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Physiological Regulatory Mechanisms: Ingestion, Water Balance, and Feeding

Physiological Regulatory Mechanisms

  • Homeostasis: process by which the body’s substances and characteristics are maintained at their optimal level.
  • System variable: variable controlled by a regulatory mechanism.
  • Set point: optimal value of the system variable.
  • Detector: signals when the system variable deviates from the set point.
  • Correctional mechanism: mechanism capable of changing the value of the system variable.
  • Negative feedback: the effect of an action serves to diminish or terminate that action.
  • Thermostat analogy:
    • Detector → Air temperature (system variable) → Heat (correctional mechanism) → Temperature setting (set point) → Negative feedback
    • Electric heater (correctional mechanism)

Fluid Compartments and Balance

  • Intracellular fluid (ICF): fluid inside cells.
  • Extracellular fluid (ECF): fluids outside cells, includes:
    • Interstitial fluid
    • Intravascular fluid (blood plasma)
  • Isotonic: equal osmotic pressure to the contents of a cell.
  • Relative sizes of fluid compartments: ext{Intracellular} = 67 ext{ extpercent}, ext{ Interstitial} = 26 ext{ extpercent}, ext Intravascular} = 7 ext{ extpercent}, ext{CSF} < 1 ext{ extpercent}

Osmotic and Hypovolemic Thirst

  • Two types of thirst:
    • Osmometric (osmotic) thirst: due to increased osmotic pressure of interstitial fluid relative to intracellular fluid.
    • Hypovolemic (volumetric) thirst: due to reduced intravascular (blood) volume.
  • Osmoreceptors detect changes in solute concentration of interstitial fluid.
  • Receptors for thirst are located in circumventricular organs near the third ventricle:
    • OVLT (organum vasculosum of the lamina terminalis)
    • SFO (subfornical organ)
  • Circumventricular organs lack a typical blood–brain barrier.
  • Stimuli and receptors:
    • Osmotic thirst: detected by osmoreceptors in OVLT/SFO.
    • Hypovolemic thirst: detected by receptors that sense blood volume and by angiotensin II signaling.
  • Angiotensin II role in thirst and salt appetite; RAAS involvement with hypovolemia.
  • Receptors for osmotic/hypovolemic signals:
    • OVLT and SFO near the third ventricle.
    • SFO contains neurons that detect circulating angiotensin II.

Hormonal Control of Water Balance

  • Vasopressin (antidiuretic hormone, ADH):
    • Released by posterior pituitary.
    • Increases blood pressure via vasoconstriction; promotes water reabsorption in kidneys; concentrates urine.
    • Helps compensate for decreased water volume.
    • Also implicated in reproductive behaviors.
    • Inhibited by alcohol.

Volumetric Thirst and RAAS

  • Hypovolemia triggers compensatory mechanisms to restore blood volume.
  • Renin: hormone secreted by kidneys that converts angiotensinogen to angiotensin II: ext{Renin} + ext{Angiotensinogen}
    ightarrow ext{Angiotensin II}
  • Angiotensin II: constricts blood vessels, promotes sodium and water retention, and stimulates thirst and salt appetite.
  • SFO contains neurons that detect circulating angiotensin II and stimulate drinking.

Osmotic vs Hypovolemic Thirst: Quick Comparison

  • Stimulus:
    • Osmotic: high solute concentration outside cells → water moves out of cells.
    • Hypovolemic: low blood volume.
  • Best relief:
    • Osmotic: water only.
    • Hypovolemic: water with solutes.
  • Receptors:
    • Osmotic: OVLT and SFO.
    • Hypovolemic: receptors measuring blood pressure in veins and the SFO.
  • Hormonal influences:
    • Osmotic: vasopressin secretion; angiotensin II increases with certain stimuli.
    • Hypovolemic: increased by angiotensin II.

Integration of Water Balance

  • Overall integration at the Median Pre-optic Nucleus (MPON): OVLT, SFO and nucleus of the solitary tract connect to the MPON.
  • Output from MPON drives drinking behavior.

Feeding and Energy Homeostasis: Short-Term Signals

  • Feast or famine concept: need for fuel and building blocks; balance with availability, metabolism, body weight, and nutrition.
  • Glucose, insulin, and glucagon:
    • Most digested fuel enters bloodstream as glucose; brain primarily uses glucose.
    • After a meal: liver converts excess glucose to glycogen; fat cells store energy as fat.
    • During low glucose: liver glycogen → glucose.
    • Insulin rises around meals and after meals; high insulin generally decreases appetite.
  • Blood chemical control of eating: pancreatic hormones regulate glucose homeostasis.

Metabolic Signals and Early Meal Initiation

  • Glucoprivation: dramatic fall in cellular glucose availability.
  • Lipoprivation: dramatic fall in fatty acids available to cells.
  • Brain uses glucose; other cells use both glucose and fatty acids.
  • Liver contains sensory neurons that detect decreases in glucose and signal the brain via the vagus nerve (cranial nerve X).

Nutrient Receptors and Long-Term Satiety

  • Receptors for glucose and lipids are located in the liver and convey signals about gluco- and lipoprivation to the brain via the vagus nerve.
  • Leptin: hormone from adipose tissue involved in long-term satiety; more fat → more leptin; low leptin increases hunger; leptin gene mutations can affect feeding.

Brain Control of Feeding: Hypothalamic Regulation

  • Lateral hypothalamus (LH): regulates hunger; feeding tends to increase when LH is activated.
  • Ventromedial hypothalamus (VMH): regulates satiety; activation reduces feeding.
  • Arcuate nucleus contains:
    • NPY/AgRP neurons: stimulate hunger.
    • POMC/CART neurons: promote satiety.
  • Other regions involved include the thalamus, cerebral cortex, and midbrain structures influencing metabolism and arousal.
  • A balance of excitation and inhibition among feeding circuits governs eating behavior.

Feeding Circuits and the Arcuate Nucleus

  • NPY neurons in the arcuate nucleus: when excited, promote hunger.
  • CART neurons (and related POMC pathways): promote satiety when activated.
  • Interactions between hunger and satiety systems determine feeding decisions.

Main Concepts

  • Balance between inhibition and excitation governs eating and drinking.
  • Interactions between brain and body tissues regulate feeding, thirst, and fluid balance.
  • Multiple regulatory mechanisms coordinate short-term signals (gastric, hormonal, metabolic) with long-term signals (adiposity/ leptin) for energy and fluid homeostasis.