CIE IGCSE Biology: 复习笔记：14.1.9 Homeostasis

• Negative feedback occurs when conditions change from the ideal or set point and returns conditions to this set point
• It works in the following way:
  • if the level of something rises, control systems are switched on to reduce it again
  • if the level of something falls, control systems are switched on to raise it again

   

• Negative feedback mechanisms are usually a continuous cycle of bringing levels down and then bringing them back up so that overall, they stay within a narrow range of what is considered ‘normal’

The negative feedback cycle

• Blood glucose levels are controlled by a negative feedback mechanism involving the production of two hormones - insulin and glucagon
• Both hormones which control blood glucose concentration are made in the pancreas
• Insulin is produced when blood glucose rises and stimulates liver and muscle cells to convert excess glucose into glycogen to be stored
• Glucagon is produced when blood glucose falls and stimulates liver and muscle cells to convert stored glycogen into glucose to be released into the blood

Negative feedback regulation of blood glucose levels

• Type 1 diabetes is a condition where the blood glucose levels are not able to be regulated as the insulin-secreting cells in the pancreas are not able to produce insulin
• This means that blood glucose levels are often far too high
• It can be treated by injecting insulin
• The extra insulin causes the liver to convert glucose into glycogen, which reduces the blood glucose level
• Symptoms of diabetes include extreme thirst, weakness or tiredness, blurred vision, weight loss and loss of consciousness in extreme cases
• People with Type 1 diabetes have to monitor their blood glucose levels throughout the day as their levels of physical activity and their diet affect the amount of insulin needed
• They can help to control their blood glucose level by being careful with their diet -  eating foods that will not cause large increases in blood glucose level, and by exercising, which can lower blood glucose levels due to increased respiration in the muscles