimulating different parts of the brain and using MRI scanning techniques. The complexity and delicacy of the brain makes investigating and treating brain disorders very difficult
• Structures of the eye and their functions

  • Retina
  • Optic nerve
  • Sclera
  • Cornea
  • Iris
  • Ciliary muscles
  • Suspensory ligaments
• Accommodation to focus on near or distant objects


  1. Ciliary muscles contract, suspensory ligaments loosen, lens is thicker and refracts light rays strongly (for near objects)

  2. Ciliary muscles relax, suspensory ligaments are pulled tight, lens is pulled thin and only slightly refracts light rays (for distant objects)
• Two common defects of the eyes are myopia (short sightedness) and hyperopia (long sightedness) in which rays of light do not focus on the retina
• Generally these defects are treated with spectacle lenses which refract the light rays so that they do focus on the retina
• Body temperature is monitored and controlled by the thermoregulatory centre in the brain
• Mechanisms to lower or raise body temperature

  • Blood vessels dilate and sweat is produced (when body temperature is too high)
  • Blood vessels constrict, sweating stops and skeletal muscles contract (shiver) (when body temperature is too low)
• The endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream
• The pituitary gland in the brain is a 'master gland' which secretes several hormones into the blood in response to body conditions
• Endocrine glands in the human body

  • Pituitary gland
  • Pancreas
  • Thyroid
  • Adrenal gland
  • Ovary
  • Testes
• Endocrine system


  Composed of glands which secrete chemicals called hormones directly into the bloodstream
• Pituitary gland


  A 'master gland' which secretes several hormones into the blood in response to body conditions
• Endocrine glands


  • Pituitary
  • Pancreas
  • Thyroid
  • Adrenal
  • Ovary
  • Testes
• Control of blood glucose concentration

  1. Pancreas produces insulin when blood glucose is too high

  2. Insulin causes glucose to move from blood into cells

  3. Excess glucose converted to glycogen for storage
• Type 1 diabetes


  Pancreas fails to produce sufficient insulin, leading to uncontrolled high blood glucose levels
• Type 2 diabetes


  Body cells no longer respond to insulin produced by pancreas
• If blood glucose concentration is too low, pancreas produces glucagon that causes glycogen to be converted into glucose and released into the blood
• Maintaining water and nitrogen balance in the body

  1. Water, ions and urea lost via lungs, skin, kidneys

  2. Kidneys filter blood and selectively reabsorb useful substances
• Digestion of proteins results in excess amino acids which are deaminated in liver to form ammonia, then converted to urea for safe excretion
• ADH


  Hormone that controls water level in body by regulating permeability of kidney tubules
• Kidney dialysis is a treatment for kidney failure
• Oestrogen


  Main female reproductive hormone produced in ovary, causes egg maturation and release
• Testosterone


  Main male reproductive hormone produced in testes, stimulates sperm production
• Menstrual cycle


  1. FSH causes egg maturation

  2. LH stimulates egg release

  3. Oestrogen and progesterone maintain uterus lining
• Contraceptive methods


  • Oral contraceptives
  • Injection/implant/patch of progesterone
  • Barrier methods
  • Intrauterine devices
  • Spermicides
  • Abstinence
  • Sterilisation
• IVF treatment involves stimulating egg maturation, collecting eggs, fertilising them in lab, and implanting embryos in uterus
• Adrenaline is produced by adrenal glands in response to fear/stress, increases heart rate and oxygen/glucose delivery
• Thyroxine from thyroid gland stimulates basal metabolic rate and plays role in growth and development
• Thyroxine levels are controlled by negative feedback
• Auxins


  Plant hormones that cause unequal growth rates in roots and shoots
• Gibberellins


  Plant hormones important in initiating seed germination
• Ethene


  Plant hormone that controls cell division and fruit ripening
• Auxins used as weed killers, rooting powders, and to promote growth in tissue culture