B1.2.2 Control in the Human Body

Controlling Internal Conditions

Internal conditions controlled by the body:
-Water content- excess water is lost from:

  • Lungs when we breathe out
  • Skin in sweat, to cool us down
  • Kidneys in urine 

-Ion content- excess ions are lost from:

  • Skin in sweat
  • Kidneys in urine

-Temperature, to maintain the temp at which enzymes work best
-Blood sugar levels, to provide cells with a constant supply of energy

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Many processes in the body are controlled by chemicals- hormones
They regulate functions of many organs and cells

1. Hormones are secreted by a gland into the blood

2. Hormones are transported around the body in the bloodstream to their target organs

3. Hormones cause a response from a target organ

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The Menstrual Cycle

Hormones secreted by the pituitary gland and the ovaries control the thickness of the uterus lining and the release of an egg.

Follicle-stimulating hormone (FSH)

  • Causes eggs in ovaries to mature
  • Stimulates ovaries to produce oestrogen
  • Secreted by the pituitary gland

Lutenising Hormone (LH)

  • Causes ovulation
  • Inhibits production of oestrogen
  • Secreted by the pituitary gland
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The Menstrual Cycle 2


  • Inhibits production of FSH so no more eggs mature that month
  • Causes uterus lining to thicken in first half of cycle
  • High levels stimulate production of LH
  • Secreted by the ovaries


  • Maintains thick uterus lining if egg is fertilised
  • High levels stop the cycle
  • Secreted by the ovaries
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FSH, LH and Oestrogen

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Reducing Fertility

Oral contraceptives

  • Oestrogen and progesterone to inhibit FSH-> no eggs mature -> no eggs are released
  • Early contraceptives contained large amounts of oestrogen caused significant side effects
  • Modern pills contain a much lower dose of oestrogen or are progesterone only, which lead to fewer side effects

 Fertility drugs

  • For women whose levels of FSH are too low to stimulate egg maturation
  • Fertility drugs contain FSH and LH


1. Mother given fertility drugs to stimulate maturation of several eggs
2. Eggs collected from ovaries and fertilised by sperm from the father
3. Fertilised eggs develop into embryos
5. When embryos are tiny balls of cells, one or two are inserted into the uterus 

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