Fluid, Electrolyte, and Acid-Base Homeostasis

Fluid Intake and Output

• Intake: 2,500 mL/day

  • Drink: 1,600 mL
  • Food: 700 mL
  • Metabolic water: 200 mL

• Output: 2,500 mL/day

  • Urine: 1,500 mL
  • Feces: 200 mL
  • Expired air: 300 mL
  • Cutaneous transpiration: 400 mL
  • Sweat: 100 mL

Body Water Composition

• Body Fluid Percentage: 55-60% of total body weight in lean adults

• Intracellular Fluid (ICF):

  • Comprises about 2/3 of body fluids located within cells.

• Extracellular Fluid (ECF):

  • Comprises the remaining 1/3, divided into two main parts:
  • Interstitial Fluid (IF): 80% of ECF with low protein anions and high Cl^-
  • Blood Plasma: 20% of ECF with high protein anions and Na⁺ ions

Water Movement Between Compartments

• Water moves primarily through two points of exchange:
  1. Cell membranes: Separate intracellular fluid from interstitial fluid.
  2. Capillary walls: Thin enough for exchange between plasma and interstitial fluid.
• Balance between water and electrolytes is crucial; they follow each other based on concentration gradients.

Types of Interstitial Fluid

• Includes specialized forms found in:
  • Lymphatic vessels
  • Cerebrospinal fluid
  • Gastrointestinal tract fluids
  • Synovial fluid
  • Ocular fluids (aqueous humor, vitreous body)
  • Ear fluids (endolymph and perilymph)
  • Lung pleural fluids
  • Heart pericardial fluids
  • Peritoneal fluids around serous membranes
  • Glomerular filtrate in kidneys

Fluid and Solute Balance

• Fluid Balance: Adequate water levels maintained in all compartments as per their physiological needs.

  • Each compartment must have appropriate water and electrolyte concentrations.

• Osmosis: Main mechanism for fluid movement across compartments; driven by solute concentrations.

Electrolytes in Body Fluids

• Electrolytes: Substances containing ions which conduct electricity (e.g., NaCl → Na⁺ + Cl^-).
  • Functions:
  1. Control water osmosis between compartments.
  2. Maintain acid-base balance for cellular functions.
  3. Carry electrical current for cellular activity (action potentials).
  4. Act as cofactors for enzyme activity.

Body Water Gain and Loss

• Body water content: 45-75% of body weight (decreases with age).
• Gain: 2,500 mL/day from ingestion and metabolic water.
• Loss: 2,500 mL/day through urine, feces, sweat, and breathing.

Water Intoxication

• Occurs when plasma Na⁺ concentration drops, swelling cells.
  • Caused by:
  • Rapid water intake exceeding kidney excretion.
  • Replacing lost fluids with plain water post-diarrhea/vomiting.
• Can lead to severe conditions like convulsions or coma without proper electrolyte balance.

Regulation of Water Gain and Loss

Water Gain

• Triggered by increased blood osmolarity or decreased blood volume/pressure, leading to thirst signals through:
  1. Osmoreceptors in hypothalamus.
  2. Renin-angiotensin system activation.

Water Loss

• Excess water or solute elimination primarily through urination.
• Hormonal regulation affecting Na⁺ and water balance:
  • Atrial natriuretic peptide (ANP) increases GFR, decreasing reabsorption.
  • Angiotensin II decreases GFR, increasing reabsorption.

Acid-Base Homeostasis

• Normal Blood pH: 7.35-7.45, crucial for metabolic processes and oxygen delivery.
• Acid and Base Definitions:
  • Acidosis: pH < 7.35 (acidic).
  • Alkalosis: pH > 7.45 (basic).
• Major Acids and Bases:
  • Acids: Carbonic acid (H₂CO₃), lactic acids, ketoacids.
  • Bases: Bicarbonate (HCO₃^-).
• Regulation of pH:
  • Lungs: Quick adjustment via CO₂ exhalation.
  • Kidneys: Slower but posts significant changes through acid or base excretion.

Imbalances and Restoration

• Constant adjustments needed to maintain pH balance.
  • Faster elimination (like urination) or reducing acid/base input (diet) restores balance.
• Respiratory and Metabolic Disorders:
  • Affect blood CO₂ levels causing respiratory acidosis/alkalosis.
  • Kidney diseases can lead to metabolic acidosis.