Fluid and Electrolyte Balance

BIO 106 – A & P II: Chapter 43 – Fluid and Electrolyte Balance

I. Fluid Compartments and Fluid Balance

  • Fluid Balance: Refers to the necessary proportioning of water and solutes among various body compartments.

    • Water: Comprises 45-75% of total body mass, varying by age and gender.

    • Infants: 75%

    • Men: 60%

    • Women: 50%

    • Electrolytes: Molecules that dissociate into charged particles (ions) when dissolved in water.

  • Fluid Compartments: Divided into two main categories:

    • Intracellular Fluid (ICF): Represents 2/3 of total body fluid found within cells.

    • Extracellular Fluid (ECF): Accounts for 1/3 of total body fluid, further divided into:

    • 80% Interstitial Fluid

    • 20% Plasma

  • Barriers: Two primary barriers separate fluid compartments:

    • Plasma Membrane: Separates intracellular fluid from interstitial fluid.

    • Blood Vessels: Separate interstitial fluid from blood plasma.

  • Processes of Fluid Exchange: Include filtration, reabsorption, diffusion, and osmosis, facilitating continuous movement of water and solutes among body compartments.

II. Water Gain and Loss

  • Cardinal Rule: Fluid balance can only be maintained if water input equals output.

  • Water Gain: Achieved primarily through two methods:

    • Ingestion: Approximately 2300 ml/day from liquids and moist foods.

    • Metabolic Water: About 200 ml/day generated through metabolism.

  • Water Loss: Must equal water gain through four routes:

    • Kidneys: Excrete approximately 1500 ml via urine.

    • Skin: Evaporates around 600 ml.

    • Lungs: Exhale approximately 300 ml as water vapor.

    • Gastrointestinal Tract (GI): Excretes about 100 ml in feces.

III. Regulation of Body Water Gain/Loss

  • Thirst Center: Located in the hypothalamus, crucial for regulating body water gain/loss.

  • Dehydration: Occurs when water loss exceeds water gain, characterized by:

    • Decreased Saliva Flow: Leading to dry mouth and pharynx.

    • Increased Blood Osmolarity: Detected by osmoreceptors in the hypothalamus.

    • Decreased Blood Volume: Resulting in lowered blood pressure.

  • Excess Water Elimination: Negative feedback results in increased water loss through urine when in excess.

  • Urinary Salt Loss: The extent of sodium chloride (NaCl) loss is a primary factor determining body fluid volume.

  • Hormonal Regulation: Three hormones play crucial roles in body water regulation:

    • ADH (Antidiuretic Hormone): Promotes water reabsorption when dehydrated.

    • Aldosterone: Enhances sodium reabsorption and consequently water retention via osmosis when dehydrated.

    • ANP (Atrial Natriuretic Peptide): Promotes the secretion and excretion of salt and water during overhydration.

IV. Factors Altering Fluid Balance

  • Abnormal Conditions:

    • Include situations like hyperventilation, vomiting, diarrhea, intestinal drainage, and dehydration.

  • Fluid Balance Disorders:

    • Hypovolemia: A state of dehydration caused by excessive water loss.

    • Causes: Fluid loss from the GI tract, inadequate fluid intake, excessive perspiration, kidney-related disorders.

    • Signs and Symptoms: Low blood pressure, reduced cardiac output, electrolyte imbalances, dizziness, lightheadedness, weakness, poor skin tone, tachycardia.

    • Hypervolemia: An increase in body fluid volume.

    • Causes: Kidney retention of large amounts of sodium and water, often due to renal failure.

    • Signs and Symptoms: Weight gain, edema, pulmonary congestion.

V. Electrolytes in Body Fluids

  • Electrolytic Nature: Electrolytes dissociate into ions when dissolved in body fluids.

  • Functions of Electrolytes:

    • Regulate osmosis of water between body fluid compartments.

    • Maintain acid-base balance.

    • Conduct electrical currents.

VI. Sodium

  • Sodium: The most abundant cation in extracellular fluid.

  • Disorders Related to Sodium:

    • Hyponatremia: Decreased sodium concentration in the body.

    • Causes: Overuse of diuretics, vomiting/diarrhea, low salt diet, aldosterone deficiency.

    • Signs and Symptoms: Muscle weakness, hypotension, tachycardia, confusion, headache.

    • Hypernatremia: Increased sodium concentration in the body.

    • Causes: Dehydration, excessive dietary sodium.

    • Signs and Symptoms: Intense thirst, hypertension, edema.

VII. Chloride

  • Chloride: The most abundant anion in extracellular fluid.

  • Chloride Disorders:

    • Hypochloremia: Decreased chloride concentration in the body.

    • Causes: Overuse of diuretics, vomiting, diarrhea, low salt intake, aldosterone deficiency.

    • Signs and Symptoms: Muscle weakness, hypotension, tachycardia, confusion, headache.

    • Hyperchloremia: Increased chloride concentration in the body.

    • Causes: Dehydration, excessive chloride intake.

    • Signs and Symptoms: Intense thirst, hypertension, edema, metabolic acidosis.

VIII. Potassium

  • Potassium: The most abundant cation in intracellular fluid.

  • Potassium Disorders:

    • Hypokalemia: Decreased potassium concentration in the body.

    • Causes: Vomiting/diarrhea, insufficient dietary potassium, hyperaldosteronism, diuretic overuse.

    • Signs and Symptoms: Muscle fatigue, flaccid paralysis, mental confusion.

    • Hyperkalemia: Increased potassium concentration in the body.

    • Causes: Renal failure, crushing injuries, transfusion of hemolyzed blood.

    • Signs and Symptoms: Vomiting, diarrhea, risk of inducing ventricular fibrillation.

IX. Calcium

  • Calcium: Primarily stored in bones; essential for blood clotting, neurotransmitter release, and muscle tone maintenance.

  • Calcium Disorders:

    • Hypocalcemia: Decreased calcium concentration in the body.

    • Causes: Increased calcium loss, reduced intake, hypoparathyroidism.

    • Hypercalcemia: Increased calcium concentration in the body.

    • Causes: Excessive vitamin D, cancer, hyperparathyroidism.

    • Signs and Symptoms: Bone pain, depression, confusion, stupor, lethargy, coma, muscle twitching, low muscle tone.

X. Other Important Electrolytes

  • Bicarbonate ( HCO3-)

  • Phosphate ( HPO4-)

  • Proteins

XI. Review of Important Electrolytes in the Body

  • Cations: Sodium, Calcium, Potassium

  • Anions: Chloride, Bicarbonate, Phosphate, Proteins

  • Comparison of Fluid Types:

    • Extracellular Fluid: Lower concentrations of potassium (K), higher concentrations of sodium (Na) and chloride (Cl); fewer proteins.

    • Intracellular Fluid: Higher concentrations of potassium (K), lower concentrations of sodium (Na) and chloride (Cl); contains many proteins.

XII. Additional Review

  • Box 43-1 on Page 1008 regarding dehydration.

  • Cycle of Life and The Big Picture on Page 1013.

  • Mechanisms of Disease and Health Matters on Pages 1014 - 1015.

NOTES ADDENDUM – FLUIDS AND ELECTROLYTE BALANCE

  • Adipose Tissue: Contains the least water of any tissue, resulting in obese individuals having less body water per kilogram of weight compared to lean individuals.

  • Body Fluid Compartments:

    • Extracellular Fluid (ECF):

    • 20% Plasma

    • 80% Interstitial Fluid

    • Transcellular Fluid: Includes cerebrospinal fluid, joint fluids, and ocular humors.

    • Intracellular Fluid (ICF): Water within cells.

  • Volume Comparisons:

    • Intracellular Fluid: Largest compartment (25 L)

    • Interstitial Fluid: Next largest (12 L)

    • Plasma: Smallest (3 L)

  • Urine Volume:

    • Glomerular Filtration Rate (GFR): Reflects the rate of fluid movement from the glomerulus into the Bowman space, generally constant except under abnormal conditions.

    • Tubular Reabsorption of Water: Fluctuates significantly to help adjust urine volume relative to GFR.