Homeostasis

• Homeostasis is defined as the maintenance of a constant internal environment
• Homeostasis means that internal conditions within the body (such as temperature, blood pressure, water concentration, glucose concentration etc) need to be kept within set limits in order to ensure that reactions in body cells can function and therefore the organism as a whole can live

  • When one of these conditions deviates far away from the normal if not brought back within set limits the body will not function properly and the eventual consequence without medical intervention will be death
• Insulin is secreted into the blood at times when blood glucose levels are high

  • This is (most often) directly after a meal
• The kidneys can only cope with a certain level of glucose in the blood

  • If the level gets too high, glucose gets excreted and is lost in the urine
  • This is like running a car with a hole in the petrol tank; valuable fuel is being wasted
• To avoid this, insulin temporarily converts excess glucose into glycogen in the liver and muscles

  • Insulin decreases blood glucose concentration
• The glycogen is converted back to glucose several hours later when blood glucose levels have dipped due to respiration in all tissues
• Negative feedback occurs when conditions change from the ideal or set point and returns conditions to this set point
• Negative feedback works in the following way:

  • if the level of something rises, control systems are switched on to reduce itagain
  • 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

  A) conditions

  B) body

  C) set point

  D) change detected

  E) corrective mechanisms

  F) conditions

  G) set point

  H) corrective mechanisms

  I) off

  
• 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
• The terms glucagon and glycogen are very often mixed up by students as they sound similar. Remember:

  • Glucagon is the hormone
  • Glycogen is the polysaccharide glucose is stored as
• 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 reducesthe 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
• Control of body temperature is a homeostatic mechanism
• Homeostasis is the maintenance of a constant internal environment

  • This means that internal conditions within your body (such as temperature, blood pressure, water concentration, glucose concentration etc) need to be kept within set limits in order to ensure that reactions in body cells can function and therefore the organism as a whole can live
• The human body maintains the temperature at which enzymes work best, around 37°C

  • If body temperature increases over this temperature, enzymes will denature and become less effective at catalysing reactions such as respiration
• The Structure of the Skin

  A) hair

  B) free nerve

  C) ending

  D) sensitive

  E) pain

  F) temperature

  G) epidermis

  H) dermis

  I) touch

  J) pressure

  K) receptors

  L) sensory neurone

  M) fatty tissue

  N) adipocytes

  O) muscles

  P) arteriole

  Q) sweat gland

  R) capillaries

  S) muscle

  T) sweat pore

  
• Regulation is controlled by the brain which contains receptors sensitive to the temperature of the blood
• The skin also has temperature receptors and sends nervous impulses to the brain via sensory neurones

  • The brain responds to this information by sending nerve impulses to effectors in the skin to maintain the temperature within a narrow range of the optimum, 37°C
  • Fatty tissue under the dermis acts as a layer of insulation to prevent too much body heat being lost through the skin
• Homeostatic responses to changes in body temperature

  A) thermoreceptors

  B) hypothalamus

  C) skin

  D) change

  E) increased

  F) vasodilation

  G) flat

  H) skin

  I) decrease

  J) thermoreceptors

  K) hypothalamus

  L) skiin

  M) change

  N) vasoconstriction

  O) shivering

  P) erect

  Q) increase

  
• Responses to changes in temperature:

  A) sweat is secreted

  B) cools

  C) evaporation

  D) heat energy

  E) liquid water

  F) water vapour

  G) lie flat

  H) air

  I) freely circulate

  J) heat transfer

  K) radiation

  L) skeletal muscles

  M) shiver

  N) involuntary

  O) energy

  P) respiration

  Q) released as heat

  R) erect

  S) trap

  T) air

  U) skin

  V) insulator

  W) heat loss

  X) radiation

  
• When we are cold blood flow in capillaries slows down because arterioles leading to the skin capillaries get narrower - this is known as vasoconstriction

  • This reduces the amount of heat lost from blood by radiation as less blood flows through the surface of the skin
• When we are hot blood flow in capillaries increases because blood vessels to the skin capillaries get wider - this is known as vasodilation

  • This cools the body as blood (which carries heat around the body) is flowing at a faster rate through the skin’s surface and so more heat is lost by radiation
• Responses in the skin when hot

  A) erector

  B) relax

  C) heat loss

  D) radiation

  E) glands

  F) excrete

  G) cools

  H) evaporates

  I) dilate

  J) increase

  K) skin

  
• Responses in the skin when cold

  A) trapped air

  B) insulator

  C) erector

  D) contract

  E) upright

  F) trapping

  G) skin

  H) constrict

  I) blood flow

  J) skin

  K) heat energy

  L) hotter

  M) cooler

  N) radiation

  O) electromagnetic

  P) conduction

  Q) convection