Lecture 21: Body Fluid and Electrolyte Balance

Chapter 21 Lecture Outline

Section 21.1 Overview and the Balance Concept

  • Electrolytes: Defined as ions dissolved in water.
  • Interdependence of Water and Electrolytes: A change in the concentration of either water or electrolytes affects the other due to their interaction in the body.
  • Important Electrolytes:
    • Sodium (Na+)
    • Potassium (K+)
    • Calcium (Ca²+)
    • Hydrogen (H+)
    • Hydroxide (OH−)
    • Chloride (Cl−)
    • Phosphate (PO₄³−)
    • Magnesium (Mg²+)
  • Homeostasis: The balance between the amounts of water and electrolytes entering and exiting the body is crucial for homeostasis.
  • Balance Maintenance:
    • Replace lost water and electrolytes through food and drink.
    • Conserve water and electrolytes when levels are low.
    • Excrete or store any excess when levels are high.

Section 21.2 Distribution of Body Fluids

  • Body Fluids: Not uniformly distributed; occupy compartments of varying volumes and compositions of water and electrolytes.
  • Body Water Composition:
    • Average adult female: approximately 52% water by weight.
    • Average adult male: approximately 63% water by weight.
    • Females typically have more adipose tissue, which is lower in water content.
    • Males typically have more muscle mass, which is higher in water content.
  • Total Water Content: Roughly 40 liters of water in the body, distributed into two major compartments:
    • Intracellular Fluid (ICF): Fluid inside cells, consists of 63% of body water.
    • Extracellular Fluid (ECF): Fluid outside of cells, consists of 37% of body water. Subcategories include:
    • Interstitial Fluid: Fluid in tissue spaces.
    • Blood Plasma: Fluid in blood vessels.
    • Lymph: Fluid in lymphatic vessels.
    • Transcellular Fluid: Separated by epithelial layers, includes cerebrospinal fluid, aqueous and vitreous humors in the eye, synovial fluid in joints, and serous fluid.
  • Fluid Composition:
    • All body fluids are solutions of electrolytes in water.
    • ECF characterized by high concentrations of Na+, Cl−, Ca²+, and HCO₃− ions.
    • ICF characterized by high concentrations of K+, Mg²+, PO₄³−, and SO₄²− ions.
  • Fluid Movement Regulation: Hydrostatic (primarily blood pressure) and osmotic (from dissolved electrolytes and proteins) pressure govern fluid movement between compartments.

Section 21.3 Water Balance

  • Water Balance: Achieved when water intake and metabolic production equal water output; essential for homeostasis.
  • Regulation of Water Intake: Controlled by the thirst centers in the brain.
  • Water Output Control: Managed by the kidneys.
  • Water Intake Averages: Daily intake averages about 2,500 mL for adults broken down into:
    • 60% from beverages.
    • 30% from moist foods.
    • 10% as a by-product of nutrient metabolism (water of metabolism).
  • Water Output: Approximately 2,500 mL/day, typically equal to intake. Methods of water loss include:
    • 60% via urine.
    • 6% via feces.
    • 6% through sweat (sensible perspiration).
    • 28% from skin evaporation and breathing (insensible perspiration).
  • Variability Factors: Output influenced by food/drink intake, electrolyte imbalances, temperature, humidity, and activity levels. Adjustments can be made based on intake changes.
  • Osmolarity: Total solute concentration in body fluid; plays a critical role in thirst regulation, stimulated by:
    • Osmoreceptors responsive to increased osmotic pressure.
    • Stretch receptors detecting blood water loss due to hemorrhage.
    • Angiotensin II, triggered by decreased blood pressure.
  • Thirst Center Inhibition: Occurs due to stomach distension after water consumption or food intake.
  • Regulation of Water Output:
    • The kidneys determine water levels excreted in urine. Distal tubules and collecting ducts are impermeable to water unless influenced by ADH (antidiuretic hormone).
    • Osmoreceptor-ADH Mechanism:
    • Dehydration triggers osmoreceptors to shrink and stimulate ADH secretion, increasing tubular permeability and water reabsorption, leading to decreased urine volume.
    • Excess water intake causes osmoreceptors to swell, inhibiting ADH, resulting in decreased water reabsorption and increased urine volume, a phenomenon regulated by diuretics.

Section 21.4 Electrolyte Balance

  • Electrolytes: Defined as molecules releasing ions in water, including: Na+, K+, Ca²+, Mg²+, Cl−, SO₄²−, PO₄³−, HCO₃−, and H+.
  • Electrolyte Balance: Achieved when electrolyte intake equals output; regulated primarily by the kidneys through urine excretion to maintain homeostasis. Excess electrolytes can be stored, such as calcium in bones.
  • Electrolyte Intake Sources:
    • Primarily from foods, supplemented by water and beverage consumption, along with by-products of metabolism.
  • Electrolyte Output Methods:
    • Primarily through urine; kidneys modulate losses to regulate body fluid composition and maintain homeostasis.
    • Loss occurs also through sweating, more pronounced during heat and exercise, and in feces.
  • Visuals: Include diagrams illustrating electrolyte balance.
  • Regulation of Positively Charged Ions (Cations):
    • Regulated for critical functions like nerve impulse conduction and muscle contraction.
    • Hormonal Regulation:
    • Aldosterone conserves sodium and excretes potassium; stimulated by high potassium levels.
    • Parathyroid hormone raises plasma calcium levels.
    • Negative ions (e.g., Cl−) typically accompany positive ions in transport.
  • Electrolyte Imbalances:
    • Hyponatremia (low Na+): Causes include prolonged sweating, vomiting, or overhydration.
    • Hypernatremia (high Na+): Caused by significant water loss; effects include CNS disturbances.
    • Hypokalemia (low K+): Associated with diuretics or kidney disease; symptoms include muscle weakness.
    • Hyperkalemia (high K+): Linked to renal disorders; can cause paralysis.

Section 21.5 Acid-Base Balance

  • Acids and Bases:
    • Acids ionize in water, releasing hydrogen ions (H+).
    • Bases combine with H+ ions, thus lowering their concentration.
  • Acid-Base Balance: Involves regulation of H+ concentrations, critical for enzyme activity, ion distribution, and hormone function.
  • pH Scale:
    • Measures acidity/alkalinity:
    • Lower pH indicates more acidity.
    • Normal bodily pH range: 7.35 to 7.45 (slightly alkaline).
  • Hydrogen Ion Sources: Primarily a by-product of metabolism.
  • Strength of Acids and Bases:
    • Strong acids (e.g., HCl) completely ionize, releasing many H+ ions; weak acids (e.g., H₂CO₃) do not.
    • Strong bases (e.g., NaOH) fully ionize and release OH−; weak bases (e.g., NaHCO₃) do not.

Section 21.6 Regulation of Hydrogen Ion Concentration

  • Mechanisms of Acid-Base Maintenance:
    • Chemical Buffer Systems:
    • Bicarbonate, phosphate, and protein buffer systems stabilize pH by neutralizing excess acids or bases.
    • Physiological Buffer Mechanisms:
    • Respiratory: Increased CO2 production lowers pH; higher breathing rates metabolize acid.
    • Renal: Kidneys excrete H+ ions, regulating body fluid acidity.
  • Time Course of Regulation:
    • Chemical buffers respond almost immediately.
    • Physiological buffers (respiratory and renal) function slower:
    • Respiratory: Minutes.
    • Renal: 1-3 days.

Section 21.7 Acid-Base Imbalances

  • Normal pH Maintenance: By chemical and physiological buffers.
  • Abnormal Conditions:
    • Acidemia: pH < 7.35, excess H+ ions, causes include acid accumulation.
    • Alkalemia: pH > 7.45, decreased H+, causes include base accumulation.
    • Disturbances Classification:
    • Respiratory Imbalances: Due to respiratory dysfunction.
    • Metabolic Imbalances: Due to other disturbances (e.g., kidneys).
  • Types of Acidosis:
    • Respiratory Acidosis: Increased CO2 leading to low pH; caused by respiratory failure.
    • Metabolic Acidosis: Accumulation of acids or loss of bases; symptoms include CNS effects.
  • Types of Alkalosis:
    • Respiratory Alkalosis: High loss of CO2 due to hyperventilation; common causes include anxiety.
    • Metabolic Alkalosis: Loss of H+ or excess bases; can arise from vomiting or use of diuretics.
  • Compensation Mechanisms:
    • Body's methods to restore pH include chemical and physiological adjustments; examples include adjustments from the lungs and kidneys based on acid-base conditions.