5. elctrolyte balance

Fluid, Electrolyte and Acid-Base Homeostasis

• Body fluid: all the water and dissolved solutes in fluid compartments.

• Mechanisms regulate: total volume, distribution, concentration of solutes, and pH.

Balance Between Fluid Compartments

• Exchange between compartments occurs only at two sites:

  • Cell membranes separate intracellular fluid from interstitial fluid.

  • Capillary walls are thin enough for exchange between plasma and interstitial fluids.

• Fluid volumes in each compartment are kept constant. Water follows electrolytes, requiring their balance.

Water Balance

• Total body water for a 150 lb male: 40L.

• Fluid compartment distribution:

  • 65% Intracellular Fluid (ICF) - 26L

  • 35% Extracellular Fluid (ECF) - 14L

    • 25% Interstitial Fluid - 10L

    • 8% Blood Plasma & Lymph - 3.2L

    • 2% Transcellular Fluid (CSF, synovial fluid, etc.) - 0.8L

Body Water Gain and Loss

• Body water comprises 45-75% of body weight, which declines with age.

• Gain from ingestion and metabolic processes (2500 mL/day).

• Loss normally equals gain through urine, feces, sweat, and breathing.

Water Loss

• Routes of loss include urine, feces, expired breath, sweat, and cutaneous transpiration.

• Loss varies with environmental conditions and physical activity.

  • Respiratory loss increases in cold/dry air or heavy exertion.

  • Perspiratory loss rises with hot/humid conditions or intense physical work.

• Types of water loss:

  • Insensible water loss: breathing and skin evaporation.

  • Obligatory water loss: minimum urine output (400 mL/day) required for excretion.

Regulation of Water Gain

• Metabolic water formation is unregulated and based on ATP needs.

• Main regulator: intake regulation affecting thirst.

• Stimulators for thirst include dry mouth, osmoreceptors in hypothalamus, decreased blood volume causing drop in BP and angiotensin II release.

• Drinking restores body water levels back to normal.

Dehydration Effects

• Causes:

  • Increased blood osmolarity, stimulated by antidiuretic hormone (ADH).

  • Thirst center activation and dry mouth sensation lead to water ingestion, resulting in blood rehydration.

• Short-term effects: inhibition of thirst upon rehydration.

• Long-term effects: sustained impact on fluid homeostasis.

Satiation Mechanisms

• Short-term inhibition: cooling and moistening of the mouth and gastrointestinal tract.

• Long-term inhibition: rehydration of blood reduces blood osmolarity, ceasing osmoreceptor response and reducing salivation.

Regulation of Water and Solute Loss

• Excess water or solute elimination primarily through urination.

• Salty meal consumption demonstrates interaction of three hormones, affecting Na+ and water balance.

  • Water follows salt; excreting Na+ leads to concurrent water excretion, decreasing blood volume.

Hormone Effects on Solutes

• Hormonal regulation includes:

  • Angiotensin II and aldosterone facilitating Na+ and Cl- reabsorption, increasing fluid volume.

  • Atrial natriuretic peptide (ANP) promotes natriuresis (Na+ excretion), decreasing blood volume.

• Increased filtration rates reduce water and Na+ reabsorption.

Hormone Regulation of Water Balance

• ADH influences thirst and assists in water reabsorption by enhancing permeability of collecting duct cells.

• ADH secretion increases with significant blood volume decreases, severe dehydration, vomiting, diarrhea, sweating, or burns.

Movement of Water

• Normally, intracellular and interstitial fluids have the same osmolarity, preventing cell swelling or shrinking.

• Water intoxication can cause swollen cells if plasma Na+ concentration falls too low due to rapid plain water consumption or improper rehydration post-diarrhea/vomiting.

Disorders of Water Balance

• Fluid Deficiency:

  • Volume depletion (hypovolemia) with normal osmolarity.

  • Common causes: hemorrhage, severe burns, chronic vomiting, and diarrhea.

• Dehydration:

  • Total body water decreases, osmolarity increases.

  • Causes include lack of water intake, diabetes, profuse sweating, and diuretics.

  • Infants are particularly vulnerable due to higher metabolic rates and surface area-to-volume ratios.

Fluid Loss & Balance Processes

1. Profuse sweating- causes fluid loss.

2. Blood volume and pressure drop while osmolarity rises.

3. Blood absorbs tissue fluid to replace loss.

4. Fluid is drawn from intracellular fluid (ICF).

Fluid Excess

• Volume excess (hypervolemia): Na+ and water retention leads to isotonic ECF.

  • Possible causes: aldosterone hypersecretion, congestive heart failure.

• Hypotonic hydration: more water retained than Na+, leading to hypotonic ECF and cellular swelling.

  • Highest concern: pulmonary and cerebral edema.

Blood Volume & Fluid Intake

• Relationship between fluid intake and kidney compensation capabilities.

• Kidneys manage excess fluid intake well, but struggle with inadequate intake.

Forms of Fluid Imbalance (Table 24.1)

• Fluid Deficiency: Volume depletion, decreased total body water, isotonic & hypertonic conditions.

• Fluid Excess: Volume excess, high total body water, isotonic & hypotonic states.

Electrolytes Concentrations and Functions

• Functions:

  • Control osmosis between compartments.

  • Maintain acid-base balance and carry electrical current.

  • Cofactors for enzymatic activity.

• Concentrations expressed in mEq/liter for plasma, interstitial, and intracellular fluids.

Comparison Between Fluid Components

• Plasma contains proteins; interstitial fluid does not, affecting blood colloid osmotic pressure.

• Na+ and Cl- are predominant in ECF.

• K+ and phosphates are prominent in ICF, alongside protein anions.

Sodium Functions

• Membrane potentials are largely sodium-driven.

• Na+ accounts for 90-95% of ECF osmolarity.

• Na+-K+ pump creates gradients for solute cotransport, generating heat.

• Sodium bicarbonate (NaHCO3) plays a significant role in pH buffering.

Sodium Homeostasis

• Primary concern: dietary excess excretion (0.5 g/day necessary; diets often contain 3-7 g/day).

• Hormones involved:

  • Aldosterone: increases renal Na+/K+ pumps, reabsorbing more Na+ and reducing K+.

  • ADH: increases water reabsorption without Na+ retention.

  • ANP promotes Na+ and water excretion, lowering BP/volume.

Sodium Imbalances

• Hypernatremia: Plasma sodium > 145 mEq/L, causing water retention, hypertension, edema.

• Hyponatremia: Plasma sodium < 130 mEq/L, resulting from excess body water, quickly corrected by excretion.

Potassium Functions

• Most abundant cation in ICF; crucial for osmolarity, membrane potential, and Na+-K+ pump function.

Potassium Homeostasis

• 90% K+ in glomerular filtrate reabsorbed by PCT; DCT and cortical collecting ducts secrete K+ based on blood levels.

Secretion and Effects of Aldosterone

• Stimulated by hypotension, hyponatremia, and hyperkalemia, leading to increased Na+ reabsorption and K+ secretion.

• Supports fluid balance and Na+ concentration for urine output.

Potassium Imbalances

• Most dangerous electrolyte imbalances.

• Hyperkalemia: Acute increases in ECF K+ make muscle cells excitable, while chronic increases result in decreased excitability.

• Hypokalemia: Results from sweating, chronic vomiting, laxatives, leading to muscle weakness, reduced reflexes, and arrhythmias.

Potassium & Membrane Potentials

• Hyperkalemia: Increases extracellular K+, causing membrane depolarization and heightened excitability.

• Hypokalemia: Lowers extracellular K+, resulting in hyperpolarization and reduced excitability, risking muscle function.

Chloride Functions

• Key for ECF osmolarity, stomach acidity, and pH regulation.

Chloride Homeostasis

• Follows Na+, K+, and Ca2+ passively to maintain balance.

Chloride Imbalances

• Hyperchloremia: Result of dietary excess IV saline.

• Hypochloremia: Typically linked to hyponatremia affecting pH balance.

Calcium Functions

• Essential for skeletal mineralization, muscle contraction, second messenger signaling, exocytosis, and blood clotting.

Calcium Homeostasis

• Regulated by PTH, calcitriol (vitamin D), and calcitonin, affecting bone metabolism and urinary excretion.

Calcium Imbalances

• Hypercalcemia: Caused by acidosis, hyperparathyroidism, inhibiting depolarization and causing weakness/arrhythmias.

• Hypocalcemia: Increased Na+ permeability leads to excitation, with severe cases resulting in tetanus and potential death.

Phosphate Functions

• Important in nucleic acids, phospholipids, ATP production, and buffering pH, primarily concentrated in ICF.

Phosphate Homeostasis

• Renal control with reabsorption influenced by parathyroid hormone; body tolerates wide variability in phosphate levels.

Bicarbonate

• Major extracellular anion acting as a primary pH buffer, regulated mainly by kidneys that synthesize and excrete bicarbonate as needed.

Magnesium

• Acts as a cofactor for metabolic processes, contributing to nerve and muscle function; urinary excretion varies based on calcium status and other factors.

Acid-Base Balance

• Critical in maintaining H+ concentration within 7.35 - 7.45 pH range.

• Regulation mechanisms include buffer systems, respiratory CO2 exhalation, and kidney H+ excretion.

Actions of Buffer Systems

• Act to prevent rapid pH changes, some transforming strong acids/bases into weaker forms; key systems include protein, carbonic acid-bicarbonate, and phosphate buffers.

Protein Buffer System

• Effective in plasma and intracellular fluids, using hemoglobin in RBCs and albumin in plasma for H+ buffering.

Carbonic Acid-Bicarbonate Buffer System

• Functions in both ECF and ICF; bicarbonate can act as a weak base, with critical roles in metabolism and respiration.

Phosphate Buffer System

• Buffers strong acids in intracellular environments and urine, maintaining optimal pH balance in accordance with metabolic needs.

Exhalation of Carbon Dioxide

• Changes in breathing rate adjust blood pH; faster respiration decreases pCO2, raising pH, while slower respiration can lower it.

Kidneys & pH Control

• Excrete H+ from metabolic processes and synthesize bicarbonate, crucial for managing blood pH and addressing acidosis or alkalosis.

Acid-Base Imbalances

• Acidosis: Blood pH < 7.35; causes depression of CNS.

• Alkalosis: Blood pH > 7.45; causes nervous system excitability leading to spasms and convulsions.

Compensation Mechanisms

• Respiratory system adjusts ventilation for rapid compensation, while renal compensation is slower but more effective for prolonged imbalances.

Diagnosis of Acid-Base Imbalances

• Evaluated by systemic arterial blood pH, bicarbonate concentration, and PCO2; identify respiratory vs metabolic causes based on these markers.

Homeostasis in Infants

• Infants have higher body water content prone to disturbances, high fluid turnover, immature kidney function, and higher metabolic demands.

Impaired Homeostasis in the Elderly

• Age-related decline in intracellular fluid volume, muscle mass, and renal function complicates fluid and electrolyte balance.