Fluid and Electrolyte Balance

Fluid and Electrolyte Balance

• Body fluid balance is maintained by controlling ECF volume and osmolarity
• Plasma composition can be modified during circulation through the lungs and kidneys, affecting interstitial fluid composition
• Intracellular fluid composition can also be changed according to cell membrane permeability

Antidiuretic Hormone (ADH)

• Synthesized by the hypothalamus and secreted by the posterior pituitary gland
• Controlled by osmotic stimuli, volume stimuli, and the renin-angiotensin system
• Osmotic stimuli:
  • Increased osmotic pressure of plasma sensed by osmoreceptors in the hypothalamus
  • Significant changes in ADH secretion occur with 1% changes in plasma osmolarity
• Volume stimuli:
  • Decreased blood volume or blood pressure stimulate ADH secretion via low-pressure receptors and baroreceptors
• Renin-angiotensin system:
  • Angiotensin II acts on the anterior hypothalamus to stimulate ADH secretion
  • ADH acts on V2 receptors to increase water permeability in the DCT and collecting ducts

Role of the Kidney in Regulating ECF Volume

• Excess water in the body:
  • Increases ECF volume
  • Increases arterial blood pressure
  • Increases GFR
  • Increases urinary excretion of Na+ and water
• Decreased water in the body:
  • Decreases ECF volume
  • Decreases arterial blood pressure
  • Decreases GFR
  • Decreases urinary excretion of Na+ and water

Renin-Angiotensin-Aldosterone System

• Decreased ECF volume:
  • Stimulates juxtaglomerular cells
  • Increases renin secretion
• Renin converts angiotensinogen to angiotensin I
• Angiotensin I is converted to angiotensin II by ACE
• Angiotensin II:
  • Stimulates aldosterone hormone secretion
  • Increases Na+ reabsorption from distal tubules and collecting ducts
  • Stimulates ADH secretion
  • Increases thirst
  • Causes vasoconstriction

Thirst Mechanism

• Regulated by the hypothalamic thirst center in response to changes in plasma osmolarity and ECF volume
• Stimulated by:
  • Changes in plasma osmolarity
  • Decreased ECF volume
  • Dryness of the mouth and pharyngeal mucous membrane

Volume Receptors

• Present in the atria and great veins
• When blood volume decreases, they cause:
  • Increased sympathetic discharge to renal vessels and juxtaglomerular apparatus
  • Increased ADH secretion

Atrial Natriuretic Peptide (ANP)

• Secreted from overstretched atrial muscles
• Decreases:
  • Aldosterone secretion
  • Renin secretion
  • Angiotensin II secretion
  • Na+ reabsorption from renal tubules

Disorders of ECF Osmolarity

• Overhydration:
  • Gain of free water without solutes
  • Decreased ECF osmolarity
  • Cell swelling (water intoxication)
• Dehydration:
  • Loss of free water without solutes
  • Increased ECF osmolarity
  • Cell shrinkage
• Isotonic fluid gain or loss:
  • No water shift between ECF and ICF compartments

Evaluation of Renal Function Tests

• Tests for assessing renal blood flow:
  • Whole renal blood flow using clearance of PAH
  • Distribution of renal blood flow using radioactive xenon 133
• Tests for assessing GFR:
  • Clearance of inulin or creatinine