Fluids and Electrolytes, Acids and Bases

Fluids and Electrolytes

• Total Body Water (TBW):

  • Approximately 60% of body weight in adults.

  • Pediatrics: 75-80% of body weight, making them susceptible to significant fluid changes, such as dehydration in newborns.

  • Aging: Decreased TBW due to decreased free fat, decreased muscle mass, renal decline, and diminished thirst perception.

• Fluid Compartments:

  • Intracellular Fluid (ICF): Fluid within cells.

  • Extracellular Fluid (ECF): Fluid outside cells.

    • Interstitial Fluid: Fluid surrounding cells.

    • Intravascular Fluid: Blood plasma.

    • Transcellular Fluids: Synovial fluid, cerebrospinal fluid (CSF), gastrointestinal (GI) fluids, pleural fluids, peritoneal fluids, and urine.

Water Movement Between Fluid Compartments

• Hydrostatic Pressure: Pushes water out of capillaries (filtration).

• Osmotic/Oncotic Pressure: Pulls water into capillaries (reabsorption).

• Starling Forces: Determine net filtration.

  • Net Filtration = Forces Favoring Filtration - Forces Opposing Filtration.

  • Forces Favoring Filtration:

    • Capillary Hydrostatic Pressure (Blood Pressure): Pushes fluid out of capillaries.

    • Interstitial Oncotic Pressure (Water-Pulling): Pulls fluid into the interstitial space.

  • Forces Favoring Reabsorption:

    • Capillary Oncotic Pressure (Water-Pulling): Pulls fluid into capillaries.

    • Interstitial Hydrostatic Pressure: Pushes fluid into capillaries.

Edema

• Accumulation of fluid within the interstitial spaces.

• Causes:

  • Increase in Capillary Hydrostatic Pressure: Due to venous obstruction, salt and water retention, or heart failure.

  • Decrease in Plasma Oncotic Pressure: Due to decreased synthesis of plasma proteins (cirrhosis, malnutrition) or increased loss of plasma proteins (nephrotic syndrome).

  • Increase in Capillary Permeability: Due to burns or inflammation, leading to loss of plasma proteins into the interstitial space and increased tissue oncotic pressure.

  • Lymph Channel Obstruction (Lymphedema): Decreased transport of capillary-filtered protein.

• Types:

  • Localized vs. Generalized.

  • Pitting Edema.

Sodium and Chloride Balance

• Sodium (Na+):

  • Primary ECF cation.

  • Regulates osmotic forces, thus water balance.

  • Roles: Nerve impulse conduction, acid-base balance, cellular biochemistry, and membrane transport.

• Chloride (Cl-):

  • Primary ECF anion.

  • Provides electroneutrality.

• Regulation:

  • Renin-Angiotensin-Aldosterone System:

    • Aldosterone leads to sodium and water reabsorption and excretion of potassium.

  • Natriuretic Peptides:

    • Cause sodium and water excretion.

Water Balance

• ADH (Antidiuretic Hormone) Secretion:

  • Increases water reabsorption into the plasma.

• Thirst Perception:

  • Osmoreceptors: Detect changes in plasma osmolality.

  • Volume Receptors: Detect changes in blood volume.

  • Baroreceptors: Detect changes in blood pressure.

Alterations in Sodium, Chloride, and Water Balance

• Isotonic Alterations:

  • Total body water change with proportional electrolyte and water change (no change in concentration).

  • Isotonic Fluid Loss: Hypovolemia.

  • Isotonic Fluid Excess: Hypervolemia.

• Hypertonic Alterations:

  • Increased osmolality.

  • Hypernatremia: Water deficit in ECF (dehydration).

• Hypernatremia:

  • Serum sodium > 145 mEq/L.

  • Related to sodium gain or water loss.

  • Manifestations: Brain cell shrinkage, altered membrane potentials, increased blood pressure.

  • Types:

    • Isovolemic: Deficit of free water and normal sodium.

    • Hypovolemic: Loss of sodium and greater loss of body water.

    • Hypervolemic: Increase of body water with a greater increase in sodium.

  • Hyperchloremia often occurs with hypernatremia.

• Hypotonic Alterations:

  • Decreased osmolality.

  • Hyponatremia: Water excess in ECF.

• Hyponatremia:

  • Serum sodium level < 135 mEq/L.

  • Related to sodium loss or water gain.

  • Manifestations: Cell swelling, altered action potentials, cerebral edema, increased intracranial pressure.

  • Types:

    • Isovolemic: Loss of sodium with normal water.

    • Hypervolemic: Increased body sodium causes increased body water.

    • Hypovolemic: Loss of body water with greater loss in sodium.

    • Dilutional: Intake of large amounts of free water which dilutes sodium.

  • Hypochloremia often occurs with hyponatremia.

Potassium

• Major intracellular cation.

• Concentration maintained by \text{Na}^+/\text{K}^+ ATPase pump.

• Regulates intracellular electrical neutrality in relation to Na+ and H+.

• Roles: Glycogen/glucose deposition, normal cardiac rhythms, skeletal and smooth muscle contraction.

Alterations in Potassium Levels

• Changes in pH affect K+ balance: Hydrogen ions accumulate in the ICF during states of acidosis; K+ shifts out to maintain a balance of cations across the membrane, resulting in hyperkalemia.

• Aldosterone, insulin, and epinephrine influence serum potassium levels.

• Kidney is most efficient regulator.

• Potassium adaptation: Slow changes tolerated better than acute.

• Hypokalemia:

  • Potassium level < 3.5 mEq/L.

  • Causes: Reduced intake of potassium, increased entry of potassium into cells, and increased loss of potassium.

  • Manifestations: Depend on rate and severity; membrane hyperpolarization causes a decrease in neuromuscular excitability, skeletal muscle weakness, smooth muscle atony, cardiac dysrhythmias, glucose intolerance, impaired urinary concentrating ability.

• Hyperkalemia:

  • Potassium level > 5.5 mEq/L.

  • Caused by increased intake, shift of K+ from ICF into ECF, decreased renal excretion, insulin deficiency, or cell trauma.

  • Manifestations: Depend on severity.

    • Mild attacks: Increased neuromuscular irritability, restlessness, intestinal cramping, and diarrhea.

    • Severe attacks: Decreases the resting membrane potential, muscle weakness, loss of muscle tone, and paralysis.

Calcium

• 99% of calcium is located in the bone as hydroxyapatite.

• Necessary for metabolic processes, structure of bones and teeth, blood clotting, hormone secretion, cell receptor function, plasma membrane stability, transmission of nerve impulses, muscle contraction.

• Parathyroid hormone, vitamin D3, and calcitonin act together to control phosphate absorption and excretion.

• Normal value = 8.8 to 10.5 mg/dl.

Alterations in Calcium Levels

• Hypocalcemia:

  • Decreased calcium concentration.

  • Causes: Inadequate intestinal absorption, deposition of ionized calcium into bone or soft tissue, blood administration, decreases in PTH and vitamin D, inadequate nutritional sources.

  • Effects: Increased neuromuscular excitability, severe cases show convulsions and tetany, Prolonged QT interval, cardiac arrest.

• Hypercalcemia:

  • Increased calcium concentration.

  • Causes: Hyperparathyroidism, bone metastases, sarcoidosis, excess vitamin D, tumors that produce PTH.

  • Effects: Many nonspecific, fatigue, weakness, lethargy, anorexia, nausea, constipation, impaired renal function, kidney stones, dysrhythmias, bone pain, osteoporosis.

Phosphate

• Like calcium, most phosphate is also located in the bone.

• Provides energy for muscle contraction.

• Parathyroid hormone, vitamin D3, and calcitonin act together to control phosphate absorption and excretion.

• Normal value = 2.5-5.0 mg/dl.

Alterations in Phosphate Levels

• Hypophosphatemia:

  • Decreased phosphate concentration.

  • Causes: Intestinal malabsorption, respiratory alkalosis, hyperparathyroidism.

  • Effects: Conditions related to reduced oxygen transport by RBCs and disturbed energy metabolism, deranged nerve and muscle function; severe cases show irritability, confusion, numbness, coma, convulsions, respiratory failure, cardiomyopathies, bone resorption.

• Hyperphosphatemia:

  • Increased calcium concentration.

  • Causes: Renal failure, chemotherapy, long-term laxative or enema use, hypoparathyroidism.

  • Effects: Conditions related to low serum calcium levels, prolonged cases show calcification of soft tissues.

Magnesium

• Intracellular cation.

• Acts as a cofactor in intracellular enzymatic reactions.

• Increases neuromuscular excitability.

• Normal value = 1.8 to 3.0 mEq/L.

Alterations in Magnesium Levels

• Hypomagnesemia:

  • Decreased magnesium concentration.

  • Causes: Malnutrition, malabsorption syndromes, alcoholism, urinary losses.

  • Effects: Behavioral changes, irritability, increased reflexes, muscle cramps, ataxia, nystagmus, tetany, convulsions, tachycardia, hypotension.

• Hypermagnesemia:

  • Increased magnesium concentration.

  • Causes: Renal insufficiency or failure, excessive intake of magnesium-containing antacids, adrenal insufficiency.

  • Effects: Skeletal smooth muscle contraction, loss of deep tendon reflexes, muscle weakness, nausea and vomiting, excess nerve function, hypotension, bradycardia, respiratory distress.

Acid-Base Balance

• Acid-base balance is carefully regulated to maintain a normal pH via multiple mechanisms.

• Small changes significantly alter biologic processes.

pH

• Negative logarithm of the H+ concentration.

• pH 7.4 is neutral for biologic fluids.

• If the H+ are high, the pH is low (acidic).

• If the H+ are low, the pH is high (alkaline).

• Acids are formed as end products of protein, carbohydrate, and fat metabolism.

  • Acids are substances that donate H+.

• To maintain the body’s normal pH, the acids must be balanced by base substances.

  • Bases are substances that accept H+.

• The bones, lungs, and kidneys are the major organs involved in the regulation of acid-base balance.

• Body acids exist in two forms:

  • Volatile: Can be eliminated as CO2 gas. \text{H}2\text{CO}3

  • Nonvolatile: Can only be eliminated by the kidneys. Lactic, sulfuric, phosphoric, and other organic acids.

Maintenance of Normal pH

• Mechanisms to maintain normal pH

  • Physiologic (chemical) buffer systems

  • Respiratory acid-base control

  • Renal acid-base control

Buffer Systems

• A buffer is a chemical that can bind excessive H+ or OH– without a significant change in pH

• Buffer systems:

  • Bicarbonate-carbonic acid buffering

    • Carbon dioxide plus water form carbonic acid

    • Carbonic acid dissociates to form one H+ (acid) and one bicarbonate (HCO3 –) (base)

    • Reversible to help maintain pH

    • Operates in the lung and the kidney

      • Lungs adjust amount of carbon dioxide

      • Kidneys reabsorb or regenerate HCO3 – and excrete H+ in urine

  • Protein buffering

    • Proteins have negative charges, so they can serve as buffers for H+

    • Hemoglobin is an excellent buffer

  • Renal buffering

    • Phosphate buffer and ammonia buffer are active in renal tubules

Acid-Base Imbalances

• Changes in H+ concentration lead to acid- base imbalances

• Acidosis

  • Systemic increase in H+ concentration or decrease in bicarbonate (base)

• Alkalosis

  • Systemic decrease in H+ concentration or increase in bicarbonate

• Renal compensation

  • Kidneys resorb HCO3 – into the plasma and excrete H+ into the urine

• Respiratory compensation

  • Lungs breath deeply and rapidly to rid the body of CO2

• Metabolic acidosis

  • Low pH, normal or low PaCO_2

  • Example, diabetic ketoacidosis

• Metabolic alkalosis

  • High pH, high HCO_3 –

  • Result of excessive loss of metabolic acids

• Respiratory acidosis

  • Low pH, high PaCO_2

  • Result of alveolar hypoventilation

• Respiratory alkalosis

  • High pH, low PaCO_2

  • Result of alveolar hyperventilation

• Total Body Water (TBW): Approximately 60% of adult body weight; higher in pediatrics, lower in elderly. Significant fluid changes can occur, especially in newborns. Aging leads to decreased TBW due to factors like reduced fat and muscle mass.

• Fluid Compartments:

  • Intracellular Fluid (ICF): Inside cells.

  • **Extracellular Fluid (ECF): Outside cells; includes interstitial fluid, intravascular fluid (blood plasma), and transcellular fluids (synovial, CSF, GI, pleural, peritoneal fluids, urine).

• Water Movement:

  • Hydrostatic Pressure: Pushes water out of capillaries.

  • Osmotic/Oncotic Pressure: Pulls water into capillaries.

  • Starling Forces: Determine net filtration (Forces Favoring Filtration - Forces Opposing Filtration).

• Edema: Fluid accumulation in interstitial spaces.

  • Causes: Increased capillary hydrostatic pressure, decreased plasma oncotic pressure, increased capillary permeability, lymph channel obstruction.

  • Types: Localized vs. Generalized, Pitting Edema.

• Sodium (Na+): Primary ECF cation; regulates water balance, nerve impulse conduction.

• Chloride (Cl-): Primary ECF anion; provides electroneutrality.

• Regulation: Renin-Angiotensin-Aldosterone System (sodium and water reabsorption, potassium excretion), Natriuretic Peptides (sodium and water excretion).

• Water Balance: ADH (increases water reabsorption), Thirst Perception (osmoreceptors, volume receptors, baroreceptors).

• Alterations:

  • Isotonic: Proportional water and electrolyte change (Hypovolemia, Hypervolemia).

  • Hypertonic: Increased osmolality (Hypernatremia).

  • Hypotonic: Decreased osmolality (Hyponatremia).

• Potassium (K+): Major intracellular cation; regulates electrical neutrality, cardiac rhythms, muscle contraction. Changes in pH, aldosterone, insulin affect K+ balance.

  • Hypokalemia: < 3.5 mEq/L; decreased neuromuscular excitability, muscle weakness.

  • Hyperkalemia: > 5.5 mEq/L; increased neuromuscular irritability, muscle weakness.

• Calcium: 99% in bone; necessary for metabolic processes, blood clotting, nerve impulses.

  • Hypocalcemia: Increased neuromuscular excitability, tetany.

  • Hypercalcemia: Fatigue, weakness, kidney stones.

• Phosphate: Mostly in bone; energy for muscle contraction.

  • Hypophosphatemia: Reduced oxygen transport, disturbed energy metabolism.

  • Hyperphosphatemia: Calcification of soft tissues.

• Magnesium: Intracellular cation; enzymatic reactions, neuromuscular excitability.

  • Hypomagnesemia: Behavioral changes, increased reflexes.

  • Hypermagnesemia: Muscle weakness, hypotension.

• Acid-Base Balance: Regulated to maintain normal pH.

• pH: Negative logarithm of H+ concentration; acids donate H+, bases accept H+.

• Maintenance: Buffer systems (bicarbonate, protein, renal), respiratory and renal control.

• Imbalances:

  • Acidosis: Increased H+.

  • Alkalosis: Decreased H+.

  • Metabolic: Acidosis (low pH), Alkalosis (high pH).

  • Respiratory: Acidosis (low pH, high PaCO2), Alkalosis (high pH, low PaCO2).