Fluid, Electrolyte and Acid/Base Balance

• Fluid and Electrolyte Balance

  • Fluid Balance:

  • Intake is erratic; needs constant adjustments for homeostasis

  • Electrolyte Balance:

  • Constant regulation necessary for normal function

  • Imbalances can occur in disease states, leading to conditions like dehydration or fluid overload.

• Percentage of Body Fluid

  • Human body fluid content ranges from 45% to 75%

  • Varies by age and body composition:

  • Infants: 75%

  • Elderly: 45%

  • Young/Middle-aged adults: 50-60%

  • Adipose tissue: 20% water; Skeletal muscle: 75% water

  • Higher muscle mass correlates with higher water percentage, individuals with lower water percentage are at risk for imbalance.

• Fluid Compartments

  • Intracellular Fluid (ICF): 2/3 of total body fluid, rich in K+, Mg²+, PO₄³⁻, proteins

  • Extracellular Fluid (ECF): 1/3 of total body fluid, consists of plasma and interstitial fluid (IF)

  • Plasma and IF are similar but vary in protein content: IF has little/no protein, plasma has proteins.

• Fluid Movement

  • Osmotic balance between compartments:

  • Water moves via osmosis depending on hypertonic (high solute) and hypotonic (low solute) conditions.

• Fluid Intake and Output

  • Fluid intake averages about 2500 mL/day

  • From drinks (1600 mL), food (700 mL), and metabolic water (200 mL)

  • Fluid output also averages 2500 mL/day through urine (60%), sweat, respiration, and feces.

  • Sensible water loss: measurable (urine, feces);

  • Insensible water loss: not measurable (evaporation, breathing)

  • Obligatory water loss maintains basic physiological functions, facultative loss is adjustable based on hydration levels.

• Fluid Imbalance

  • Can be categorized as:

  • Volume depletion

  • Volume excess

  • Dehydration

  • Hypotonic hydration

  • Fluid sequestration occurs with abnormal fluid distribution, leading to conditions like edema.

• Regulation of Fluid Balance

  • Monitored through blood volume, blood pressure, osmolarity

  • Thirst center activated by: dry mouth, increased plasma osmolality, low blood pressure

  • Stimulus for ADH: promotes water retention

  • Aldosterone and ANP (Atrial Natriuretic Peptide) regulate sodium and fluid balance.

• Electrolyte Balance

  • Salts like Na⁺ are vital for cell function, fluid movement, and muscle activity.

  • Sodium is the principle cation in ECF affecting osmotic pressure and blood pressure.

  • Potassium (K⁺): primarily intracellular, critical for neuromuscular function; imbalances can be lethal.

  • Calcium (Ca²⁺): essential for muscle contraction, blood clotting; 99% stored in bones.

• Acid-Base Balance

  • Normal pH: arterial blood: 7.4, venous blood: 7.35

  • Mechanisms:

  • Chemical buffers (seconds)

  • Respiratory adjustments (minutes)

  • Renal control (hours to days)

  • Acidosis and alkalosis depend on shifts in H⁺ concentration.

• Chemical Buffer Systems

  • Function to resist changes in pH:

  • Bicarbonate buffer system

  • Protein buffer system

  • Phosphate buffer system

  • These buffers can mitigate swings in pH effectively.

• Physiological Buffering Systems

  • The respiratory system manages acute pH changes primarily through CO₂ adjustments; kidneys can manage longer-term imbalances.

  • Understanding how these mechanisms interact helps maintain homeostasis across various conditions.

Chemical Buffer Systems function to resist changes in pH, utilizing three main systems:

• Bicarbonate buffer system: Helps maintain the pH of blood and other fluids by neutralizing excess acids or bases.

• Protein buffer system: Involves proteins that can bind or release H⁺ ions, thus stabilizing pH levels.

• Phosphate buffer system: Operates mostly in the intracellular fluid, maintaining pH within cells through phosphate ions.

The respiratory system plays a role in managing acute pH changes primarily through the adjustment of carbon dioxide (CO₂) levels, which can affect the concentrations of carbonic acid in the blood. The kidneys handle longer-term pH imbalances by excreting or retaining H⁺ and bicarbonate ions over hours to days.

Fluid Compartments

• Intracellular Fluid (ICF): Comprises about 2/3 of total body fluid, is rich in potassium (K⁺), magnesium (Mg²⁺), phosphate (PO₄³⁻), and proteins. It is essential for cellular function.

• Extracellular Fluid (ECF): Makes up about 1/3 of total body fluid and includes plasma and interstitial fluid. Plasma contains proteins, whereas interstitial fluid has little to no proteins.

Sodium, Potassium, and Calcium

• Sodium (Na⁺): The principal cation in extracellular fluid that is crucial for fluid movement, osmotic pressure, and overall blood pressure regulation.

• Potassium (K⁺): Found primarily inside cells, it is vital for neuromuscular function; imbalances may have lethal consequences.

• Calcium (Ca²⁺): Key for muscle contraction and blood clotting, with 99% stored in bones.

Balancing Loss Versus Gain

Fluid intake averages about 2500 mL/day, gathered from drinking (1600 mL), food (700 mL), and metabolic processes (200 mL). Conversely, fluid output also averages about 2500 mL/day via urine (approximately 60%), sweat, respiration, and feces. Recognizing the balance dictates bodily function and avoids imbalances:

• Sensible water loss: Measurable losses, such as urine and feces.

• Insensible water loss: Unmeasured, such as evaporation through skin and breathing.

• Obligatory water loss: Necessary to maintain physiological function; facultative loss can adjust based on hydration levels.

Blood Pressure and Control Mechanisms

Fluid balance is monitored through blood volume, blood pressure, and osmolarity. The thirst center is activated by a dry mouth, increased plasma osmolality, or low blood pressure. Hormonal responses include:

• ADH (Antidiuretic Hormone): Promotes water retention by the kidneys.

• Aldosterone: Regulates sodium and thus fluid balance, leading to blood pressure maintenance.

• Atrial Natriuretic Peptide (ANP): Responds to increased blood volume by promoting the excretion of sodium, leading to decreased blood pressure.

Imbalances

Fluid imbalances can manifest in various conditions, including volume depletion or excess, dehydration, hypotonic hydration, and fluid sequestration, which can lead to localized edema. Aging can affect homeostatic mechanisms, increasing sensitivity to imbalances, especially in infants and older adults.