Homeostasis Notes

Homeostasis

  • Definition: Homeostasis is a self-regulating process by which biological systems maintain stability by adjusting to changes in their external environment.

Dehydration

  • Causes:
    • Blood has too little water due to increased sweating, high temperatures, or insufficient water intake.
  • Negative Feedback Mechanism:
    1. Low water levels in blood and tissue fluids are detected.
    2. The hypothalamus (via osmoreceptors) senses the changes.
    3. The pituitary gland releases Anti-Diuretic Hormone (ADH).
    4. ADH is transported to the kidneys via blood.
    5. Increases the permeability of the distal convoluted tubule and collecting duct.
    6. More water is absorbed from the filtrate into the blood.
    7. Blood becomes more diluted, producing less concentrated urine.

Overhydration

  • Causes:
    • Blood has too much water due to little exercise, low sweating, cooler temperatures, or excessive water intake.
  • Negative Feedback Mechanism:
    1. Increased water levels dilute blood and tissue fluids.
    2. The hypothalamus (via osmoreceptors) detects the changes.
    3. The pituitary gland secretes less or stops ADH secretion.
    4. ADH travels to the kidneys via blood.
    5. Decreases permeability of distal convoluted tubule and collecting duct.
    6. Less water is absorbed into the blood.
    7. More dilute urine is excreted.

Role of Aldosterone: Control of Salt Homeostasis

  • Definition: Aldosterone is a steroid hormone secreted by the adrenal glands.
  • Function: Controls sodium ions (Na+) in the body.
Low Salt Levels (Hypotonic)
  • Detection:
    1. Low Na+ levels in blood and tissue fluids are detected.
    2. Receptor cells in afferent and efferent arterioles respond.
  • Responses:
    1. The adrenal gland secretes aldosterone.
    2. Aldosterone is transported to the kidneys.
    3. Promotes reabsorption of Na+ from filtrate back into blood.
    4. Increases Na+ concentration in blood.
    5. Less sodium is excreted in urine.
Elevated Salt Levels (Hypertonic)
  • Detection:
    1. Elevated Na+ levels in blood and tissue fluids are detected.
    2. Receptor cells in afferent and efferent arterioles respond.
  • Responses:
    1. The adrenal glands secrete less or stop aldosterone secretion.
    2. Less or no aldosterone reaches kidneys.
    3. Na+ is not reabsorbed into blood.
    4. No additional Na+ is available in blood.
    5. More sodium is excreted in urine.

Homeostatic Control of Blood pH

  • Low Blood pH (Acidity):

    • High concentration of hydrogen ions decreases blood pH.
    • Hydrogen ions move from the blood into distal convoluted tubules, decreasing the amount of hydrogen ions and returning pH to normal (pH 7.4).

  • High Blood pH (Alkalinity):

    • High concentration of bicarbonate ions increases blood pH.
    • Bicarbonate ions move from blood into distal convoluted tubule, decreasing the bicarbonate ions and returning pH to normal (pH 7.4).

Anti-Diuretic Hormone (ADH)

  • Production: Secreted by the pituitary gland and produced by the hypothalamus.
  • Role:
    • Homeostatic control of water.
    • Has osmoreceptors to monitor blood osmolarity (number of particles per unit of solution).
    • Water reabsorption primarily occurs in proximal convoluted tubule and descending loop of Henle.