Lecture 4: Fluid Homeostasis & Imbalances

TOTAL BODY WATER (TBW):

• Sum of all fluids in the body

• ~60% of adult body weight

• 1 liter of water = 1 kg = 2.2 lb

• Percentage varies by:
  ○ Age (decreases with aging)
  ○ Sex (lower in females due to higher fat content)

• Muscle contains more water than fat

BODY FLUID COMPARTMENTS:

Intracellular fluid (ICF)

• Fluid inside cells

• ~2/3 of total body water

• Major electrolyte: potassium (K⁺)

Extracellular fluid (ECF)

• Fluid outside cells

• ~1/3 of total body water

• Major electrolyte: sodium (Na⁺)

Types of ECF

• Interstitial fluid (between cells)

• Intravascular fluid (plasma)

• Transcellular fluid (CSF, pleural, peritoneal, synovial)

FLUID HOMEOSTASIS:

• Maintenance of water and electrolyte balance

• Necessary for normal physiologic function

• Primary goal:
  ○ Maintain adequate vascular volume
  ○ Ensure tissue perfusion

REGULATION OF FLUID BALANCE:

• Fluid intake

• Fluid absorption

• Fluid distribution

• Fluid excretion

FLUID INTAKE & ABSORPTION:

Primary route

• Oral intake (thirst)

• Regulated by hypothalamus

Triggers for thirst

• Increased extracellular fluid osmolality (osmoreceptors)

• Decreased blood volume or blood pressure (baroreceptors, angiotensin II)

Older adults

• Reduced thirst response

• Increased risk for dehydration

Other routes

• Intravenous (IV)

• GI tubes

• Subcutaneous or bone marrow infusion

• Rectal intake (enema)

FLUID DISTRIBUTION:

• Movement of fluid between compartments:
  ○ Vascular ↔ interstitial
  ○ Interstitial ↔ intracellular

• Governed by Starling forces

STARLING FORCES:

Capillary (vascular) side

• Hydrostatic pressure
  ○ Pushes fluid out of capillaries

• Oncotic pressure
  ○ Pulls fluid into capillaries
  ○ Primarily due to albumin

Interstitial side

• Hydrostatic pressure
  ○ Pushes fluid into capillaries

• Oncotic pressure
  ○ Pulls fluid into tissues
  ○ Normally weak

FLUID MOVEMENT & OSMOSIS:

• Water moves freely across cell membranes

• Electrolytes require transport mechanisms:
  ○ Diffusion
  ○ Facilitated diffusion
  ○ Active transport (requires ATP)

• Osmosis:
  ○ Water moves toward higher solute concentration
  ○ Purpose is to equalize osmolality

TONICITY:

Isotonic

• Same osmolality as cell

• No net water movement

• Cell size unchanged

Hypotonic

• Lower osmolality outside cell

• Water moves into cell

• Cell swells

Hypertonic

• Higher osmolality outside cell

• Water moves out of cell

• Cell shrinks

IV FLUIDS:

Isotonic

• 0.9% NaCl

• Lactated Ringer’s

• D5W (initially isotonic)

Hypotonic

• 0.45% NaCl

• Used for water replacement

Hypertonic

• 3% NaCl

• D10W

• 25% albumin

Serum osmolality

• Normal: 275–295 mOsm/kg

FLUID EXCRETION:

Sensible fluid loss (measurable)

• Urinary tract
  ○ Largest source
  ○ Minimum ~0.5 L/day
  ○ GFR ~1 mL/kg/hr

• GI tract
  ○ Increases with diarrhea

Insensible fluid loss (not measurable)

• Lungs (exhalation)

• Skin (perspiration)

HORMONAL REGULATION OF FLUID BALANCE:

Antidiuretic hormone (ADH)

• Synthesized in hypothalamus

• Released from posterior pituitary

• Increases water reabsorption in kidneys

Effects of ADH

• High ADH:
  ○ Decreased urine volume
  ○ Concentrated urine

• Low ADH:
  ○ Increased urine volume
  ○ Dilute urine

• Alcohol inhibits ADH

Aldosterone

• Produced by adrenal cortex

• Causes kidneys to:
  ○ Reabsorb sodium and water
  ○ Excrete potassium

• Activated by RAAS

• Expands extracellular fluid volume

ADH vs aldosterone

• ADH = water hormone

• Aldosterone = salt hormone

Natriuretic peptides (ANP & BNP)

• Released when blood volume increases

• Promote sodium and water excretion

• Cause vasodilation

• Decrease blood pressure

• Oppose RAAS

ALBUMIN

• Plasma protein synthesized by liver

• Maintains oncotic pressure

• Keeps fluid in vasculature

Hypoalbuminemia

• Decreased oncotic pressure

• Fluid shifts into tissues

• Edema

• Poor wound healing

FLUID IMBALANCES OVERVIEW:

ECF volume imbalances

• Problem with amount of saline

• Isotonic changes

• Serum osmolality unchanged

• Includes:
  ○ ECF volume deficit
  ○ ECF volume excess

ECF concentration imbalances

• Problem with sodium concentration

• Osmolality altered

• Includes:
  ○ Hyponatremia
  ○ Hypernatremia

ECF VOLUME IMBALANCES:

• Problem with the amount of isotonic saline

• Sodium and water are gained or lost together

• Serum osmolality remains normal

• Types:

  • ECF volume deficit

  • ECF volume excess

ECF CONCENTRATION IMBALANCES

• Problem with sodium concentration

• Sodium and water are not proportional

• Serum osmolality changes

• Includes:

  • Hyponatremia

  • Hypernatremia

ECF VOLUME DEFICIT (FLUID VOLUME DEFICIT)

• Loss of sodium-containing fluid from extracellular space

Causes

• Vomiting

• Diarrhea

• Excessive sweating

• Diuretics

• Polyuria

• Third spacing

• Inadequate fluid intake

Assessment findings

• Sudden weight loss

• Orthostatic hypotension

• Tachycardia

• Flat neck veins

• Oliguria

• Concentrated urine

• Dry mucous membranes

• Decreased skin turgor

• Sunken eyes

• Hard stools

Infant finding

• Sunken fontanel

ECF VOLUME EXCESS (FLUID VOLUME OVERLOAD)

• Excess sodium-containing fluid in extracellular space

Causes

• Excess IV isotonic fluids

• Aldosterone excess

• Renal failure

• Heart failure

Assessment findings

• Rapid weight gain

  • 1 L fluid = 1 kg = 2.2 lb

• Edema

• Jugular venous distention (JVD)

• Bounding pulse

• Crackles

• Dyspnea

• Orthopnea

• Pink, frothy sputum (pulmonary edema)

Infant finding

• Bulging fontanel

EDEMA

• Accumulation of excess fluid in interstitial spaces

Mechanisms

• Increased capillary hydrostatic pressure

• Decreased plasma albumin

• Increased capillary permeability

• Lymphatic obstruction

DEHYDRATION

• Combination of:

  • ECF volume deficit

  • Hypernatremia

• Occurs when fluid loss exceeds intake

Clinical manifestations

• Hypotension

• Tachycardia

• Weight loss

• Dry mucous membranes

• Poor skin turgor

• Confusion

• Seizures

• Coma

HYPONATREMIA

• Serum sodium <135 mEq/L

• Relative excess of water compared to sodium

• ECF becomes hypotonic → water moves into cells

Clinical manifestations

• Headache

• Nausea

• Confusion

• Seizures

• Coma

• Cerebral herniation

HYPERNATREMIA

• Serum sodium >145 mEq/L

• Relative water deficit

• ECF becomes hypertonic → water leaves cells

Clinical manifestations

• Thirst

• Oliguria

• Confusion

• Seizures

• Coma

• Death

SYNDROME OF INAPPROPRIATE ADH (SIADH)

• Excess ADH secretion

• Excessive water reabsorption

• Dilutional hyponatremia

• ECF volume excess

Management

• Fluid restriction

• Hypertonic saline (severe cases)

DIABETES INSIPIDUS (DI)

• insufficient or ineffective ADH

• Inability to concentrate urine

• Excessive water loss

• Hypernatremia

• ECF volume deficit

Types

• Central DI (lack of ADH)

• Nephrogenic DI (renal resistance to ADH)

Management

• Central: desmopressin

• Nephrogenic: low-salt diet, thiazide diuretics, treat cause

AGE-RELATED VARIATIONS

INFANTS

• Higher total body water percentage

• Poor urine-concentrating ability

• High insensible fluid losses

• Thirst expressed by crying

OLDER ADULTS

• Lower total body water

• Reduced thirst sensation

• Reduced renal concentrating ability

• Skin turgor unreliable

KEY LAB VALUES

• Sodium: 135–145 mEq/L

• Serum osmolality: 275–295 mOsm/kg

• Urine specific gravity: 1.003–1.030

• Hematocrit:

  • Male: 40–50%

  • Female: 37–47%

KEY EXAM POINTS:

• Sodium controls extracellular fluid volume

• Water follows sodium

• Potassium is the primary intracellular electrolyte

• Albumin maintains oncotic pressure

• 1 L fluid change = 1 kg = 2.2 lb

• Isotonic changes do not alter serum osmolality

• Hyponatremia = excess water relative to sodium

• Hypernatremia = water deficit relative to sodium

• ADH regulates water balance

• Aldosterone regulates sodium balance

• SIADH → hyponatremia + volume excess

• Diabetes insipidus → hypernatremia + volume deficit