Fluid Balance and Electrolytes / Acid and Base Balances

Normal Electrolyte Values and Imbalances

• Potassium (K+): Normal range 3.5-5.0 mmol/L
  • Hyperkalemia: > 5.0 mmol/L causing muscle weakness, cramping, and cardiac arrhythmias
  • Hypokalemia: < 3.5 mmol/L causing muscle weakness, fatigue, and heart dysrhythmias
• Sodium (Na+): Normal range 135-145 mmol/L
  • Hypernatremia: > 145 mmol/L causing thirst, confusion, muscle twitching, and increased BP
  • Hyponatremia: < 135 mmol/L causing lethargy, seizures, and confusion due to cerebral edema
• Chloride (Cl-): Normal range 98-106 mmol/L
  • Hyperchloremia: > 106 mmol/L, usually linked to hypernatremia
  • Hypochloremia: < 98 mmol/L, often linked to hyponatremia

Solution Osmolality

• Isotonic: Solutions with the same osmolality as body cells, like 0.9% saline, used for fluid replacement without affecting fluid balance
• Hypertonic: Solutions with higher osmolality than body cells, like 3% saline, draw water out of cells causing them to shrink
• Hypotonic: Solutions with lower osmolality than body cells, like 0.45% saline, cause water to move into cells, potentially leading to cell swelling

Edema

• Edema is the fluid accumulation in interstitial spaces, caused by:
  • Increased capillary hydrostatic pressure (e.g., venous obstruction, heart failure)
  • Loss of plasma proteins (e.g., kidney disease)
  • Increased capillary permeability (e.g., inflammation)
  • Lymphatic obstruction (e.g., tumors)

Intracellular Fluid (ICF) and Extracellular Fluid (ECF)

• ICF: Fluid inside cells, comprising two-thirds of the body's total body water (TBW)
• ECF: Fluid outside cells, including interstitial fluid, plasma, and other body fluids, comprising one-third of the TBW
• TBW varies with age, gender, and body composition

Osmotic and Hydrostatic Pressure / Starling Forces

• Osmotic Pressure: The force pulling water into an area of higher solute concentration
• Hydrostatic Pressure: The force pushing water out of the capillaries into interstitial spaces
• Starling Forces: Capillary hydrostatic pressure, plasma oncotic pressure, interstitial hydrostatic pressure, and interstitial oncotic pressure, controlling fluid movement between capillaries and tissues

Water Balance: Renin-Angiotensin System (RAAS) and Antidiuretic Hormone (ADH)

• RAAS: Activated when blood pressure or sodium levels drop.
  • Kidneys release renin, activating angiotensin I.
  • Angiotensin I converts to angiotensin II, causing vasoconstriction and stimulating aldosterone release, increasing sodium and water reabsorption by the kidneys
• ADH: Secreted when plasma osmolality increases, promoting water reabsorption by the kidneys, reducing urine output

Acid-Base Balance

• Normal pH: 7.35-7.45

• Acidosis: Systemic increase in H+ concentration or a loss of bicarbonate

  • Respiratory Acidosis: Caused by hypoventilation (PaCO2 > 45 mmHg, pH < 7.35)
  • Metabolic Acidosis: Caused by an increase in noncarbonic acids or loss of bicarbonate (HCO3 < 22 mmol/L, pH < 7.35)

• Alkalosis: Systemic decrease in H+ concentration or an increase in bicarbonate

  • Respiratory Alkalosis: Caused by hyperventilation (PaCO2 < 35 mmHg, pH > 7.45)
  • Metabolic Alkalosis: Caused by an increase in bicarbonate or excessive loss of metabolic acids (HCO3 > 26 mmol/L, pH > 7.45)