Integrated Pharmacotherapy I: Fluid, Electrolytes & Hematology

Integrated Control of Volume, Osmolarity, and Blood Pressure

Theological and Symbolical Context of Water

• Biblical References: The Bible, across both the Old and New Testaments, contains many references to water and fluids.

  • Genesis 1:2: "The earth was without form and void, and darkness was over the face of the deep. And the Spirit of God was hovering over the face of the waters."

  • Statistical Frequency: Water is mentioned $722$ times in the Bible.

  • Symbolism: Water is used in numerous verses to symbolize the Spirit of God and life itself.

Learning Objectives for Fluid and Electrolyte Homeostasis

• Integration of Control: Identify factors contributing to the integrated control of volume, osmolarity, and blood pressure.

• Mitigation Strategies: Incorporate strategies to mitigate or prevent the clinical effects of hypovolemia and hypervolemia.

• Regulatory Roles: Describe the role of fluid and electrolyte homeostasis and the specific functions of the kidneys and lungs in disease management.

Standardized Terminology and Definitions

• Amount Quantification: An extensive language exists to standardize descriptions of fluid, electrolyte, and blood disorders.

  • Deficiency (Prefix -hypo or -ia):

    • Hypovolemia: Low blood volume.

    • Anemia: Low red blood cell count or hemoglobin.

    • Hypoproteinemia: General term for total protein levels below normal.

  • Excess (Prefix -hyper or -cytosis):

    • Hypervolemia: High fluid volume (occasionally referred to as edema).

    • Erythrocytosis: Increase in red blood cell count.

• Qualitative vs. Quantitative Changes:

  • Qualitative Example: Hypoalbuminemia (focus on specific protein type).

  • Quantitative Example: Anuria, defined as diuresis of less than $200\,ml$.

Core Principles of Homeostasis

• Etymology: Derived from Greek, meaning stability in a general sense.

• Definition: The maintenance of the internal environment in a relatively constant state compatible with life.

• Homeostatic Parameters:

  • Total Water Volume: $60-70\%$ of Total Body Water (TBW).

  • Blood Volume: $7-10\%$ of TBW.

  • Hematocrit (Hct): $45\%$.

  • Viscosity of Blood: $5\,U$.

  • Blood pH: $7.4$.

  • Cellular Components: Stability of Red Blood Cells (RBC), White Blood Cells (WBC), and Platelets (PL).

  • Biochemical Levels: Stability of protein and albumin; ions including $Na^+$, $K^+$, $Cl^-$, $Ca^{2+}$, and $Mg^{2+}+$

• Influencing Factors: Homeostasis is directly related to age, gender, and Body Mass Index (BMI).

Fluid Compartments and Distribution

• Abundance: Water is the most abundant molecule in the body. In a $70\,kg$ man, it constitutes approximately $60\%$ of body weight.

• Variation Ranges: This value can range from $40-80\%$ based on:

  • Body Fat: Adipose tissue contains very little water ($\sim 10\%$), whereas other soft tissue is approximately $75\%$ water.

  • Age: Water contribution decreases as an individual ages.

  • Gender: Differences exist due to estrogen’s role in promoting adipose deposition.

• Compartmental Division: Separated by cellular membranes into two main areas:

  • Intracellular Fluid (ICF): Comprises approximately $2/3$ of the total body water.

  • Extracellular Fluid (ECF): Comprises approximately $1/3$ of total body water; includes plasma, interstitial fluid, lymph, and transcellular fluid.

  • Redistribution Factor: The key factor in water redistribution is the permeability of the membrane.

Factors Affecting Fluid Distribution

1. Capillary Membrane Permeability: Increased by inflammation, injury, and burns, leading to "leakiness."

2. Hydrostatic Pressure (Blood Pressure): Driven by the Cardiovascular System (CVS). Capillary blood pressure pushes fluid out of the vasculature. Venous congestion can raise capillary pressure.

3. Oncotic (Colloid Osmotic) Pressure: Driven by total protein levels, primarily albumin.

   • Function: Albumin pulls fluid into the vasculature.

   • Clinical Decline: Falls during liver disease (low synthesis), malnutrition (low substrate), or renal loss.

4. Sodium and Water Balance: Regulated by cell membranes and the $Na^+-K^+$ pump.

5. Lymphatic Drainage: Functions to remove excess interstitial fluid and protein. Obstruction leads to fluid accumulation.

Clinical Case 1: Allergic Reaction

• Scenario: Local swelling (edema) following a bee sting allergic reaction.

• Analysis Questions:

  1. What fluid compartment volume increased?

  2. What is the mechanism of redistributing fluids?

• Key Consideration: Severe cellulitis at the site of an insect bite involves erythema, swelling, and painful movement (loss of ankle function).

Pathophysiology of Dehydration and Edema

• Dehydration Classifications:

  • Isotonic: Equal loss of water and $Na^+$ (e.g., hemorrhage, vomiting, diarrhea).

  • Hypotonic: $Na^+$ loss exceeds water loss; plasma osmolarity decreases ($\downarrow$). This causes cells to swell (e.g., adrenal insufficiency).

  • Hypertonic: Water loss exceeds $Na^+$ loss; plasma osmolarity increases ($\uparrow$). This causes cells to shrink (e.g., fever, diabetes insipidus).

• Etiology of Edema:

  • Increased Hydrostatic Pressure: Heart failure, venous obstruction.

  • Decreased Oncotic Pressure: Hypoalbuminemia from liver disease or nephrotic syndrome.

  • Increased Capillary Permeability: Inflammation, burns, allergic reactions, toxins, bacterial infections, prolonged ischemia, or immune reactions like vasculitis (releasing histamine).

  • Lymphatic Obstruction.

Hypovolemia vs. Hypervolemia

• Hypovolemia (Reduced ECF volume):

  • Causes: Hemorrhage, vomiting, diarrhea, burns, excessive sweating, diuretic overuse, third-spacing.

  • Signs & Symptoms: Tachycardia, hypotension, decreased skin turgor, dry mucous membranes, low urine output, thirst.

• Hypervolemia (Increased ECF volume):

  • Causes: Renal failure, heart failure, excess IV fluid administration, hyperaldosteronism, Cushing syndrome.

  • Signs & Symptoms: Peripheral edema, weight gain, hypertension, jugular venous distension, pulmonary congestion, dyspnea.

• Euvolemia: Normal ECF volume.

Body Water Balance and Excretion Routes

• Excretion Channels:

  • Kidneys: Primary route for ions and water.

  • Feces and Sweat: Minor loss of water and ions.

  • Lungs: Loss of water and bicarbonate via $CO_2$.

• Behavioral Mechanisms: Thirst and salt cravings.

• System Integration:

  • Respiratory System: Rapid response; chemical/neural control (pH and temperature).

  • Kidneys: Slow response; endocrine and neuroendocrine control.

• Daily Balance Statistics (Total $2.5\,L/day$):

  • Water Gain: Ingestion of food and drink ($2.2\,L/day$) + Metabolic production ($0.3\,L/day$).

  • Water Loss: Urine ($1.5\,L/day$), Skin sensible/insensible loss ($0.9\,L/day$), Lungs ($0.3\,L/day$), Feces ($0.1\,L/day$).

• Critical Constraint: Kidneys can only conserve volume; they cannot restore lost volume. Lost volume can only be replaced by input from outside the body.

Functional Anatomy and Physiology of the Kidney

• Blood Flow: Kidneys receive $25\%$ of cardiac output.

• Filtration Volume: They filter $200\,liters$ of blood daily.

• Urine Production: Normal daily diuresis is $1.5 - 2.0\,L/day$.

• Nephron Distribution:

  • $7/8$ of nephrons are cortical nephrons.

  • $1/8$ of nephrons are juxtamedullary nephrons.

• Renal Terms:

  • Oliguria: Reduced urine production.

  • Polyuria: Large production (>2.5-3\,L over $24\,hours$ in adults).

  • Renal Secretion: Moving solutes from plasma into urine.

  • Renal Reabsorption: Moving solutes from urine back into plasma.

  • Uricosuric Agents: Drugs that increase the mass of excreted uric acid.

• Non-urinary Functions:

  • Gluconeogenesis during prolonged fasting.

  • Renin production (Blood pressure regulation).

  • Erythropoietin (EPO) production (Stimulates RBC production).

  • Activation of Vitamin D production.

Glomerular Filtration Rate (GFR) and Kidney Assessment

• GFR Markers: Measured using inulin, PAH, or creatinine.

• Inulin/Creatinine Properties: Freely filtered by the glomerulus, not absorbed, not secreted, not metabolized, and do not alter GFR.

• GFR Formulas:

  • $\text{Amount Filtered} = \text{Amount Excreted}$

  • $GFR \times P_{in} = U_{in} \times \frac{dV}{dt}$

  • $GFR = \frac{U_{in} \times \frac{dV}{dt}}{P_{in}}$

  • Reference Values: If $P_{inulin} = 1\,mg/ml$, $U_{inulin} = 125\,mg/ml$, and $V = 1\,ml/min$, then $GFR = 125\,ml/min$.

• Blood Urea Nitrogen (BUN): Measures urea nitrogen in blood. Normal range: $6 - 24\,mg/dL$ ($2.1 - 8.5\,mmol/L$).

Urinalysis Reference Values (Averages)

Vasopressin (Arginine Vasopressin / AVP / ADH)

• Mechanism: Vasopressin makes the collecting duct epithelium permeable to water by stimulating aquaporin (AQP) insertion into the apical membrane through membrane recycling.

• Triggers for Release:

  • High plasma osmolarity (>280\,mOsM) sensed by hypothalamic osmoreceptors.

  • Decreased blood pressure sensed by carotid and aortic baroreceptors.

  • Decreased atrial stretch due to low blood volume.

• Properties of AVP:

  • Chemical Nature: $9$-amino acid peptide.

  • Origin: Hypothalamic neurons in paraventricular and supraoptic nuclei; released from the posterior pituitary.

  • Half-Life: $15\,min$.

  • Target: Renal collecting duct via $V2$ receptor/cAMP.

  • Circadian Pattern: Less urine is produced at night.

Intestinal Water Dynamics and CFTR

• Absorption Capacity: Small intestine absorbs $7-8\,L/day$ (capacity exceeds $20\,L$).

• Regional Statistics:

  • Small Intestine: Ingestion ($1.5\,L$) + Secretions ($7-9\,L$) = Total ($8.5-10\,L$); Reabsorbs $6.5-9\,L$.

  • Large Intestine: Receives $1.5-2\,L$; Reabsorbs $1.3-1.7\,L$.

  • Rectum: Contains $200\,ml$.

• CFTR (Cystic Fibrosis Transmembrane Conductance Regulator):

  • Function: $Cl^-$ channel protein on apical membranes. Actively pumps $Cl^-$ out of epithelial cells.

  • Secretion Mechanism: Chloride transport is regulated by cAMP. Sodium follows intercellularly to maintain electroneutrality, dragging water with it.

• Pathologies:

  • Loss-of-Function (Cystic Fibrosis): Reduced chloride/water secretion leads to thick, dehydrated mucus, intestinal obstruction (meconium ileus), and pancreatic insufficiency.

  • Overactivation (Cholera Toxin): Toxin locks CFTR in an "open" state, causing massive uncontrolled salt and water secretion resulting in severe secretory diarrhea.

• Therapies:

  • Cystic Fibrosis: CFTR modulators (e.g., Trikafta/ivacaftor), airway clearance, and nutritional support.

  • Cholera: Oral Rehydration Solution (ORS) uses glucose-sodium cotransport to bypass the CFTR defect. IV fluids (Lactated Ringer's) and antibiotics.

Plasma Protein Equilibrium

• Albumin: Most abundant plasma protein. Generates oncotic pressure because it is too large to cross capillary walls easily.

• Starling Balance:

  • Hydrostatic pressure pushes fluid out.

  • Oncotic pressure (Albumin) pulls fluid back in.

• Normal Ranges:

  • Albumin: $3.5 - 5.0\,g/dL$.

  • Total Protein: $6.0 - 8.3\,g/dL$.

  • Fibrinogen: $200 - 400\,mg/dL$.

• Hypoalbuminemia Impact: Reduced oncotic pressure causes fluid to leak into the interstitium faster than it returns, resulting in edema.

Hemolytic Osmotic Resistance

• RBC Stability: Normal resistance occurs in isotonic ($0.9\%\,NaCl$) conditions.

• Osmotic Thresholds: Minimal resistance at $0.42 - 0.48\%\,NaCl$; maximum resistance at $0.30 - 0.34\%\,NaCl$.

• Toxicity Note: Certain drugs, such as Rifampin, can cause hemolytic anemia.

Pharmacology of Loop Diuretics

• Administration: Can be oral or IV.

• Onset of Action:

  • Oral: $1\,hour$ (Torsemide) to $2\,hours$ (Furosemide).

  • IV Furosemide: Vasodilation in $5\,minutes$; diuresis in $20\,minutes$.

• Protein Binding: Loop diuretics are extensively bound to plasma proteins.

• Half-lives: $45\,minutes$ (Bumetanide) to $3.5\,hours$ (Torsemide).

• Excretion:

  • Furosemide: $65\%$ eliminated by the kidney; remainder metabolized.

  • Torsemide: $20\%$ eliminated by the kidney; $80\%$ metabolized.

Clinical Case 3: Integrated Evaluation

• Patient Profile: $53$-year-old male with alcohol use disorder and hyperlipidemia.

• Laboratory Data:

  • $Na: 142\,mEq/L$

  • $K: 3.4\,mEq/L$

  • $Cl: 88\,mEq/L$

  • $HCO_3: 36\,mEq/L$

  • Phosphate: $6\,mg/dl$

  • BUN: $12\,mg/dl$

  • Creatinine: $1.1\,mg/dl$

  • Albumin: $30\,g/l$

• Analysis: Identifying which values indicate a water imbalance.

Summary: Disturbances in Volume and Osmolarity

Questions & Discussion (True/False Review)

• Q: Homeostasis is the relative balance of parameters of the internal environment?

  • A: True.

• Q: Female adults have more water than male adults?

  • A: False.

• Q: Edema is a disorder related to water imbalance (increased ECF)?

  • A: True.

• Q: Potassium ($K$) is the key electrolyte for ICF?

  • A: True.

• Q: Hypoproteinemia cannot be a cause of edema?

  • A: False.

• Q: Water lost by lungs and skin is about $1.5 - 2.0\,L$?

  • A: False.

• Q: Kidneys cannot contribute to water balance?

  • A: False.

• Q: Proteins can easily pass the glomerular filtration barrier?

  • A: False.

• Q: The kidney has metabolic function?

  • A: True.

• Q: Glucose and Sodium reabsorption occur mostly in the PCT?

  • A: True.