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GCSE Biology Summary Notes Homeostasis
Summary Notes on the GCSE Biology topic Homeostasis, specifically curated for the AQA Biology Exam Board.
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HomeostasisIntroduction
Cells in the body can only survive within narrow physical chemical limits. They require a
constant temperature and pH as well as a constant supply of dissolved food and water. In
order to do this the body requires control systems that constantly monitor and adjust the
composition of the blood and tissues.
Internal Conditions
Homeostasis is the maintenance of a constant internal environment. This is vital as change
in temperature/ pH can leave enzymes dormant or denatured. High sugar levels would
mean water would travel by osmosis out of the cell and shrink; low sugar levels mean they’d
burst. Cells need a constant supply of glucose for respiration.
Too Hot
When our body temperature exceeds 37o thermoreceptors in skin and hypothalamus detect
an increase in temperature in blood. An electrical impulse informs the hypothalamus
(thermoregulatory centre) which acts as the co-ordinator. An electrical impulse causes
sweat glands to secrete sweat which, when it evaporates takes away some of the heat
energy. Muscles around arterioles relax so blood vessels near skin can dilate, so more blood
flows near the skin’s surface which increases heat loss by radiation. Erector muscles relax so
hairs lie flat causing the body temperature to return to normal.
Sweating increases
More water evaporates leading to cooling
Vasodilation of arterioles on skin surface
More heat loss by radiation
Hair erector muscles relax
Hair lies flat
Less air trapped
Voluntary action – take off clothes
Too Cold
When our body temperature drops under 37o thermoreceptors in skin and hypothalamus
detect a decrease in temperature in blood. An electrical impulse informs the hypothalamus
(thermoregulatory centre) which acts as the co-ordinator. An electrical impulse cause
muscles around arterioles near skin’s surface to constrict so less blood flows near the skin
surface which limits heat loss by radiation and conserves heat. Erector muscles contract so
hairs stand on end which traps an insulative layer of air. Skeletal muscles contract rapidly
which increases respiration and is exothermic (shivering).
Shivering
, Pale skin
Hair erector muscles contract
Hair are raised
More air trapped
Vasoconstriction of arterioles on skin surface
Less heat loss by radiation
Sweating decreases
Less water evaporates leading to less heat loss
Voluntary action – put on clothes
Hormones
A chemical messenger, made from protein, travels in the blood stream and is secreted by a
gland. Hormones are slower, travel in the blood stream as a chemical messenger, and are
secreted by glands and received by the target organ, their effects are much longer lasting.
Pituitary – releases FSH and LH which are fertility hormones
and TSH (thyroid stimulating hormone) and ADH which
controls the uptake of water in the kidneys.
Thyroid – thyroxine which controls metabolism.
Adrenal – adrenaline which is the fight or flight hormone.
Testes – testosterone which is the male sex hormone.
Ovaries - estrogen and progesterone which are fertility
hormones.
Pancreas – insulin and glucagon which controls blood sugar levels.
Blood Sugar Levels
It’s important that blood sugar levels don’t rise or fall so osmosis isn’t affected. Too much
glucose in the blood and cells would lose too much water and vice versa. There also needs
to be a constant supply of glucose for respiration.
Blood sugar levels may be high after eating and drinking, carbohydrates and sugars. Blood
sugar levels may be low first thing in the morning after not eating for hours, or after
exercise.
When glucose levels are too high, it is detected by the cells in the pancreas which releases
insulin. The insulin produced by the pancreas causes glucose to move into cells. This causes
excess glucose to be converted into glycogen in the liver and muscle cells.
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