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.