Water, Electrolyte, Acid-Base Balance

Water, Electrolyte, Acid-Base Balance

1. Equilibrium

• Dynamic Balance: Input equals output in the body.
• Water and electrolyte balance are interdependent.
• Body Fluid Distribution: Not equal throughout the body.

2. Fluid Compartments

• The average adult female is about 52% water by weight.
• The average adult male is 63% water by weight.
• Women generally have more fat, which holds less water; men have more muscle, which holds more water.
• Total Water in the Body: About 40 liters.

2.1 Intracellular and Extracellular Fluids

• There are two primary fluid compartments:
  • Intracellular Fluid (ICF):
  • Water and electrolytes within cells.
  • About 63% of total body water.
  • Extracellular Fluid (ECF):
  • Water and electrolytes in plasma, lymph, and interstitial spaces.
  • Specialized fluids:
    • Cerebrospinal fluid
    • Aqueous and vitreous humors
    • Synovial fluids
    • Serous fluids
    • Glandular secretions
  • About 37% of total body water.

2.2 Electrolytes Distribution

• Concentration by compartment:
  • Intracellular:
  • High concentrations of: K extsuperscript{+}, Mg extsuperscript{2+}, PO extsubscript{4} extsuperscript{3-}
  • Extracellular:
  • High concentrations of: Na extsuperscript{+}, Cl extsuperscript{-}, HCO extsubscript{3} extsuperscript{-}

3. Fluid Movement Between Compartments

• Movement is driven by two forces:
  • Hydrostatic Pressure:
  • Moves fluid from plasma to interstitial spaces.
  • Osmotic Pressure:
  • Primarily causes net movement due to ion concentration changes.

4. Water Balance

• Input must equal output:
  • Input: Average of 2,500 ml/day from:
  1. Fluids: ~1000 ml
  2. Foods: ~1200 ml
  3. Respiration: ~300 ml
  • Output: Average of 2,500 ml/day from:
  1. Urine: ~1200 ml
  2. Feces: ~150 ml
  3. Sweat: ~750 ml
  4. Lungs: ~400 ml
  • Kidneys control output and thirst centers in the brain control input.
• Regulation of Intake and Output:
  • Intake Regulation: Triggered by osmoreceptors detecting loss of body water; thirst sensation is created by the hypothalamus.
  • Output Regulation: Controlled by kidneys, with hormonal influence from ADH (Antidiuretic Hormone) to increase water reabsorption.

4.1 ADH Mechanism

• ADH leads to increased water reabsorption in convoluted tubules and decreases urine output.

5. Electrolyte Balance

• Importantly regulated electrolytes include Na extsuperscript{+}, Ca extsuperscript{2+}, K extsuperscript{+}, and others.

• Intake:

  • Primarily from foods and beverages.

• Output: Via urine, sweat, and feces.

• Regulation:

  • Na+: Regulated by kidneys and Aldosterone.
  • K+: Regulated by kidneys in conjunction with Aldosterone.
  • Ca extsuperscript{2+}: Regulated by Parathyroid Hormone (PTH) and calcitonin.

6. Acid-Base Balance

• H+ Ion Regulation: Maintains proper pH levels in the blood.
• Normal blood pH is between 7.35 and 7.45.
• Sources of H+:
  • Aerobic and anaerobic respiration, fatty acid oxidation, and breakdown of certain biological molecules.

6.1 Strength of Acids and Bases

• Determined by ease of ionization:
  • Strong Acids: Completely ionize (e.g., HCl).
  • Weak Acids: Do not ionize easily (e.g., H extsubscript{2}CO extsubscript{3}).
• Buffer Systems:
  • Bicarbonate Buffer System:
  • H extsuperscript{+} + HCO extsubscript{3} extsuperscript{-} ⇌ H extsubscript{2}CO extsubscript{3}
  • Phosphate Buffer System:
  • H extsuperscript{+} + HPO extsubscript{4} extsuperscript{2-} ⇌ H extsubscript{2}PO extsubscript{4} extsuperscript{-}
  • Protein Buffer System: Utilizes protein gradients to stabilize pH.
• Regulation Mechanisms: Utilize kidneys and lungs for regulation of H+ losses.

6.2 Buffer System Examples

• Bicarbonate: In ECF (Extracellular Fluid).
• Phosphate: In ICF (Intracellular Fluid).
• Protein Buffer: Found in both compartments.

Summary

• The body maintains homeostasis through various feedback mechanisms for fluid, electrolytes, and acid-base balance, regulating input and output to sustain proper physiological function.