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Biology · Homeostasis

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Biology · OCR Paper 1

Homeostasis

Homeostasis and Control Systems

Homeostasis keeps the internal environment constant using receptors, coordination centres and effectors in negative feedback loops.

  • Homeostasis The regulation of the internal conditions of the body to keep them constant.
  • Internal environment The conditions inside the body, such as the blood and tissue fluid.
  • Receptor A cell or organ that detects a change.
A flow chart of a negative feedback loop from a change, to a receptor, to a coordination centre, to an effector, to a return to normal.
In negative feedback the response reverses the change.

Nervous and hormonal control

  • Nervous control Nerve impulses; Very fast response; Used for reflex actions and quick changes
  • Hormonal control Hormones in the blood; Slower response, but longer-lasting; Used for slower changes

Temperature Control and the Skin (Separate only)

The hypothalamus coordinates temperature control by negative feedback: sweating and vasodilation cool the body, and shivering and vasoconstriction warm it.

  • Thermoregulation The control of body temperature.
  • Hypothalamus The part of the brain that monitors and controls body temperature.
  • Sweat gland A gland in the skin that releases sweat.
A section through the skin showing the epidermis, hair, hair erector muscle, sweat gland, sweat duct, blood vessels, temperature receptor and fat layer.
The skin contains the receptors and the effectors for temperature control.

Too hot and too cold

  • Too hot Sweating increases; Blood vessels widen; Hairs lie flat
  • Too cold Shivering; Blood vessels narrow; Hairs stand up

Blood Glucose and Diabetes

Insulin lowers the blood glucose concentration by making the liver store glucose as glycogen, and for Higher tier glucagon raises it by making the liver release glucose.

  • Blood glucose concentration The amount of glucose in the blood.
  • Insulin The hormone that lowers the blood glucose concentration.
  • Glucagon The hormone that raises the blood glucose concentration (Higher tier).
High blood glucose makes the pancreas release insulin, which makes the liver store glucose, and low blood glucose makes the pancreas release glucagon, which makes the liver release glucose.
Insulin lowers the blood glucose concentration and glucagon raises it.

Insulin and glucagon

  • Insulin Released when blood glucose is too high; Released by the pancreas; Liver converts glucose to glycogen
  • Glucagon (Higher tier) Released when blood glucose is too low; Released by the pancreas; Liver converts glycogen to glucose

The Kidneys and Water Balance (Separate only)

The kidneys filter the blood, reabsorb what is needed and remove urea and excess water and ions.

  • Kidney An organ that filters the blood and makes urine.
  • Nephron The filtering unit of the kidney.
  • Urea A poisonous waste substance made from excess amino acids and removed by the kidneys.
A red blood cell that swells in a dilute solution, stays normal in an equal solution and shrivels in a concentrated solution.
With no cell wall, an animal cell can burst in a dilute solution.

Dilute urine and concentrated urine

  • Too much water in the body Less ADH is released; Collecting duct less permeable; Less water is reabsorbed
  • Too little water in the body More ADH is released; Collecting duct more permeable; More water is reabsorbed