homeostasis

• Homeostasis is the process where the body’s internal environment is maintained within tolerance limits
• Aspects of the internal environment that needs regulating:

  • core body temperature
  • pH and concentration of dissolved substances
  • concentration of glucose in blood
  • O2 and CO2 concentration in blood and other fluids
  • blood pressure
  • concentration of metabolic wastes
• Tissue fluid:

  • bathes ALL cells
  • represents the cells’ immediate environment
  • provides optimum conditions for cell function
• Homeostasis operates through feedback systems
• Features of a feedback system:

  • stimulus - change in the environment causing the system to operate
  • receptor - detects the change
  • modulator - processes information
  • effector - carries out a counteracting/enhancing response
  • feedback - original stimulus changed by the response
• Feedback systems:

  • can be negative or positive
  • circular situation where the body responds to a stimulus
  • response alters original stimulus → feedback provided
• Homeostatic mechanisms are controlled by both the nervous and endocrine system
• Negative feedback: (steady state control systems)

  • response reduces or eliminates the stimulus
  • dynamic equilibrium - state of rates of reverse and forward changes are equal
  • set point - point which conditions fluctuate
  • tolerance limits - upper and lower limits between fluctuation levels
• Positive feedback:

  • has no role in homeostasis
  • response to stimulus intensifies it
  • eg. childbirth, blood clots
• Heat gain and loss:

  (To maintain optimum temperature, heat gained by the body must equal heat loss)

  • 36.8 degrees - constant temperature
  • 37 degrees - optimum temperature
  • heat gain = heat loss
• Increased body temperature leads to:

  • nerve malfunction
  • structual changes of proteins
  • death
• Heat input:

  • metabolism (respiration)
  • conduction and radiation
• Heat output:

  • conduction and convection
  • water evaporation from skin and lungs
  • warm air breathed out
  • warm urine and faeces
• Heat production:

  • food oxidised → energy released
  • metabolic rate = energy release rate
• Factors affecting metabolic rate:

  • exercise - increase up to 40x
  • stress - noradrenaline increases rate
  • body temperature - 1 degree rise → 10% rate increase
• Thermoreceptors:

  • peripheral - skin and mucous membranes
  • detect temperature in external environment
  • sends information to hypothalamus
  • central - hypothalamus
  • detects temperature in internal environment
  • other locations - spinal cord and abdominal organs
• Temperature regulation: (heat transfer)

  • radiation - no direct contact (sunlight)
  • convection - air passes over the body (fan)
  • conduction - direct contact
  • evaporation - liquid → gas (absorbs heat)
• Blood vessels:

  • vasodilation - heat loss rate increases
  • vasoconstriction - heat loss rate decreases
• Sweating:

  • active secretion of fluid via sweat glands
  • stimulated by sympathetic nerves
  • sweat - water containing dissolved substances
  • effect - cooling effect
• Shivering:

  • due to increase in skeletal muscle tone
  • produces rhythmic muscle tremors (10-20/s)
  • heat produced from muscles → released
• Preventing body temperature from falling:

  • vasoconstriction → less heat loss
  • adrenaline and noradrenaline secreted
  • shivering → increases body heat production
  • increased T4 production
  • behavioural response (jumper, shelter)
• Preventing body temperature from rising:

  • vasodilation → more heat loss
  • sweating → more heat loss (inaffective when humid)
  • decreased metabolic rate → less heat produced
  • behaviour response (fan, stripping)
• Temperature tolerance:

  • heat stroke - regulatory mechanisms cease
  • heat exhaustion - blood pressure and output decreases
  • hypothermia - heat production cannot replace heat loss (33 degrees)
• Extracellular fluid includes the:

  • intravascular fluid - blood plasma
  • interstitial fluid - between the cells
  • transcellular fluid - in specific body regions
• Water in:

  • food - 0.7L
  • drink - 1.6L
  • metabolic water - 0.2L
• Water out:

  • urine - 1.5L
  • sweating - 0.5L
  • lungs - 0.3L
  • faeces - 0.2L
• Maintaining fluid balance:

  • 2.5L lost per day
  • fluid gain = fluid loss
  • fluids are lost through kidneys, skin, lung surface and alimentary canals
• The kidneys:

  • 99% of water is reabsorbed
  • osmosis - PCT and LH
  • active reabsorption - DCT and CD
  • level of active reabsorption controlled by ADH (permeability)
  • aldosterone acts on DCT and CD via active transport (sodium-potassium pump)
• Aldosterone is secreted in response to a:

  • decrease in concentration of sodium ions in blood
  • increase in concentration of potassium ions in blood
  • decrease in blood volume and pressure
• Dehydration:

  • not enough water for bodily functions
  • symptoms appear after 2% body water loss
  • symptoms: thirst, low blood pressure, dizzyness, headache
• Water intoxication:

  • when body fluids become diluted
  • may occur when loss is replaced with normal water
  • symptoms: lightheaded, headache, vomiting, collapse
• Glucose and glycogen:

  • glucose is stored as glycogen
  • 500g of glycogen is stored
  • 100g in liver and remainder in skeletal muscles
• Role of the liver:

  • converts glucose into glycogen or vice versa
  • blood supply mostly comes from the hepatic portal vein
  • liver has first chance to absorb nutrients from food
• When at the liver glucose may:

  • be removed from blood to provide energy for the liver
  • be converted to glycogen for storage
  • continue to circulate in the blood
  • be converted into fat for long-term storage
• Glycogenesis - glucose → glycogen (to store excess glucose)

  Glycogenolysis - glycogen → glucose (released into the blood)
• Role of the pancreas:

  • blood levels above normal → chemoreceptors in beta cells stimulates insulin secretion
  • blood levels below normal → chemoreceptors in alpha cells stimulates glucagon secretion
• Insulin decreases blood glucose levels by:

  • accelerating glucose transport from blood to cells
  • accelerating glycogenesis
  • stimulating protein synthesis and lipogenesis
• Glucagon increases blood glucose levels by:

  • stimulating glycogenolysis and glucogenesis
  • having a mild stimulating effect on protein catabolism
• Cortisol:

  • stimulates glycogen to glucose conversion
  • stimulates protein catabolism
  • stimulates amino acids to glucose conversion
• Adrenaline and noradrenaline:

  • stimulates glycogen catabolism
  • stimulates release of glucose into blood