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.