Homeostasis Notes

Homeostasis

Regulation of Water Balance in Vascular Plants

  • This topic is part of the study design.

Regulation of Body Temperature, Blood Glucose, and Water Balance in Animals

  • Homeostatic mechanisms, including stimulus-response models, feedback loops, and associated organ structures, regulate these factors.

Malfunctions in Homeostatic Mechanisms

  • Examples include type 1 diabetes, hypoglycemia, and hyperthyroidism.

Endocrine System

  • Composed of glands producing chemical messengers called hormones.
  • Hormones travel through the bloodstream to target organs.

Endocrine Glands

  • Pituitary gland
  • Thyroid gland
  • Parathyroid glands
  • Thymus
  • Adrenal glands
  • Pancreas
  • Ovary and Testis

Homeostasis Definition

  • Maintenance of a relatively stable internal environment despite external changes.
  • Essential for complex animals and maintained by organ systems.
  • Examples of inputs and outputs: Heart, Cells, Food, Metabolic wastes (urine), Unabsorbed matter, CO<em>2CO<em>2, O</em>2O</em>2

Homeostatic Control Systems

  • Three functional components:
    • Receptor: Detects change.
    • Control center (modulator): Processes information.
    • Effector: Produces a response.
  • Examples: Brain, Muscle cells, Rod or cone cells

Thermostat Analogy

  • Heater with sensors (receptor) monitors room temperature.
  • Control center processes data and activates the effector (heating unit).
  • Maintains constant temperature by switching on when cold and off when hot.

Thermostat Function

  • Regulates temperature to maintain it near a setpoint.
  • When room temperature is below the setpoint, it increases.
  • When room temperature is above the setpoint, it decreases.
  • Setpoint is reached in both scenarios.

Maintaining Homeostasis: Organ Systems

  • Circulatory system: Distributes gases, nutrients, and hormones.
  • Digestive system: Digests and absorbs food, providing nutrients.
  • Respiratory system: Exchanges gases with the environment, providing oxygen and removing carbon dioxide.
  • Urinary system: Disposes of waste, regulates ion balance, and controls blood volume and pressure.

Feedback Systems

  • A stimulus from one part of the body invokes a response from another to alter the original stimulus.
  • Regulated by positive or negative feedback systems.
  • Components: Stimulus, Receptors, Control center, Effectors, Response.

Negative Feedback

  • Most common in the body.
  • Restores conditions to a steady state.
  • The response reduces or stops the original stimulus.
  • Components: Stimulus disrupts homeostasis, Receptors, Control center, Effectors, Response returns to homeostasis.

Stomach Emptying

  • Example of negative feedback.
  • Stretching of the stomach wall triggers muscle movement, leading to emptying and return to the original state.
  • Components: Food entering, Stretch receptors, Stomach muscles, Submucosal plexus, Mixing and emptying.

Positive Feedback

  • Amplifies or speeds up a physiological response.
  • Uncommon in the body.
  • Causes a large departure from the original condition.
  • Enhances the effect of the stimulus, leading to escalation.
  • Components: Stimulus disrupts homeostasis, Receptors, Control center, Effectors, Response

Childbirth

  • Example of positive feedback.
  • Oxytocin stimulates stronger contractions, leading to more oxytocin release.
  • Cycle ends with the delivery of the infant and expulsion of the placenta.
  • Components: Labor contractions, Hypothalamus and pituitary, Stretch receptors, Uterine contractions.

Thermoregulation

  • Maintaining internal environment at about 37°C

Learning Objectives

  • Understand thermoregulation.
  • Understand how the body detects temperature changes.
  • Understand how the body restores normal temperature.

Success Criteria

  • Relate thermoregulation to negative feedback loops.
  • Describe how corrective mechanisms work.

Hypothalamus

  • Located below the thalamus, above the brainstem and pituitary gland.
  • Controls and integrates physiological activities:
    • Temperature regulation
    • Food and fluid intake
    • Sleep

Source of Body Heat

  • Endothermic: Mammals and birds gain heat internally.
  • Ectothermic: Reptiles gain heat externally.
  • Energy requirement: Reptiles require only 10% of the energy of mammals with the same body mass.

Thermoregulation Control

  • Controlled by the hypothalamus, which has a set point of 36.7°C in humans.
  • Responds to core temperature changes and nerve impulses from skin receptors.
  • Coordinates nervous and hormonal responses.

Skin and Thermoregulation

  • Thermoreceptors in the dermis detect changes in skin temperature.
    • Hot thermoreceptors: detect rises above 37.5°C.
    • Cold thermoreceptors: detect drops below 35.8°C.
  • Negative feedback loop.
  • Blood vessels dilate (vasodilation) to promote heat loss and constrict (vasoconstriction) to reduce heat loss.
  • Sweat glands produce sweat for evaporative cooling.
  • Fat insulates organs against heat loss.
  • Hairs raise or lower to adjust the insulating air layer.

Heat Transfer Mechanisms

  • Convection.
  • Radiation.
  • Conduction.
  • Evaporation.

Stimulus-Response Model for Thermoregulation

  • Stimulus: Change in internal or environmental temperature.
  • Receptor: Thermoreceptors.
  • Modulator: Hypothalamus.
  • Effector: Variety of cells and tissues.
  • Response: Change that alters heat transfer in the body.

Responding to High Temperature

  • Stimulus: Increase in body temperature.
  • Receptors: Thermoreceptors.
  • Modulator: Hypothalamus.
  • Effectors and Responses:
    • Sweat glands produce more sweat for evaporation.
    • Vasodilation increases surface blood flow.
    • Cerebral cortex changes behavior.
    • Arrector pili muscles relax.
    • Metabolic processes slow down.

Counteracting Heat Loss

  • Factors affecting heat loss:
    • Thermal gradient increased by wind (wind chill), cold temperatures, and inadequate clothing.
    • Being wet or in cold water accelerates conductive heat loss.
    • Dehydration and circulatory shock lower blood volume, reducing heat retention.
  • Responses:
    • Heat-promoting center in the hypothalamus monitors temperature falls below 35.8°C.
    • Thyroxine and adrenaline increase metabolic rate.
    • Muscle activity produces internal heat.
    • Erector muscles contract to raise hairs.
    • Blood flow to skin decreases.

Responding to Low Temperature

  • Stimulus: Decrease in body temperature.
  • Receptors: Thermoreceptors.
  • Modulator: Hypothalamus.
  • Effectors and Responses:
    • Skeletal muscles cause shivering to increase metabolic processes.
    • Vasoconstriction reduces surface blood flow.
    • Cerebral cortex changes behavior.
    • Arrector pili muscles contract, causing goosebumps.
    • Metabolic processes increase.

Counteracting Heat Gain

  • Factors causing heat gain:
    • Warmer environment.
    • High humidity reduces evaporative cooling.
    • Excessive fat deposits.
    • Heavy clothing.
  • Responses:
    • Heat-losing center in the hypothalamus monitors rises above 37.5°C.
    • Sweating increases cooling.
    • Muscle tone and metabolic rate decrease.
    • Blood flow to skin increases.
    • Erector muscles relax.

Body Shape and Heat Loss

  • Animals with a lower surface area to volume ratio lose less heat.
  • Animals in cooler climates have shorter limbs to conserve heat.
  • Animals in warmer climates have longer limbs to increase surface area for heat loss.

Departures from Normal

  • Hyperthermia: Temperatures above normal cause metabolic problems and can lead to death.
  • Mild hypothermia: Shivering, vasoconstriction, hypertension, and cold diuresis.
  • Moderate hypothermia: Muscle incoordination, slow movements, blue extremities, mental confusion, paradoxical undressing.
  • Severe hypothermia: Speech failure, irrational mental processes, stupor, terminal burrowing, organ failure, and death.

Stimulus-Response Model for Homeostatic Regulation

  • Involves monitoring a variable, detecting movements, and making adjustments with feedback.
  • Stimulus: Change in the variable.
  • Receptor: Detects the change.
  • Modulator: Evaluates changes and sends information to the effector (hypothalamus).
  • Effector: Adjusts the output.
  • Response: Corrective action.
  • Negative feedback: Response counteracts the change.

Negative Feedback Loop Example

  • Fall in body temperature leads to shivering.

Drop in Core Body Temperature

  • Decrease in core body temperature.
  • Detected by receptors in the skin, organs, and hypothalamus.
  • Hypothalamus sends signals via nerve and hormonal systems to effectors.
  • Effectors: Skeletal muscles, blood vessels, body cells, cerebral cortex.
  • Shivering, vasoconstriction, increased metabolic rate, behavioral changes.

Rise in Core Body Temperature

  • Increase in core body temperature.
  • Detected by receptors in the skin, organs, and hypothalamus.
  • Hypothalamus sends signals via nerve and hormonal systems to effectors.
  • Effectors: Sweat glands, blood vessels, body cells, cerebral cortex.
  • Sweating, vasodilation, increased metabolic rate, behavioral changes.

Hypothermia

  • Core temperature drops below 35°C.
  • Ranges from mild to severe.
  • Severe: Mental confusion, inability to speak, amnesia, organ failure, and death.

Hypothermia and Exposure

  • Factors determining onset:
    • Water temperature.
    • Body shape: Shorter, stockier people lose heat more slowly.

Hyperthermia

  • Core temperature rises above normal (around 38°C).
  • Caused by absorbing or generating heat faster than it can be dissipated.
  • Severe: Enzymes and organ systems fail.
  • Can be induced by drugs and used in cancer treatment.

Thyroid Gland

  • Large endocrine gland in the neck.
  • Secretes thyroid hormones, mainly thyroxine (T4).
  • Thyroxine levels are regulated by negative feedback.
  • TRH (thyrotropin-releasing hormone) from the hypothalamus stimulates thyroxine release.
  • Thyroxine increases metabolic rate and temperature.
  • Changes detected by the hypothalamus counteract further TRH and T4 release.

Thyroid Hormone Regulation

  • The hypothalamus releases TRH.
  • The pituitary gland releases TSH (thyroid-stimulating hormone).
  • The thyroid gland releases thyroxine.
  • Thyroxine inhibits TRH and TSH release (negative feedback).

Response to Cold Environment

  • Heat loss increases.
  • Body temperature falls.
  • Hypothalamus receives information about the temperature fall.
  • Thermostat in the hypothalamus activates 'warming-up' mechanisms
  • Neurosecretory cells in the hypothalamus produce TRH
  • Anterior pituitary gland produces TSH
  • Thyroid produces thyroxine
  • Skeletal muscles are activated; shivering generates heat
  • Skin arterioles constrict, diverting blood to deeper tissues, reducing heat loss from the skin surface
  • General increase in metabolism
  • Behavioural change such as adding
    clothing or jumping up and down

Hyperthyroidism and Thermoregulation

  • Overactive thyroid produces too much thyroxine.
  • Disrupts temperature regulation.
  • Common cause: Graves disease, where thyroid stimulating immunoglobulin (TSI) bypasses the negative feedback loop.
  • People often have goiter (enlarged thyroid gland).