Homeostasis

• Thermoregulation


  Homeostatic process of maintaining a constant, internal temperature within narrow limits (36.5 - 37.5C)
• Homeostasis


  The body's ability to maintain a stable internal environment, within narrow limits

  • principles of homeostasis are the stimulus receptor response and a negative feedback loop
• Maintaining Homeostasis

  • Organisms can detect changes in their internal and external environment and coordinate responses to these changes to restore balance
  • changes are detected by receptors inside and outside of the cell
  • in animals, the endocrine and nervous system are the major systems responsible for the control and coordination of homeostasis
  • homeostasis is maintained through the interaction between the nervous and endocrine system
  • the nervous system may stimulate the release of hormones
• Negative feedback loop


  The response counteracts or negates the stimulus.

  • the original stimulus detected is reduced by the response for the body to re-achieve equilibrium (within narrow limits)
  • examples: thermoregulation, osmoregulation, blood glucose regulation
• Stimulus-response model


  Stimulus → Receptor → Modulator (control centre) → Effectors → Response

  • within the body, if there is a change in the level of a variable of the external or internal environment (a stimulus), that change is often detected by a cell or organ (receptor). the receptor communicates via a control centre with another organ or cell (an effector) which produces a response.
• Regulation of body temperature - 2 types

  Homeothermy - the regulation of body temperature through homeostatic mechanisms. homeotherms

  Poikilothermy - the regulation of body temperatures through structural adaption and behaviours alone. without physiology. poikilotherms
• Regulation of body temperature - Reducing body temperature

  • sweating
  • panting
  • vasodilation of arterioles near the skin surface and extremities.
  • arrector pili muscles relax (pilorelaxation)
  • decrease the secretion of thyroxin to reduce metabolic rate
  • increasing SA:V, so more heat is lost through radiation (stretching body out)
• Sweating


  Sweating reduces internal body temperature by secreting water onto the surface of the skin. This water is evaporated and takes heat energy with it as it is converted from a liquid to a gas.
• Endothermic vs Exothermic


  Endotherms (homeotherms) use internally generated heat to maintain body temperature. Their body temperature tends to stay steady regardless of the environment.

  Ectotherms (poikilotherms) depend mainly on external heat sources, and their body temperature changes with the temperature of the environment.
• Regulation of body temperatures - Increasing body temperature

  • shivering of skeletal muscles, other muscular activity
  • vasoconstriction of blood vessels near the skin surface and extremities (decrease heat lost due to radiation)
  • contraction of arrector pili muscles in the skin (traps a layer of warm air around the body - insulation)
  • basic metabolic processes
  • eating more → increases metabolic rate → convert chemical energy to heat
  • increased secretion of thyroxine (stimulates thyroid which is in charge of the metabolic rate)
• Thermoregulation Increase in temp: Stimulus-response model

  • Stimulus (increase in body temp)
  • Receptor ( thermoreceptors in skin or hypothalamus)
  • Control Centre (CNS)
  • Effector (skeletal muscle, skin arterioles, thyroid produces less thyroxine)
  • Response (decreased muscle activity, panting, sweating, decreased metabolism)

  Negative Feedback (decrease in body temp).
• Thermoregulation Decrease in temp: Stimulus-response model

  • Stimulus (decrease in body temp)
  • Receptor (thermoreceptors in skin or hypothalamus)
  • Control Centre (CNS)
  • Effector (skeletal muscle, skin arterioles, thyroid produces more thyroxine
  • Response (increased muscle activity, shivering, vasoconstriction, piloerection)

  Negative Feedback (increase in body temp)
• Blood glucose 


  Glucose plays many key roles and must be maintained within a narrow range for the body to function correctly. Maintains constant blood glucose levels by releasing insulin to lower blood glucose levels and glucagon to increase blood sugar levels.

  
  

  average is 4-5.9 mmol/L
• Regulating Blood glucose levels - Islets of Langerhans

  The islets of Langerhans are endocrine tissue in the pancreas that measures blood glucose concentration and releases 2 hormones

  • insulin from beta cells
  • glucagon from alpha cells

  Is both a receptor and modulator/control centre
• Insulin

  • Secreted by beta cells in the Islets of Langerhans (pancreas)
  • Targets liver cells, skeletal muscle, and fat cells to convert glucose into glycogen and store that inside themselves
  • Inserts glucose transporters into cell membranes
  • in response to a high blood glucose level (>5mmol/L)
• Glucagon

  • Secreted by alpha cells in the Islets of Langerhans (pancreas)
  • Targets mostly liver cells to convert glycogen to glucose, and release that into the bloodstream
  • in response to a low blood glucose level (<5mmol/L)
• blood glucose levels are high: Stimulus-response model

  • Stimulus (blood glucose level >5mmol/L)
  • Receptors AND modulator (islets of Langerhans detect this, alpha cells respond by producing less glucagon, beta cells respond by producing more insulin)
  • Effectors (Liver is stimulated to store more glycogen, release less glucose)
  • Response (blood glucose levels fall back within the narrow limits, 4-5.9 mmol/L)

  Negative feedback
• blood glucose levels are low: Stimulus-response model

  • Stimulus (blood glucose <5mmolL)
  • Receptors AND modulator (islets of langerhans detect this, alpha cells respond by producing more glucagon, beta cells respond by producing less insulin)
  • Effectors (Liver is stimulates to release glucose into bloodstream)
  • Response (blood glucose levels rise back within the narrow limits)

  Negative feedback
• Type 1 Diabetes

  • caused by an autoimmune disorder in which cytotoxic T cells (Tc) inappropriately target and kill the beta cells in the islets of Langerhans
  • thus, it is a disease resulting from insufficient or no insulin production
  • onset is sudden and typically in younger people
  • people with type 1 diabetes must take insulin injections
  • can lead to hyperglycemic if untreated, or death
• Type 2 Diabetes

  • usually develops over a long period, later in life
  • liver cells and other cells become resistant to insulin and do not reduce glucose levels appropriately
  • blood sugar levels stay high because the liver cells and skeletal cells do not respond
  • beta cells respond by producing greater amounts of insulin since they can detect that the blood sugar levels are still high. this wears the beta cells out and they start to die.
  • now there's insulin resistance and lowered levels of insulin production
  • can be treated through diet, exercise and several medications. not insulin injections
• Symptoms of Diabetes 1

  • Beta cells are destroyed
  • The body's cells without insulin cannot take up glucose
  • lack an energy source for cell metabolism
  • Kidneys increase the volume of urine to clear the body of excess glucose
  • constant thirst to compensate for excessive urine production, increased hunger and weight gain
  • muscles are weak and fatigued due to the inability to utilise glucose
• Hypoglycaemia

  • blood sugar level is below normal (<4mmol/L)
  • most common cause in mediation for insulin. often, an increase in insulin (through eating less than usual, exercising more than usual, drinking alcohol, injected too much insulin) can lead to low blood sugar levels
  • also caused by kidney disease, liver disease, starvation, infections, tumours.
• Hypoglycaemia symptoms

  • clumsiness
  • trouble walking
  • confusion
  • loss of consciousness
  • death
• Hyperglycaemia


  An increase in blood sugar can happen to anyone due to sugary foods.Dry mouth, Increased thirst, weakness, headache, blurred vision, Frequent urination
• Hyperthyroidism

  • a condition that results from an overactive thyroid gland. the thyroid gland produces thyroxine which controls numerous body functions, especially metabolic rate
  • most common cause of hyperthyroidism is an autoimmune disorder called Graves' disease
  • b lymphocytes mistakenly produce antibodies against the TSH receptors on thyroid cells. when the antibodies bind to the TSH receptors, they stimulate them to produce thyroxine, just as TSH does
  • can also be caused by increased iodine consumption and medications for hypothyroidism
• Hyperthyroidism Symptoms

  • too much thyroxine
  • can cause anxiety, excessive sweating, weight loss, an increased heart rate, difficulty sleeping, and goitre (swollen thyroid gland) since the metabolism is too fast
  • also, unexplained weight loss, sensitivity to heat
• Hypothyroidism

  • The thyroid gland is underreactive and fails to secrete enough hormones into the bloodstream.
  • Causes include autoimmune condition Hasimate's disease and insufficient dietary iodine
  • Treatment is lifelong hormone replacement with thyroxine tablets.
• Hypothyroidism Symptoms

  • too little thyroxine
  • can cause cold intolerance, decreased sweating, depression and irritability,
  • also, slow heart rate, muscle or joint pain, weight gain
• Osmoregulation

  The process by which an organism regulates the water balance in its body and maintains the homeostasis

  It includes controlling excess water loss or gain and maintaining the fluid balance and the osmotic concentration, that is, the concentration of electrolytes. Osmoregulation is a method of water balance in which receptors detect the concentration of solutes in the blood.
• How water enters the body

  • drinking
  • eating
  • metabolic water from cellular respiration
  • equation is O2 + C6H12O6 --> CO2 + H2O

  drinking and eating can be adjusted to increase or decrease water balance
• How water exits the body

  • sweating
  • breathing (water vapour)
  • urination
  • defaecation
• Regulating water balance

  • most of the water lost from the body is lost in ways that cannot be regulated (can't regulate how much you breathe, how much you sweat if your body temperature is too high, your metabolic water).
  • the amount of water lost in urine can be regulated though!
  • 2 stimulus-response pathways
• Osmoregulation - ADH stimulus-response model pathway DECREASE

  • Stimulus (decrease in water levels)
  • Receptors (osmoreceptors detect increases in osmolality)
  • Modulator (hypothalamus causes a release of ADH from the posterior pituitary gland)
  • Effectors (aquaporins inserted into the distal convoluted tubule and collecting duct of kidneys, stimulation of cells in thirst centre)
  • Response (increased reabsorption of water from kidney filtrate, and increased consumption of fluid → increased water level)

  Negative feedback
• Osmoregulation - ADH stimulus-response model pathway INCREASE

  • Stimulus (increase in water levels)
  • Receptors (osmoreceptors detect decreases in osmolality)
  • Modulator (hypothalamus suppresses the release of ADH from the posterior pituitary gland)
  • Effectors (decreased aquaporins insertion into the distal convoluted tubule and collecting duct of kidneys, thirst centre cells are suppressed)
  • Response (decrease in reabsorption of water from kidney filtrate, and decreased consumption of fluid → decreased water level)

  Negative feedback
• Osmoregulation - Renin secretion stimulus-response model pathway INCREASE

  • Stimulus (increase in water levels)
  • Receptors (baroreceptors detect increases in blood pressure and volume)
  • Modulator (hypothalamus suppresses the release of ADH from the posterior pituitary gland)
  • Effectors (decreased aquaporins insertion into the distal convoluted tubule and collecting duct of kidneys, thirst centre cells are suppressed)
  • Response (decrease in reabsorption of water from kidney filtrate, and decreased consumption of fluid → decreased water level)

  Negative feedback
• Osmoregulation - Renin secretion stimulus-response model pathway DECREASE

  • Stimulus (decrease in water levels)
  • Receptor (baroreceptors detect fall in blood pressure and volume)
  • Modulator (release of renin from kidney cells, causing release of aldosterone, and hypothalamus causes release of ADH from posterior pituitary gland
  • Effector (activation of sodium-potassium pumps, increasing reabsorption of sodium + ADH pathway effectors)
  • Response (same as ADH pathway)

  Negative feedback
• Renin

  • if the blood pressure falls, renin is secreted within the kidney. it initiates the release of aldosterone from adrenal cortex, which acts on distal tubules actively absorbing sodium ions from tubules, water follows and blood pressure rises
• Postive Feedback

  A positive feedback loop occurs in nature when the product of a reaction leads to an increase in that reaction