Homeostatic control

HOMEOSTASIS

HOMEOSTATIC CONTROL OF WATER AND SALTS

  • Kidneys continuously regulate chemical composition of blood to maintain homeostasis.

HOW DO MOLECULES OR IONS MOVE

  • Molecules/Ions move passively from high concentration to low concentration (down a concentration gradient).

    • Osmosis: Water moves from hypotonic (high water concentration) to hypertonic (low water concentration) through a semi-permeable membrane.

    • Solutes: Move from hypertonic (high solute concentration) to hypotonic (low solute concentration) areas.

  • Water follows salt solution.

  • Solutions with equal concentrations are isotonic.

  • Active Transport: Molecules move against concentration gradient with energy, combining with a carrier molecule.

OSMOREGULATION

  • Maintains balance between water and solute content in body fluids.

  • Primarily controlled by kidneys and skin.

  • Vital constants for cell functions:

    • Amount of Water:

      • Too little water leads to dehydration of cells.

      • Too much water causes cells to burst.

    • Concentration of Solutes (e.g., glucose, salts):

      • Determines osmolarity (osmotic pressure) of body fluids.

      • Higher solute concentration increases osmolarity, affecting water movement by osmosis.

  • Osmolarity: Number of solute particles in one liter of water.

HOMEOSTATIC CONTROL OF WATER – THE ROLE OF ADH

  • 99% of body's water is reabsorbed from kidneys into bloodstream.

  • Normal conditions: Most water reabsorbed in proximal tubule and descending limb of the loop of Henle.

  • When fluid intake is low, kidneys conserve water by forming concentrated urine:

    • Sodium ions actively pumped out of the loop of Henle, creating hypertonic tissue fluid in the medulla.

    • Results in steep concentration gradient between hypotonic filtrate in distal and collecting ducts and hypertonic tissue fluid.

    • Water moves by osmosis from filtrate into the blood, conserving water and reducing urine.

WHAT IS ADH

  • Antidiuretic Hormone (ADH): Secreted by hypothalamus and stored in posterior pituitary.

    • Anti: Opposed to or preventing.

    • Diuretic: Causes increased output of urine.

WHEN IS ADH SECRETED

  • Osmoreceptors in hypothalamus monitor blood osmolarity.

  • Stimulation occurs when osmolarity increases due to dehydration:

    • Activates thirst center in hypothalamus (resulting in increased fluid intake).

    • Stimulates pituitary gland to release ADH.

WHAT ROLE DOES ADH PLAY

  • ADH dissolves in blood plasma, carried throughout the body.

  • At kidneys (target organ), ADH:

    • Enlarges pores of collecting ducts, increasing water permeability.

    • Results in:

      • More water reabsorbed by osmosis from filtrate into blood.

      • Normalization of blood volume and composition.

      • Production of less but more concentrated urine.

  • When osmolarity decreases, hypothalamic receptors deactivate, reducing ADH secretion; kidneys then excrete more urine.

  • This mechanism exemplifies a negative feedback system.

REGULATION OF WATER CONTENT IN BLOOD

Increase in Water Content

  • Increase in water can occur due to:

    • Cold conditions (low sweating)

    • High fluid intake.

  • Hypothalamus detects the increase:

    • Sends message to pituitary to slow/stopped ADH release.

    • Less ADH leads to kidneys conserving less water, producing large volumes of dilute urine, returning water levels to normal.

Decrease in Water Content

  • Decrease can occur due to:

    • Insufficient fluid intake

    • Excessive sweating.

  • Hypothalamus detects low water levels:

    • Sends signal to pituitary gland to release ADH.

    • ADH travels to kidneys for increased water reabsorption.

    • Results in smaller volume of more concentrated urine, restoring water levels.

    • Thirst center may also stimulate increased fluid intake.

SALT REGULATION

  • Aldosterone is secreted from adrenal glands to regulate sodium and potassium ions in blood.

LEVEL OF SODIUM CONCENTRATION

Too Low Sodium

  • Adrenal glands secrete aldosterone:

    • Sodium reabsorbed from distal convoluted tubules and collecting ducts.

    • Increases sodium in blood and decreases in urine.

Too High Sodium

  • Adrenal glands secrete less aldosterone:

    • Less sodium reabsorbed; higher sodium in urine.

ADRENAL EFFECTS ON POTASSIUM AND SODIUM LEVELS

  • Increased potassium leads to aldosterone release from adrenal cortex, targeting kidneys.

  • This causes increased sodium reabsorption and decreased potassium in blood, normalizing levels.

FINAL SUMMARY

  • Homeostasis depends on the delicate balance of water and electrolytes in the blood, regulated by hormones like ADH and aldosterone.

  • Proper functioning of kidneys is essential in maintaining this balance through reabsorption mechanisms.