BI 211 Fluids and Electrolytes Vocabulary

Anatomic and Functional Fluid Compartments

• Anatomic Compartments

  • Defined by physical, structural boundaries within the body.
  • Includes major body cavities: thoracic cavity, abdominopelvic cavity, and cranial cavity.
  • Includes anatomical lumens within hollow organs and blood vessels.
  • Includes specialized joint spaces (synovial cavities).

• Intracellular Organellar Compartments

  • Physical separation inside cells created by membrane-bound organelles.
  • Allows isolation of incompatible biological processes: specific organelles specialize in synthesis (e.g., endoplasmic reticulum), while others specialize in degradation (e.g., lysosomes).
  • Membrane entry dynamics determine metabolic efficiency: the ability of substrate molecules to cross the mitochondrial membrane determines whether respiration proceeds aerobically or anaerobically, dictating the net energy ($\text{ATP}$) derived per sugar molecule.

• Functional Fluid Compartments

  • Classification based on fluid distribution and the specific solutes dissolved inside and outside cells.
  • Total Body Water ($\text{TBW}$): Approximately $44\,\text{L}$ in an average adult.
  • Intracellular Fluid ($\text{ICF}$):
  • The larger of the two primary functional compartments, accounting for $65\% - 70\%$ of total body water.
  • Contains approximately $28\,\text{L}$ of fluid.
  • Consists of all fluid contained within cellular plasma membranes.
  • Extracellular Fluid ($\text{ECF}$):
  • Contains approximately $16\,\text{L}$ of fluid.
  • Consists of all fluid residing outside of cellular plasma membranes.
  • Subdivided into two primary sub-compartments:
    • Intravascular fluid (blood plasma within vessels).
    • Interstitial fluid (fluid surrounding cells in tissues).

Microcirculation and Capillary Hydrodynamics

• Capillary Fluid Movement Principles

  • Capillary fluid exchange is governed by the balance between Capillary Hydrostatic Pressure ($\text{CHP}$) and Blood Colloidal Osmotic Pressure ($\text{BCOP}$).
  • $Net\,Filtration\,Pressure\,(NFP) = CHP - BCOP$

• Arterial End Dynamics (Filtration)

  • Capillary Hydrostatic Pressure ($\text{CHP}$): $35\,\text{mm Hg}$
  • Blood Colloidal Osmotic Pressure ($\text{BCOP}$): $25\,\text{mm Hg}$
  • Net Filtration Pressure ($\text{NFP}$): $+10\,\text{mm Hg}$
  • Physiological Result: Fluid exits the capillary bed into the interstitial space because hydrostatic pressure pushing fluid out exceeds colloidal osmotic pressure pulling fluid in.

• Mid-Capillary Dynamics (Equilibrium)

  • Capillary Hydrostatic Pressure ($\text{CHP}$): $25\,\text{mm Hg}$
  • Blood Colloidal Osmotic Pressure ($\text{BCOP}$): $25\,\text{mm Hg}$
  • Net Filtration Pressure ($\text{NFP}$): $0\,\text{mm Hg}$
  • Physiological Result: Zero net fluid movement between intravascular and interstitial spaces.

• Venous End Dynamics (Reabsorption)

  • Capillary Hydrostatic Pressure ($\text{CHP}$): $18\,\text{mm Hg}$
  • Blood Colloidal Osmotic Pressure ($\text{BCOP}$): $25\,\text{mm Hg}$
  • Net Filtration Pressure ($\text{NFP}$): $-7\,\text{mm Hg}$
  • Physiological Result: Fluid re-enters the capillary from the interstitial space because blood colloidal osmotic pressure exceeds hydrostatic pressure.

Cell Membrane Dynamics and Osmotic States

• Membrane Permeability Rules

  • Fat-Soluble Substances: Pass directly through the phospholipid bilayer via simple diffusion. Includes steroid hormones, fat-soluble vitamins (A, D, E, K), and lipid-soluble gases ($\text{O}_2$, $\text{CO}_2$).
  • Water-Soluble Substances: Cannot cross the lipid core unassisted. Require specialized membrane structures such as gated channels, leak channels, pumps, or carrier proteins, rendering the plasma membrane selectively permeable.
  • Water Movement: Moves rapidly across cellular membranes through specialized channel proteins called aquaporins. Aquaporin-mediated water movement occurs rapidly to equalize osmolarity across compartments, which can drastically alter cell volume.

• Cellular Effects of Solution Tonicity

  • Hypotonic Solution: Solute concentration in the extracellular environment is lower than inside the cell. Water flows down its concentration gradient into the cell ($\text{H}_2\text{O}$ influx), leading to cell swelling and eventual lysis.
  • Isotonic Solution: Solute concentration in the extracellular environment equals that inside the cell. Equal rates of water influx and efflux yield no net change, maintaining normal cell volume and morphology.
  • Hypertonic Solution: Solute concentration in the extracellular environment is higher than inside the cell. Water leaves the cell ($\text{H}_2\text{O}$ efflux), causing cellular dehydration and shriveling (crenation).

• Equilibrium vs. Disequilibrium States

  • Osmotic Equilibrium: Total concentration of dissolved solutes inside versus outside cells is virtually equal, maintaining an overall isotonic state.
  • Chemical Disequilibrium: While overall solute numbers are equal, specific chemical species are unevenly distributed between the $\text{ICF}$ and $\text{ECF}$.
  • Electrical Disequilibrium: The intracellular environment carries a net negative ($-$) charge relative to the positively ($+$) charged extracellular environment, despite the human body being electrically neutral as a whole.
  • Physiological Role: Chemical and electrical disequilibria store potential energy utilized for crucial physiological processes, including action potential generation, neuron depolarization, cardiac conduction, and muscle depolarization.

Ion Distribution and Physiological Roles

• Compartmental Specificity of Ions

  • Extracellular Fluid ($\text{ECF}$):
  • Major Cation: Sodium ($\text{Na}^+$)
  • Major Anions: Chloride ($\text{Cl}^-$), Bicarbonate ($\text{HCO}_3^-$)
  • Intracellular Fluid ($\text{ICF}$):
  • Major Cations: Potassium ($\text{K}^+$), Magnesium ($\text{Mg}^{++}$)
  • Major Anions: Phosphate ($\text{PO}_4^{3-}$), Anionic Proteins ($\text{Protein}^-$)

• Sodium ($\text{Na}^+$)

  • Function: Major extracellular cation. Exerts the primary control over total body water volume and fluid distribution between functional compartments. Drives membrane depolarization (alongside $\text{Ca}^{++}$) for neuronal transmission, cardiac conduction, and muscle contraction.
  • Osmotic Principle: Water passively follows $\text{Na}^+$ movement across fluid compartments.
  • Hormonal Regulation:
  • Atrial Natriuretic Peptide (ANP): Promotes renal excretion of $\text{Na}^+$, causing passive water excretion in urine.
  • Aldosterone: Increases renal reabsorption and retention of $\text{Na}^+$, causing passive water retention in blood.
  • Antidiuretic Hormone (ADH): Increases direct water reabsorption in renal collecting ducts without directly transporting $\text{Na}^+$, thereby diluting $\text{ECF}$ sodium concentration.

• Potassium ($\text{K}^+$)

  • Function: Major intracellular cation. Primary determinant of cellular resting membrane potential ($\text{RMP}$).
  • Clinical Significance: Potassium imbalances represent the most acutely dangerous electrolyte disturbances due to their capacity to induce fatal cardiac arrhythmias.
  • Hormonal Regulation: Direct endocrine control via local stimulation. Elevated serum $\text{K}^+$ levels directly stimulate the adrenal cortex to synthesize and secrete aldosterone, which increases renal secretion and urinary excretion of $\text{K}^+$.

• Calcium ($\text{Ca}^{++}$)

  • Function: Essential structural component of bone hydroxyapatite ($\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$). Essential for excitation-contraction coupling in muscle tissue, vascular tone regulation, exocytosis of synaptic neurotransmitters, and coagulation cascades.
  • Compartmentalization: Primary extracellular cation. Kept at extremely low intracellular concentrations to prevent precipitation with high intracellular phosphate ($\text{PO}_4^{3-}$) concentrations.
  • Hormonal Regulation:
  • Calcitonin: Secreted by parafollicular C-cells of the thyroid gland to lower serum calcium levels.
  • Parathyroid Hormone (PTH): Secreted by parathyroid glands to raise serum calcium levels.

• Chloride ($\text{Cl}^-$)

  • Function: Most abundant extracellular anion. Required for gastric hydrochloric acid ($\text{HCl}$) synthesis. Participates in the chloride shift, exchanging with $\text{HCO}_3^-$ across red blood cell membranes during systemic $\text{CO}_2$ transport.
  • Regulation: Passively follows $\text{Na}^+$ electrical transport gradients; retained when $\text{Na}^+$ is retained and excreted when $\text{Na}^+$ is excreted.

Fluid and Electrolyte Disturbances

• Regulation of Intake and Output

  • Fluid Intake: Driven primarily by thirst, regulated by osmoreceptors in the hypothalamus. Secondary sources include ingested food and metabolic water production. Thirst sensations are temporarily suppressed by gastric distension prior to complete systemic absorption.
  • Fluid Output: Controlled by renal tubular responses to aldosterone, ADH, and ANP. Unregulated output occurs via feces, cutaneous evaporation/sweating, and pulmonary respiration.
  • Pathological Losses: Conditions like severe diarrhea induce massive fluid and electrolyte output, severely compromising system volume and balance.

• Hypovolemia vs. Dehydration

  • Volume Depletion (Hypovolemia): Proportional loss of both water and sodium ($\text{Na}^+$).
  • Causes: Acute hemorrhage, extensive burns, severe vomiting, chronic diarrhea, and Addison's disease (hypoaldosteronism).
  • Clinical Consequences: Severe hypotension, reduced tissue perfusion, and hypovolemic circulatory shock.
  • Dehydration: Loss of body water without proportional loss of $\text{Na}^+$, causing elevated $\text{ECF}$ osmolarity.
  • Causes: Diabetes Insipidus (ADH insufficiency), profuse diaphoresis (sweating), or inadequate fluid intake (e.g., bedridden patients).
  • Clinical Consequences: Cellular dehydration, hypotension, and circulatory shock.

• Fluid Excess States

  • Volume Excess: Retention of equal proportions of water and $\text{Na}^+$.
  • Causes: Hypersecretion of aldosterone (e.g., functional adrenal cortical tumors) or renal failure.
  • Clinical Consequences: Circulatory strain, expanded vascular volume, and hypertension.
  • Hypotonic Hydration (Water Intoxication): Retention of excess water without adequate $\text{Na}^+$ intake.
  • Causes: Over-ingestion of free water following strenuous physical activity or excessive ADH release (SIADH).
  • Clinical Consequences: Cellular swelling, dilutional hyponatremia, circulatory stress, pulmonary edema, and cerebral edema.

• Sequestration, Edema, and Third Spacing

  • Fluid Sequestration: Pathological accumulation and isolation of excess fluid in a localized space.
  • Edema: Sequestration specifically within interstitial spaces. Caused by increased capillary hydrostatic pressure, decreased plasma colloid osmotic pressure, increased capillary permeability (inflammation), or lymphatic obstruction.
  • Effusion: Fluid accumulation within anatomical potential spaces, such as pleural effusion occurring between the visceral and parietal pleura.
  • Third Spacing: Shift of significant fluid volumes from the intravascular space (second space) into interstitial or non-functional body compartments (third space).
  • Reduces intravascular blood volume, causing cardiovascular strain and severe hypotension.
  • Fluid trapped in non-exchangeable compartments (e.g., burn blisters) is unavailable for normal exchange.

Clinical Pathologies and Electrolyte Imbalances

• Osmolality Conditions

  • Hyperosmolality: Abnormally high solute concentration in body fluids.
  • Causes: Excessive intake of salty or sweet foods, reduced renal clearance (renal failure), Diabetes Mellitus, excessive water loss (Diabetes Insipidus, overuse of loop diuretics), and severe dehydration.
  • Hyposmolality: Abnormally low solute concentration in $\text{ECF}$.
  • Causes: Excessive unreplaced water consumption (hypotonic hydration) and hyponatremia.

• Diabetes Mellitus (DM)

  • Osmotic Diuresis: Glucosuria exceeds renal tubular transport maximums. High urinary glucose concentration exerts osmotic force, inhibiting renal water reabsorption and producing marked polyuria.
  • Hyperosmotic ECF: Severe hyperglycemia raises $\text{ECF}$ osmolarity, drawing water out of cells (intracellular dehydration) and diluting extracellular electrolytes. Triggers constant thirst (polydipsia).
  • Ketoacidosis: Systemic accumulation of ketoacids increases extracellular $\text{H}^+$. Cellular uptake of $\text{H}^+$ displaces intracellular $\text{K}^+$ into the $\text{ECF}$, producing hyperkalemia.
  • Secondary Shifts: Polyuria and therapeutic insulin administration (which drives glucose and $\text{K}^+$ into cells) can subsequently lead to severe hypokalemia. Progressive DM-induced renal failure impairs $\text{1,25-dihydroxyvitamin D}$ activation, inducing hypocalcemia.

• Diabetes Insipidus (DI)

  • Central DI: Deficient production, storage, or secretion of ADH from the hypothalamic-pituitary system due to head trauma, tumors, or elevated intracranial pressure.
  • Nephrogenic DI: Failure of renal collecting duct principal cells to respond to circulating ADH.
  • Clinical Features: Polydipsia, massive polyuria ($10\times - 20\times$ normal daily urine output), extreme hypovolemia, hypotension, produce of very dilute urine, and hypernatremia (excessive free water clearance sparing $\text{Na}^+$).

• Syndrome of Inappropriate ADH (SIADH)

  • Pathology: Unregulated hypersecretion of ADH resulting in pathological fluid retention.
  • Causes: Side effect of medications, post-neurosurgical procedures, traumatic brain injury, central nervous system inflammation/infections, cerebrovascular accidents (stroke), hypothalamic/pituitary tumors, substance abuse disorders, and ectopic ADH synthesis by malignant tumors (e.g., small cell lung carcinoma).
  • Clinical Features: Highly concentrated urine, dilutional hyponatremia, fatigue, malaise, confusion, memory impairment, abnormal behavior, nausea, vomiting, and seizures. Notably, hypertension is typically absent.

• Addison's Disease

  • Pathology: Adrenal cortical insufficiency causing marked hypoaldosteronism.
  • Clinical Features: Severe orthostatic hypotension, hyponatremia, and hyperkalemia.
  • Dermatologic Sign: Deficient cortisol and aldosterone feedback increases pituitary synthesis of Adrenocorticotropic Hormone (ACTH). High ACTH levels cross-stimulate cutaneous melanocytes, causing systemic hyperpigmentation.

• Hyperparathyroidism

  • Primary Hyperparathyroidism: Pathological hypersecretion of PTH due to parathyroid adenomas or gland hyperplasia.
  • Features: Elevated serum calcium (hypercalcemia), increased bone resorption leading to fragile bones, and nephrolithiasis (kidney stones).
  • Secondary Hyperparathyroidism: Compensatory response to chronic kidney disease. Impaired renal phosphate excretion and loss of active vitamin D synthesis induce chronic hypocalcemia, continuously stimulating PTH release.

• Liver Cirrhosis

  • Pathology: Irreversible hepatic fibrosis and destruction, frequently secondary to chronic alcoholism.
  • Fluid Disturbances: Portal hypertension paired with impaired albumin synthesis (hypoalbuminemia) forces massive fluid displacement from blood vessels into the peritoneal cavity (ascites).
  • Systemic Impact: Intravascular volume depletion causes prerenal failure due to decreased renal perfusion, provokes complex electrolyte imbalances, and pathologically hyperactivates the Renin-Angiotensin-Aldosterone System (RAAS).

• Acid-Base Disturbances and Potassium Shifts

  • Acidosis: Excess extracellular $\text{H}^+$ enters cells to be buffered. Intracellular $\text{K}^+$ exits into the $\text{ECF}$ to preserve electrical charge neutrality, causing hyperkalemia. Concurrently, renal tubules prioritize excretion of $\text{H}^+$ over $\text{K}^+$, promoting potassium retention.
  • Alkalosis: Extracellular $\text{H}^+$ deficiency causes intracellular $\text{H}^+$ to exit cells into $\text{ECF}$. $\text{ECF}$ potassium shifts into cells to maintain electrical balance, producing hypokalemia.

• Burns and Crush Injuries

  • Burns: Destruction of the epidermal barrier combined with systemic inflammatory capillary permeability causes massive loss of plasma fluid and proteins into interstitial spaces (hypovolemia). Insensible water loss drives hyponatremia. Severe direct tissue necrosis releases large intracellular stores of $\text{K}^+$ into the $\text{ECF}$, causing hyperkalemia.
  • Crush Injuries: Extensive cell destruction, rhabdomyolysis, or intravascular hemolysis releases intracellular contents into $\text{ECF}$, producing severe hyperkalemia and hyperphosphatemia.

• Burn and Ulcer Categorization

  • 1st Degree (Superficial): Involves epidermis only. Characterized by erythema, localized edema, and pain without blister formation.
  • 2nd Degree (Superficial & Deep Partial-Thickness): Extends through the epidermis into variable depths of the dermis. Characterized by blister formation, moist/weeping bases, and severe pain.
  • 3rd Degree (Full-Thickness): Extends completely through the epidermis and dermis into underlying subcutaneous tissue, fascia, muscle, or bone. Appears white, charred, or leathery; insensitive to pain at the center due to complete destruction of nociceptive nerve terminals.
  • Complications: Hypovolemic shock, overwhelming sepsis, severe electrolyte derangements (hyperkalemia, hyponatremia), hypothermia, and joint contractures.