Fluid, Electrolytes, and pH - In Depth Notes

Body Composition Overview

• Body composed of solids and two fluid compartments: Intracellular Fluid (ICF) and Extracellular Fluid (ECF).
  • ICF: Cytosol inside cells.
  • ECF: All other body fluids, including:
  • Blood plasma
  • Interstitial fluid
  • Lymph
  • Cerebrospinal fluid (CSF)
  • Synovial fluid
  • Serous fluids
  • Aqueous humor
  • Perilymph/endolymph

Distribution of Water in Body

• Water Composition:
  • Adults: Water = 60% (Males) or 50% (Females)
  • Body distribution:
  • ICF: 27% (Males), 18% (Females)
  • ECF: 33% (Males), 21.5% (Females)
  • Plasma: 4.5%, Other body fluids: ≤1%
• Solid Components of the Body:
  • Accounts for 40-50% of body mass, including:
  • Proteins
  • Lipids
  • Carbohydrates
  • Minerals

Fluid Balance

• Definition: Fluid balance exists when water gain equals water loss.
• Water Gain:
  • Absorption from digestive tract (primary method)
  • Metabolic processes
• Water Loss:
  • Urine (over 50%)
  • Feces and evaporation (via skin/lungs)
• Regulatory Mechanisms:
  • Fluid shifts occur between ICF and ECF in response to osmotic gradients.

Dehydration and Its Effects

• Dehydration: Water loss exceeds water gain, concentrating ECF.
  • Results in:
  • Thirst
  • Skin dryness
  • Possible circulatory shock due to drop in blood volume/pressure.

Mineral and Electrolyte Balance

• Mineral Balance:
  • Maintaining equal ion absorption and excretion (e.g., Na, K, Ca).
  • Minerals absorbed across small intestine; excreted by kidneys.
  • Key minerals such as sodium and potassium are fundamental to homeostasis.
• Electrolyte Solutions:
  • Includes electrolytes like sodium ions (Na+), potassium (K+), etc.
  • Movement mechanisms: channel-mediated diffusion, active transport, etc.

Sodium Balance & Its Impact

• Sodium balance impacts fluid balance; regulated by:
  • Water following Na+ by osmosis.
  • Hormones such as ADH regulate water retention.
• Hyponatremia: Low Na+ concentration leads to water intoxication.
• Hypernatremia: High Na+ concentration, commonly due to dehydration.

Potassium Balance

• Potassium (K+):
  • Absorption from digestive tract ~100 mEq/day.
  • Controlled mainly through kidney excretion stimulated by aldosterone.
• Imbalances:
  • Hypokalemia: Low K+ levels leading to weakness/paralysis.
  • Hyperkalemia: High K+ levels leading to muscular spasms / heart arrhythmias.

Acid-Base Balance

• Acid-Base Homeostasis: Balance of H+ production and loss to maintain pH (normal pH: 7.35-7.45).
• Buffer Systems: Provide short-term pH stability (e.g. bicarbonate buffer system).
• Types of Acids:
  • Fixed acids (sulfuric acid)
  • Metabolic acids (pyruvic acid)
  • Volatile acids (carbonic acid).
• Disorders:
  • Metabolic acidosis: Excessive H+ lowers pH.
  • Respiratory disorders affect CO2 removal, causing acidosis or alkalosis.

Summary of Key Definitions

• Acidosis: pH < 7.35.
• Alkalosis: pH > 7.45.
• Regulatory Responses include respiratory responses (changing respiratory rate) and renal adjustments (H+ secretion, bicarbonate reabsorption).