Chapter 1-6: Fluid Balance and IV Fluids Review

Chapter 1 – Foundations of Fluid Balance

• Objective: Understand how water is distributed in the body and how balance is maintained; move toward clinical reasoning beyond memorization.
• Body water content
  • Most adults are about 60\% water by weight: \text{TBW} \approx 0.60 \times \text{body weight}
• Fluid compartments
  • Intracellular fluid (ICF): inside cells; accounts for roughly two-thirds of TBW: \text{ICF} \approx \frac{2}{3}{\text{TBW}}
  • Extracellular fluid (ECF): outside cells; about one-third of TBW: \text{ECF} \approx \frac{1}{3}{\text{TBW}}
  • Interstitial fluid: outside the plasma, not inside cells
  • Plasma: the free water component of blood; vessels carry a lot of water in plasma
  • Transcellular fluid: small amount, not between cells or in plasma; not a major focus here
• Electrolytes and charges
  • Electrolyte = charged molecule (ion); can be cation (+) or anion (−)
  • Inside the cell (ICF): main ions = \text{K}^+ and \text{Mg}^{2+} (phosphate also inside; mentioned for later)
  • Outside the cell (ECF): main cation = \text{Na}^+; main anion outside = \text{Cl}^-; bicarbonate \text{HCO}_3^- mentioned as important for future acid–base topics
  • Plasma contains albumin and other proteins influencing oncotic (colloid osmotic) pressure
• Forces moving fluid
  • Two broad forces in vascular space: push (hydrostatic) and pull (oncotic/osmotic)
  • Hydrostatic pressure pushes fluid out of vessels, especially arterial side
  • Oncotic (osmotic) pressure pulls fluid into intravascular space; prominent in veins; largely driven by albumin
• Albumin and pharmacokinetics
  • Albumin in plasma binds drugs; free drug is the portion not bound to protein
  • Hypoalbuminemia (low albumin) reduces oncotic pull; more free drug distribution; increased risk of fluids leaving vasculature into interstitial space leading to edema
• Lymphatic return
  • Lymphatic system helps return excess fluid to circulation
• Clinical takeaways (context and implications)
  • When low albumin is seen on labs, suspect risk of fluid leakage into interstitial space and edema
  • Hormonal regulation (brief mention): hormones help regulate water and sodium balance; detailed discussion in later modules

Chapter 2 – Amount of Fluid

• Fluid movement is governed by the same basic compartments introduced in Chapter 1
• Interplay between compartments explains why shifts in volume affect intravascular volume and interstitial space
• Exam strategy example (conceptual): when evaluating a multiple-choice item about fluid distribution, reason through each option rather than memorize blindly; focus on why the correct option is correct and why others are not
  • Example reasoning from transcript: most body water is inside cells (ICF), not outside (ECF/plasma)
  • Within ECF, sodium is the primary extracellular cation; potassium is predominant intracellular
  • Transcellular fluid is a small fraction of total body water

Chapter 3 – Low Solute Concentration and Diffusion/Osmosis

• Diffusion basics (no energy required): movement of solutes from high to low concentration across a permeable membrane
• Osmosis basics (diffusion of water): water moves from areas of low solute concentration to high solute concentration to balance solute levels
• Energy requirements
  • Movement of solutes against their gradient requires energy (ATP) via cellular pumps
  • The Na⁺/K⁺ pump is the most important pump discussed for maintaining intracellular/extracellular balance
• Cellular injury and energy failure
  • Etiology and mechanism: injury that reduces ATP production impairs pumps (e.g., Na⁺/K⁺-ATPase), leading to disrupted fluid/electrolyte balance
  • If pumps fail, cells may undergo necrosis (unplanned cell death) rather than apoptosis
• Hormonal regulation (high-level view)
  • Hormones are key regulators of water and sodium balance; detailed discussion deferred to later modules
• Thirst and osmolality
  • Thirst is a subjective drive controlled by the hypothalamus
  • Osmolality = solute per unit measurement of fluid; high plasma osmolality indicates higher solute concentration (less free water relative to solute)
  • Thirst can be blunted in older adults, increasing risk of imbalance
• Diurnal and clinical implications
  • When ADH is excessive, the body retains water, potentially causing hyponatremia if sodium balance is not appropriately managed
  • Diabetes insipidus is the opposite: insufficient ADH leading to excessive water loss and potential hypernatremia

Chapter 4 – Hormonal Regulation and Fluid Balance (Regulators and Imbalances)

• Antidiuretic hormone (ADH/vasopressin)
  • Promotes water retention by the kidneys; high ADH -> more water reabsorbed; decreased plasma osmolality can result if water retention is excessive
• Aldosterone (mineralocorticoid)
  • Increases sodium (and thus water) reabsorption in kidneys; when sodium is retained, water follows, increasing extracellular fluid volume
• Natriuretic peptides (BNP, ANP)
  • Released in heart failure; promote natriuresis (sodium excretion) and diuresis to reduce volume
• Thirst mechanism and aging
  • Thirst signals via hypothalamus; aging can blunt the thirst response, complicating fluid balance
• Summary implication for nursing care
  • Imbalances can arise from hormonal dysregulation, organ failure, and medications; monitoring is essential

Chapter 5 – Intravenous Fluids: Tonicity and Product Types

• Tonicity and fluid shifts
  • Tonicity describes a solution's effect on water movement across a membrane
  • Three types: isotonic, hypotonic, hypertonic
• Crystalloids vs colloids
  • Crystalloids: small molecules that move between plasma and cells; most fluids used clinically fall in this category
  • Isotonic crystalloids: exert the same tonicity as blood; no net fluid shift across membranes
  • Hypotonic crystalloids: lower solute concentration than plasma; draw water into cells
  • Hypertonic crystalloids: higher solute concentration than plasma; draw water out of cells
  • Colloids: contain larger molecules (e.g., proteins) that affect oncotic pressure; examples include albumin; act by increasing intravascular oncotic pressure to pull fluid into vessels
• Clinical notes on IV fluids
  • Isotonic fluids (e.g., normal saline, lactated Ringer’s) are typically used to treat hypovolemia without changing cell size dramatically
  • Colloids can pose allergy risks; careful monitoring is necessary
  • All IV fluids require monitoring for potential fluid overload (pulmonary edema, edema, hypertension) and for urine output (UOP) and lung sounds
• Normal saline and lactated Ringer’s
  • Normal saline (0.9% NaCl): electrolyte composition similar to plasma sodium and chloride; preserves intravascular volume without changing tonicity relative to plasma
  • Lactated Ringer’s: isotonic crystalloid; contains electrolytes plus lactate buffer; used similarly to NS
• Practical nursing considerations
  • Lung auscultation and urine output are key monitoring parameters when infusing IV fluids to detect overload or under-resuscitation
  • Isotonic fluids are the default for hypovolemia; risks of edema if given in excess

Chapter 6 – Conclusion and Practical Implications for Fluid Therapy

• Isotonic fluids for hypovolemia
  • The goal is to restore intravascular volume without shifting water into or out of cells; isotonic fluids accomplish this without changing cell size significantly
• Risks of fluid therapy
  • Even isotonic fluids can cause hypervolemia if given in excess; watch blood pressure and edema
  • Edema often manifests in dependent regions due to gravity (often “third spacing” where fluid collects in interstitial spaces rather than within cells or vessels)
• Third spacing
  • Fluid is trapped in interstitial spaces (not in cells or plasma) and is not readily available for metabolic needs
• Indications for hypotonic fluids
  • If cells are dehydrated but intravascular volume is adequate, hypotonic fluids can draw water into cells to rehydrate intracellularly
  • This strategy creates a deliberate osmotic gradient to favor water movement into cells
• Hypotonic fluid and hypernatremia management (example)
  • In hypernatremia, consider hypotonic fluids to lower plasma osmolality; an example cited is half-normal saline (0.45% NaCl)
• Summary principle
  • Fluid therapy is about balancing tonicity, electrolyte status, and volume status while monitoring organ function (lungs, kidneys) to avoid fluid overload or inadequate resuscitation

Quick reference: Key definitions and concepts

• TBW: Total body water; ~60% body weight
• ICF: Intracellular fluid (~2/3 of TBW)
• ECF: Extracellular fluid (~1/3 of TBW)
• Plasma: Free water in blood; component of ECF
• Interstitial fluid: Fluid between cells, outside plasma
• Transcellular fluid: Small, additional fluid compartment
• Electrolytes: Ions with electrical charges; drive fluid movement
• Major ions
  • Inside cell: \text{K}^+, \text{Mg}^{2+} (also phosphate)
  • Outside cell: \text{Na}^+, \text{Cl}^-, \text{HCO}_3^-
• Oncotic pressure: Pulls fluid into intravascular space; mainly due to albumin
• Hydrostastic pressure: Pushes fluid out of vessels, esp. arteries
• Thirst regulation: Hypothalamus
• Osmolality: Solute concentration per unit fluid; high osmolality = high solute, relatively less free water
• Hormonal regulators: ADH, aldosterone, BNP/ANP
• Fluid types
  • Crystalloids: small solutes; isotonic/hypotonic/hypertonic; NS or LR are common isotonic examples
  • Colloids: large molecules (e.g., albumin) that raise oncotic pressure
• IV therapy safety checks: monitor lung sounds, urine output (UOP), blood pressure, edema
• Third spacing: fluid accumulation in interstitial spaces not readily usable for perfusion