Physiology of Fluid Movement and Metabolic Disorders

Fluid Balance

• Total body water comprises 60% of an individual's weight
• Intracellular fluid (ICF) accounts for 2/3 of total body water (TBW), found inside cells
• Extracellular fluid (ECF) comprises 1/3 of TBW, located outside cells
• Interstitial fluid is found between cells
• Intravascular fluid is equivalent to blood plasma
• Other fluids include lymph, synovial, intestinal, cerebrospinal fluid, sweat, urine, pleural, peritoneal, pericardial, and intraocular fluid

Fluid Balance: Cellular Level

• Cells exist in a fluid environment with electrolyte and acid-base concentrations within a narrow range
• Changes or shifts in these concentrations severely affect metabolism and pose a life-threatening risk

Sodium/Chloride Balance

• Sodium (Na+) is the primary cation in the ECF, mostly found outside cells
• It regulates osmotic forces within the ECF
• Key roles of Na+ include neuromuscular excitability, acid-base balance, cellular reactions, and membrane transport processes
• Chloride (Cl-) is the primary anion in the ECF
• Cl- mimics the function of Na+, maintaining electroneutrality

Sodium and Water Balance

• Water balance is maintained through the intricate relationship between sodium (Na+) and water (H₂O)
• Water follows sodium; an increased or decreased concentration of salt leads to an increase or decrease in water concentration
• Tonicity refers to the change in concentration of solutes (salt) relative to the solvent (water)

Tonicity

• Isotonic solutions have the same solute concentration as the intracellular fluid
• Hypertonic solutions have a higher solute concentration than the intracellular fluid
• Hypotonic solutions have a lower solute concentration than the intracellular fluid

Sodium (Na+) Levels

• The normal range of Na+ in blood is 135-145 mEq/L
• Hypernatremia: Na+ levels above 145 mEq/L, usually caused by loss of water or increased intake of sodium
• Hyponatremia: Na+ levels below 135 mEq/L, typically resulting from an increased volume of water or loss of sodium

Hypernatremia Causes

• IV therapy (acidosis)
• Cushing's Syndrome
• fever
• respiratory infection
• diabetes
• diarrhea
• decreased water intake

Hypernatremia Manifestations

• Dehydration of cells, manifested by convulsions, thirst, fever, muscle twitching, hypertension, and hyperreflexia

Hyponatremia Causes

• Vomiting
• diarrhea
• GI suction
• burns
• diuretics
• D5W (isotonic) replacement

Hyponatremia Manifestations

• Lethargy
• confusion
• depressed reflexes
• seizures
• coma
• hypotension
• tachycardia
• decreased urine output.
• Most serious is cerebral edema and increased intracranial pressure

Chloride (Cl-)

• Chloride is the primary anion of the ECF, and plays a critical role in maintaining electroneutrality in the body
• Hypochloremia is often linked with hyponatremia

Potassium (K+)

• Potassium (K+) is the major intracellular electrolyte, with a typical range of 3.5-5.0 mEq/L
• It plays a vital role in regulating resting potential, nerve impulses, normal cardiac rhythm, skeletal, and smooth muscle contractions

Potassium and Action Potentials

• Hypokalemia reduces excitability, whereas hyperkalemia enhances excitability

Hypokalemia

• Low K+ level (< 3.5 mEq/L)
• Caused by reduced intake, increased loss, or increased movement into cells
• Impacts resting potentials; membrane hyperpolarization, impaired neuromuscular excitability
• Manifestations include skeletal muscle weakness, smooth muscle atony, cardiac dysrhythmias

Hyperkalemia

• High K+ level (> 5.0 mEq/L)
• Often due to shift from ICF (acidosis), reduced renal excretion or insulin deficiency / cell trauma
• Elevates resting potential; enhanced neuromuscular excitability
• Manifestations include tingling of lips and fingers, restlessness, intestinal cramps/diarrhea, and cardiac dysrhythmias

Potassium and pH

• pH changes significantly affect K+ balance
• Acidosis: H+ moves out of cells and into the ECF causing K+ to shift out of cells
• Alkalosis: Opposite occurs so K+ moves into cells

Calcium (Ca2+)

• Calcium (Ca2+) is critical for bone health, blood clotting, muscle contraction, neurotransmitter release, hormone secretion, and cell receptor function
• Hypocalcemia decreases threshold potential; enhances muscle excitability
• Hypercalcemia reduces threshold potential; decreases muscle excitability

Calcium and Action Potentials

• Changes in calcium (Ca2+) levels influence excitability

Edema

• Edema is the abnormal accumulation of fluid in interstitial spaces
• Four primary causes:
• Increased capillary hydrostatic pressure
• Decreased capillary oncotic pressure
• Increased capillary permeability
• Lymphatic obstruction

Acid-Base Balance

• pH is a measure of hydrogen ion concentration
• Physiological fluids maintain a pH range of 7.35 to 7.45
• Body acids exist as volatile (H2CO3) and nonvolatile (sulfuric, phosphoric) forms
• The lungs regulate volatile acids; kidneys regulate nonvolatile acids
• Buffer systems (e.g., carbonic acid-bicarbonate) are crucial for maintaining pH stability in blood

Acid-Base Balance: Compensation

• Respiratory compensation relates to changes in blood CO2 levels (hypo or hyperventilation).
• Renal compensation involves changes in H+ and HCO3- excretion in the urine.

Acid-Base Disorders

• Metabolic acidosis
• Metabolic alkalosis
• Respiratory acidosis
• Respiratory alkalosis

Arterial Blood Gas (ABG) Analysis

• A diagnostic tool used to evaluate respiratory and metabolic functions of the body
• Key components of ABG reports: pH, partial pressure of oxygen (pO2), partial pressure of carbon dioxide (pCO2), and bicarbonate (HCO3-)
• Interpreting ABG reports involves using a "tic-tac toe" approach, referencing the normal values.

ABG Analysis: Interpretations