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Human Biology > Senescence
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The Decline of Physiological Effectiveness

  • Rate of cell division and number of cells reduce
    • All cells are capable to divide during embryological development
    • Cells lose ability to divide after birth or have a lower growth rate
    • Born with a fixed number of neurones → cannot divide/be replaced
  • Decline in functional effectiveness of cells and organ systems
    • Deterioration in cells / slower responds to stimuli / slows homeostatic mechanism / increases change of dysfunction and death
  • Ageing is controlled by genes but can be slowed down by
    • Regular (and adequate) sleep, (well balanced) meals, exercise
    • Refrain from smoking and alcohol
    • Keep body mass close to desirable mass for your height

Effect of Age on

  • BMR
    • Number of cells decreases during ageing → lowers BMR
    • BMR decreases by ≈ 5% every 10 years above the age of 55
    • 10-20 years - rapid decrease associated with adolescent growth spurt
    • 20-35 - no change as body same size / same level of activity
    • 30-70 - slow decrease associated with loss of muscles / gain of fat / reduced activity
  • CARDIAC OUTPUT = STROKE VOLUME  x  HEART RATE
    • Cardiac output decreases even though heart rate does not decline
    • Due to cardiac muscle fibres weaken (mainly left ventricle)
    • Decreases stroke volume of ventricles/volume of blood pumped per beat/cycle
  • NERVE CONDUCTION VELOCITY
    • Cells in peripheral nervous system and brain get less
      • Neurones (nerve cells) are lost and cannot divide
      • Effect of cell loss depends on cells location
      • Brain loses ≈25% of cells that control muscular movement but hardly any that control speech → changes muscle coordination but not ability to speak
    • LOSS OF MYELIN: no saltatory conduction / impulses cannot jump from node to node / impulses must pass through greater amount of membrane
    • INCREASED WIDTH OF SYNAPSES: longer needed for diffusion/movement/greater distance to receptors/further to stimulate post-synaptic membrane/further diffusion distance of transmitter (across synapse)
    • SLOWER SYNAPTIC TRANSMISSION: presynaptic neurones produce less neurotransmitter
  • Female reproductive capacity → MENOPAUSE (45-55 year old women)
    • Ovaries gradually become insensitive to FSH / secretion of oestrogen becomes less / ovulation becomes less / menstrual cycle becomes less / vagina walls become thinner / woman is infertile when oestrogen secretion stops
    • Levels of gonadotrophins (FSH, LH) rise to a peak after menopause
      • At menopause, oestrogen no longer secreted
      • FSH and LH no longer inhibited by negative feedback
    • SYMPTOMS: due to loss of oestrogen
      • Intense sweating / uncomfortable warmth / psychological problems
      • Increase risk of osteoporosis (loss of bone tissue) and heart diseases
    • TREATMENT: Hormone replacement therapy (HRT)
      • Postmenstrual woman take in small doses of oestrogen and progesterone
      • As tablets (orally) or apply implants beneath skin (skin patches)