homeostasis

• Homeostasis is the maintenance of a constant internal environment
• Basic principles of homeostasis:

  • Stimulus results from a change in the internal environment
  • Corrective mechanism and negative feedback are involved
• Sequence of events in the negative feedback loop:

  1. Receptors sense the change in the body’s internal environment

  2. Receptors send a signal to the control centre

  3. Control centre sends a signal to the effectors

  4. Effectors carry out corrective responses to reverse the stimulus

  5. Body condition returns to normal
• Importance of homeostasis:

  • Maintains a constant internal environment
  • Allows organisms to be independent of changes in the external environment
• Internal conditions that should be kept constant include:

  • Body temperature
  • Blood pH
  • Blood water potential
  • Blood glucose concentration
  • Thyroid regulation
  • Rate of photosynthesis in response to increased CO2 concentrations
• Enzymes require an optimum temperature and pH to function properly
• Composition of tissue fluid must be maintained to ensure water potential of cells is constant
• Glucose in food is required for tissue respiration which releases energy for cells
• Principles of homeostasis in a negative feedback loop:

  • Stimulus
  • Receptors
  • Corrective mechanism
  • Feedback to the receptors that the condition has been restored back to normal state
• Negative feedback loop components:

  • Normal or set point
  • Stimulus
  • Receptors
  • Corrective mechanism
• Pancreas plays a role in blood glucose regulation by secreting insulin and glucagon
• Negative feedback loop prevents overcorrection
• Components of negative feedback mechanism:

  • Set point
  • Stimulus
  • Receptor
  • Corrective mechanism
• Blood glucose is needed for cellular respiration, a process which releases energy for cells to carry out their vital activities
• Drastic changes in blood glucose concentration can be dangerous
• Normal blood glucose concentration is 70-90 mg of glucose per 100 cm3 of blood
• When blood glucose concentration decreases:

  • Islets of Langerhans in the pancreas detect the stimulus
  • Islets of Langerhans release more glucagon into the bloodstream
  • Liver converts stored glycogen to glucose for release into the bloodstream
  • Glucose concentration of blood increases
  • Negative Feedback
• When blood glucose concentration increases:

  • Islets of Langerhans in the pancreas detect the stimulus
  • Islets of Langerhans release more insulin into the bloodstream
  • Increase cell permeability to glucose
  • Liver and muscle cells convert excess glucose to glycogen for storage
  • Glucose concentration of blood decreases
  • Negative Feedback
• Effects of Insulin:

  • Stimulating the liver and muscle cells to convert glucose into glycogen for storage
  • Increasing the oxidation of glucose during tissue respiration
  • Making cell membranes more permeable to glucose
  • Increasing the rate of glucose uptake by cells
  • Insulin decreases blood glucose concentration
• Effects of Glucagon:

  • Stimulating liver cells to convert glycogen to glucose
  • Stimulating liver cells to convert fats and amino acids to glucose
  • Glucagon increases blood glucose concentration
• Insufficient insulin secretion leads to persistently high levels of glucose in the blood, causing glucose to be excreted in urine
• Diabetes mellitus is a metabolic disorder in which the body is unable to regulate blood glucose levels
• Types of diabetes:

  • Type I diabetes: Insufficient or no insulin production, treated by insulin injections
  • Type II diabetes: Insulin is produced but target cells do not respond well to insulin or insulin is insufficiently produced, treated by controlling dietary intake and exercising
• Diabetes mellitus symptoms include consistently high blood glucose concentration, presence of glucose in urine, slow healing of wounds, frequent urination, and weight loss
• Core body temperature increases above 40 ⁰C and the body is unable to lose the extra heat
• Common symptoms of heat stroke include: rapid pulse, disorientation, nausea, vomiting
• Thermoregulation helps to maintain the human body temperature at about 37 °C
• Heat gain processes:

  • Metabolic activities such as respiration within the body
  • Vigorous physical activities
  • Consumption of hot food and drinks
  • Being in warm environments
• Heat loss processes:

  • From the skin surface through convection, radiation and conduction
  • Evaporation of sweat
  • Faeces and urine
  • Air that is exhaled
• Hypothalamus monitors and regulates body temperature
• When body temperature increases above normal:

  • Vasodilation of arterioles near the skin’s surface
  • Increased production of sweat
  • Decreased metabolic rate
  • Increased heat loss + decreased heat production = Body temperature decreases
• When body temperature decreases below normal:

  • Vasoconstriction of arterioles near the skin’s surface
  • Decreased production of sweat
  • Increased metabolic rate
  • Shivering occurs
  • Decreased heat loss + increased heat production = Body temperature increases
• Vasodilation of arterioles, constriction of shunt vessels leads to more blood to the skin capillaries resulting in more heat removed by conduction, convection, and radiation
• Sweat glands become more active and secrete more sweat, leading to more latent heat of vaporization being removed when water in sweat evaporates
• Rate of metabolic activities slow down when body temperature increases, resulting in less heat produced within the body
• Sweat glands become less active when body temperature decreases, leading to less sweat being produced and less latent heat being removed when water in sweat evaporates
• Rate of metabolic activities increase when body temperature decreases, resulting in more heat being produced within the body
• Shivering occurs when more heat production is needed to prevent a drop in temperature, which generates heat released via respiration to increase body temperature to normal
• Hair erector muscles contract when body temperature decreases, causing hairs to "stand up" and trap an insulating layer of warm air over the skin
• Sweating regulates temperature by the evaporation of sweat, which takes away heat through evaporation