Homeostatic Mechanisms in Humans - Osmoregulation

Introduction

  • Metabolism in humans is controlled by homeostatic mechanisms.
  • These mechanisms maintain and ensure survival.
  • Water/salt balance (osmoregulation) is crucial.

Osmoregulation

  • Maintaining water balance controls salt concentrations.
  • Salts exist as ions in solution.
  • Cells need specific ion concentrations for effective biochemical processes.
  • Ions regulate the pH of body fluids, which affects enzyme function.
  • Regulation of water and salt balance maintains ion concentrations.

Osmosis and Water Movement

  • Osmosis causes net water movement, regulated by solute concentrations.
  • Water moves from areas of low solute concentration to areas of high solute concentration across a semipermeable membrane.
    • In a salty solution, water exits red blood cells (RBCs).
    • In a solution with low salt, water enters RBCs.

Factors Affecting Water Loss/Gain

  • Water loss/gain varies based on exercise, temperature, humidity, and fluid intake.

Water Gain and Loss

  • Fluid intake varies (2-16 liters).
  • Water is gained through:
    • Eating food
    • Drinking water
    • Metabolic water (cellular respiration byproduct)
  • Water is lost through:
    • Urination
    • Sweating
    • Feces
    • Evaporation from the respiratory system

Salt Gain and Loss

  • Salt intake depends on diet.
  • Major salt groups: potassium, sodium, and calcium.
  • Salts are lost mainly in urine, with some in sweat and feces.
  • The kidney filters excess salts from the blood, excreting them into the urinary system.
  • Most salts are reabsorbed into the blood for recirculation.

The Kidney

  • Unfiltered blood enters the kidney.
  • Filtered blood exits the kidney.
  • Nephrons filter blood, removing toxins and waste as urine.
  • Urine exits via the ureter to the bladder.
  • Urine drains into a central collection channel.

Hormonal Control of Water Balance

  • Osmolarity: concentration of particles (sodium and chloride ions) affecting osmosis.
  • Water and solute concentrations are monitored by:
    • Osmoreceptors in the hypothalamus (detect blood solute concentrations)
    • Baroreceptors in the heart atria (detect blood pressure/volume changes)

Osmolarity and Fluid Compartments

  • Cell membranes are water-permeable.
  • Extracellular fluid osmolarity = intracellular fluid (cytosol) osmolarity.
  • Cytosol:
    • High in potassium (K^+) and magnesium (Mg^{2+}) ions
    • Low in sodium (Na^+) and chloride (Cl^-) ions
  • Extracellular fluid:
    • High in sodium (Na^+) and chloride (Cl^-) ions

Antidiuretic Hormone (ADH)

  • Also known as Vasopressin
  • Regulates water reabsorption
  • Synthesized in the hypothalamus, stored in the posterior pituitary gland
  • When osmoreceptors detect high blood osmolarity, they signal the posterior pituitary to release ADH.

ADH Action on Kidneys

  • ADH increases the permeability of distal tubules and collecting ducts in the kidneys.
  • These run through the kidney medulla, which contains high salt levels (high osmotic potential).
  • Water is reabsorbed from the tubules back into the blood via osmosis.
  • Results in more concentrated/darker urine.
  • As water is reabsorbed, osmolarity returns to normal, stopping ADH release (negative feedback loop).

ADH and Blood Osmolarity

  • Decreased blood osmolarity (low salt, high water):
    • Osmoreceptors signal the posterior pituitary to reduce or stop ADH release.
  • Increased blood osmolarity (high salt, low water):
    • Osmoreceptors signal the posterior pituitary to increase ADH release.

Summary of ADH Regulation

  • Low blood water content (high osmolarity) due to salt intake or sweating:
    • Pituitary releases lots of ADH
    • Kidneys reabsorb high volume of water
    • Low volume of concentrated urine is produced
  • High blood water content (low osmolarity) due to drinking too much water:
    • Pituitary releases little ADH
    • Kidneys reabsorb low volume of water
    • High volume of dilute urine is produced
  • Normal blood water content:
    • Pituitary releases ADH at normal rate.

Aldosterone and Blood Volume

  • Low blood volume (high osmolarity) stimulates renin release.
  • Renin leads to aldosterone release from the adrenal glands.
  • Aldosterone causes sodium (Na^+) to be reabsorbed into the blood, and potassium (K^+) to be excreted in the urine.
  • More water is reabsorbed into the blood, increasing blood volume and pressure.

Disruptions to Water Balance

  • Substances like nicotine, alcohol, and narcotics disrupt the feedback control of water balance.