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HBIO4 > Growth & Ageing
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Patterns of Human Growth

Growth rate

  • GROWTH RATE = (SIZE AT TIME 2 - SIZE AT TIME 1) / (TIME 2 - TIME 1)
  • Highest in the first year
  • Constant and low during childhood
  • Girls and boys have similar height until 10yrs → growth spurt occurs
    • Growth spurt is later but larger in boys
    • Therefore, boys are taller than girls at 14yrs
  • Growth stops by the age of 18

Relatives growth rate of organs

  • Different parts of the body grow at different rates
  • Head and brain develop first
    • 90% of its adult size at 5yrs
    • Before growth of bones and muscles in limbs → change in body proportion
  • Reproductive system develops latest
    • Remains below 20% until puberty
    • Extended childhood (link to 3-2-4(a))
  • Curve of the whole body is similar to an absolute growth graph as height is an indicator of body size

Puberty

  • Triggered by the release of gonadotrophin releasing factor (GnRF)
    • GnRF is released from the hypothalamus
    • Travels to the pituitary gland where it releases gonadotrophins
      • Luteinising hormone (LH) - also called interstitial cell stimulating hormone (ICSH) in males
      • Follicle stimulating hormone (FSH)
    • FSH and LH travel in blood to gonads (→ovaries or testes)
    • There, they cause the release of sex hormones
      • Testosterone in males
      • Oestrogen + progesterone in females
  • Release of GnRF in females
    • Improved diet
      • Females grow faster and reach stage of maturity at younger age
      • Age of menarche (→first period) is earlier than it was in the 19th century
      • Relates to changes in female's muscle:fat ratio
    • Menstrual cycle in females relates to the proportion of fat to muscle
      • Girls with low body fat tend to have a later menarche
      • Anorexia nervosa → body loses fat → periods stop
      • Female athletes who have a high muscle:fat ratio often have irregular periods
  • Sex hormones
    • Trigger development of reproductive organs
    • Trigger development of 2° sexual characteristics
      • Increased activity of sweat and sebaceous glands (blocked sebaceous glands cause acne)
      • Growth of pubic and axillary hair
    • Testosterone
      • Expands shoulders (growth of cartilage in thorax and pectoral girdle)
      • Causes growth of muscles
    • Oestrogen
      • Expands the hips (growth of cartilage in pelvis)
      • Causes an accumulation of fat

Table: Summary of hormones in puberty

Name of Hormone

Site of Release

Actions

Gonadotrophins

Anterior pituitary gland
(situated under brain)

 

  - FSH

 

- Development of follicles
- Secretion of oestrogen by follicle cells in ovaries

  - LH (females)

 

- Triggers ovulation
- Forms corpus luteum
- Thickens uterus lining
- Produces milk in breasts

  - LH (males)

 

- Secretion of testosterone by testes
- Spermatogenesis in testes

Oestrogen

Ovaries

- Growth of 2° sexual characteristics
- Thickening of uterus lining

Progesterone

Corpus luteum

- Growth of breasts
- Increases blood supply to uterus lining

Testosterone

Testes

- Growth of 2° sexual characteristics
- Sperm production

Effects and Diseases of Ageing

Decline in physiological functions

  • BMR
    • Number of cells decreases during ageing → lowers BMR
    • BMR decreases by ≈ 5% every 10yrs above the age of 55
    • 10-20yrs → rapid decrease associated with adolescent growth spurt
    • 20-35yrs → no change as body same size / same level of activity
    • 30-70yrs → slow decrease associated with loss of muscles / gain of fat / reduced activity
  • Cardiac output (CO) = stroke volume (SV) x  heart rate (HR)
    • CO decreases even though HR does not decline
    • Due to cardiac muscle fibres weaken (mainly left ventricle)
    • Decreases SV of ventricles / volume of blood pumped per beat/cycle
  • Nerve conduction velocity
    • Cells in peripheral nervous system and brain decline
      • Neurones (nerve cells) are lost and cannot divide
      • Effect of cell loss depends on cells location
      • Brain loses ≈25% of cells that control muscle coordination but hardly any that control speech
    • Nerve conduction is slowed by
      • 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 (45-55yrs)
    • 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)

Alzheimer's disease

  • Most common type of dementia
    • Progressive decline of cognitive function (thinking, memory, orientation)
  • Causes
    • Development of
      • Amyloid plaques between neurones
      • Tangles (→twisted fibres) inside neurones
    • Leads to the destruction of cerebral cortex
    • Lack of acetylcholine (ACh) in nerve synapses
  • Symptoms
    • Memory loss (recent memory is lost first)
    • Decline in language (cannot find the right word)
    • Confusion
    • Change of mood and behaviour (depression, anxiety, aggression)´
  • Risk factors
    • Age (more common >65yrs)
    • Genetics (family history)
    • Whiplash and head injuries (boxers)
    • Cardiovascular (smoking, alcohol, obesity, hypertension, cholesterol, no exercise)
  • No cure available - but drugs that ↑ACh in the brain are used in SEVERE dementia to delay progression
  • True diagnosis only possible after death
    • Piece of brain tissue must be looked at under the microscope to identify tangles and plaque

Cancer

  • Damage to DNA is more likely in the elderly
    • Longer exposure to environmental risk factors
    • Accumulation of mutations
  • Evaluate whether screening should be offered
    • Think of
      • ?false positives
      • ?anxiety
    • Very slowly growing cancers
      • ?more likely to die from other causes
      • ?surgery to remove cancer causes more damage than doing nothing
    • Young age
      • ?less common
      • ?harder to detect tumour
      • ?cost effective
  • Risk factors
    • Genetic (family history)
    • Environment (alcohol, smoking, hormones from contraception, radiation)