Growing up, growing old and passing on your genes

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

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
  • Never 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)