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Biology · Homeostasis

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Biology · Edexcel Paper 2

Homeostasis

Maintaining a Constant Internal Environment

Whatever the weather, and whether you are asleep or sprinting, your body temperature stays close to 37 °C.

  • Homeostasis The regulation of the conditions inside the body, so that they stay constant.
  • Internal environment The conditions inside the body, such as temperature and blood glucose.
  • Stimulus A change that is detected by a receptor.
A control loop with a change from the normal level, a receptor, a coordination centre, an effector and a return to normal, with a negative feedback arrow back to the start.
Homeostatic control systems work by negative feedback.

Nervous and hormonal control

  • Nervous control Uses electrical impulses along neurones; Very fast response; The effect usually lasts a short time
  • Hormonal control Uses hormones carried in the blood; Slower response; The effect usually lasts longer

Why Homeostasis Matters: Enzymes and Osmosis (Separate only)

Two things go wrong very quickly if the internal environment is not controlled: the enzymes stop working, and the cells gain or lose water.

  • Thermoregulation The control of body temperature.
  • Osmoregulation The control of the water content of the body.
  • Optimum temperature The temperature at which an enzyme works fastest.
A graph of enzyme activity against temperature, which peaks at 37 degrees Celsius and falls to zero at high temperatures.
Enzyme activity is greatest at the optimum temperature, and the enzyme is denatured at high temperatures.

Too hot and too cold, and too dilute and too concentrated

  • Temperature problems (enzymes) Too cold: enzymes work slowly, because of fewer collisions; Too hot: enzymes are denatured; Denaturing is not reversed
  • Water problems (animal cells) Too dilute: cells swell and may burst; Too concentrated: cells shrink and shrivel; Caused by osmosis

Thermoregulation and the Skin (Separate only)

When you are hot your skin sweats, and when you are cold you shiver.

  • Epidermis The thin outer layer of the skin.
  • Dermis The layer of the skin below the epidermis, which contains sweat glands, blood vessels and receptors.
  • Hypothalamus The part of the brain that monitors body temperature and coordinates the response.
A section through the skin showing the epidermis, the dermis with a hair, a hair erector muscle, a sweat gland, a sweat duct, a temperature receptor and blood vessels, and the fat layer.
The skin has an epidermis, a dermis and a layer of fat, and the dermis contains the sweat glands and blood vessels.

Too hot and too cold

  • Too hot Sweat glands release more sweat; Sweat evaporates, cooling the skin; Hairs lie flat
  • Too cold Sweat glands release little sweat; Muscles shiver, releasing heat from respiration; Hairs stand up, trapping air

Vasoconstriction and Vasodilation (Higher tier) (Separate only)

The skin of a hot, tired runner is red and flushed, while the fingers of a person in the snow turn white.

  • Vasodilation The widening of the blood vessels in the skin, so that more heat is lost.
  • Vasoconstriction The narrowing of the blood vessels in the skin, so that less heat is lost.
  • Radiation The transfer of heat from a warm object to its cooler surroundings.
Two cross-sections of the skin, one with wide blood vessels near the surface and more heat lost, and one with narrow blood vessels and less heat lost.
In vasodilation the vessels widen and more heat is lost, and in vasoconstriction they narrow and less heat is lost.

Vasodilation and vasoconstriction

  • Vasodilation (too hot) Muscles in the vessel walls relax; Vessels get wider; More blood near the skin surface
  • Vasoconstriction (too cold) Muscles in the vessel walls contract; Vessels get narrower; Less blood near the skin surface
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Biology · Edexcel Paper 2

Homeostasis continued

Insulin and Blood Glucose

After a meal, a lot of glucose enters the blood, but the level soon returns to normal.

  • Insulin A hormone that is made in the pancreas, which lowers the blood glucose concentration.
  • Pancreas A gland that makes insulin, glucagon and digestive enzymes.
  • Blood glucose concentration The amount of glucose in the blood, in mmol per dm³.
A flow chart in which a rise in blood glucose makes the pancreas release insulin, which makes the liver, muscles and other cells take up glucose.
Insulin makes cells take up glucose from the blood, which lowers the blood glucose concentration.

Before and after insulin

  • Blood glucose too high Glucose rises after a meal; Pancreas releases insulin; Cells take up glucose
  • Blood glucose back to normal Blood glucose falls; Less insulin is released; Glycogen is stored in the liver

Glucagon and Blood Glucose (Higher tier) (Higher tier)

Between meals, and during exercise, the blood glucose concentration can fall, and then a second hormone comes into play.

  • Glucagon A hormone made in the pancreas that raises the blood glucose concentration.
  • Glycogen A storage carbohydrate, made from glucose, in the liver and muscles.
  • Insulin A hormone made in the pancreas that lowers the blood glucose concentration.
A diagram of the pancreas releasing insulin when the blood glucose is too high and glucagon when it is too low, with the liver storing glucose as glycogen or releasing glucose.
Insulin lowers the blood glucose, and glucagon raises it, and both act on the liver.

Insulin and glucagon

  • Insulin Released when the blood glucose is too high; Made in the pancreas; Makes cells take up glucose
  • Glucagon Released when the blood glucose is too low; Made in the pancreas; Acts on the liver

Type 1 and Type 2 Diabetes

Diabetes is one of the most common long-term diseases in the world, and its two main forms have different causes and different treatments.

  • Diabetes A condition in which the blood glucose concentration is not controlled properly.
  • Type 1 diabetes Diabetes in which the pancreas does not produce enough insulin.
  • Type 2 diabetes Diabetes in which the body cells no longer respond to insulin.
  • Insulin A hormone that lowers the blood glucose concentration.

Type 1 and type 2 diabetes

  • Type 1 The pancreas makes little or no insulin; The immune system destroys insulin-making cells; Usually begins in childhood
  • Type 2 The cells do not respond to insulin; Linked to obesity and an inactive lifestyle; Usually begins in adults

Body Mass, BMI and Type 2 Diabetes

People who are very overweight are much more likely to develop type 2 diabetes.

  • Body mass index A measure of mass for height, calculated as mass in kilograms divided by height in metres squared.
  • Waist to hip ratio The waist circumference divided by the hip circumference.
  • Obesity A condition in which a person has a large amount of excess body fat, with a BMI of 30 or more.
  • Correlation A link between two variables.

Strengths and limits of the evidence

  • Evidence for a link Large studies show a positive correlation; A mechanism is known: insulin resistance; Weight loss often improves control
  • Limits of the evidence A correlation does not prove a cause; BMI does not show where the fat is; Muscular people can have a high BMI but little fat