Hyperhydration of the organism, edema, ascites, hydrothorax

Starling Forces & Capillary Exchange

Fluid movement across a capillary wall is driven by the Starling pressures across the wall and is described by the Starling equation:

J_v= K_f[(P_c-P_i)-(𝜋_c-𝜋_i)]

where:

• J_v: Fluid movement (mL/min)
• K_f: Hydraulic conductance (mL/min per mm Hg) → water permeability across the wall - varies w/ tissue type, capillary wall, increases w/ capillary injury
• P_c: Capillary hydrostatic pressure (mm Hg) → favours filtration out of capillary; highest at arteriolar end, lowest at venous end
• P_i: Interstitial hydrostatic pressure (mm Hg) → opposes filtration, nearly zero
• 𝜋_c: Capillary oncotic pressure (mm Hg) → force opposing filtration due to presence of plasma proteins (i.e., albumin)
• 𝜋_i: Interstitial oncotic pressure (mm Hg) → force favouring filtration, determined by the interstitial fluid protein concentration (low in physiological situations)

The direction of fluid movement can be either into or out of the capillary - when net fluid movement is out of the capillary into the interstitial fluid, it is called filtration; when net fluid movement is from the interstitium into the capillary, it is called absorption

Hyperhydration

Definition: excessive amount of water in the body

Types of hyperhydration:

1. Isotonic: ↑ ECT volume, normal osmolarity
   1. Causes: excessive infusion of isotonic solutions, heart failure, liver cirrhosis (ascites), nephrotic syndrome (Na+ and water retention)
   2. Consequences: edema (pulmonary, systemic), hypertension
2. Hypotonic: ↑ volume of ECT,↓ osmolarity of ECT → movement of water into cells (cellular edema)
   1. Causes: excessive intake of hypotonic fluids (water drinking) when the kidneys are unable to excrete free water (oliguric phase of renal failure, SIADH - syndrome of inadequate secretion of ADH), psychogenic polydipsia
   2. Consequences: brain edema (headache, confusion, convulsions, coma), hyponatremia
3. Hypertonic: ↑ ECT volume,↑ ECT osmolarity → water transfer from cells (cellular dehydration)
   1. Causes: excessive infusions of hypertonic solutions (NaCl, mannitol), drinking seawater, primary hyperaldosteronism (Conn's syndrome)
   2. Consequences: hypertension, hypernatremia, symptoms of cellular dehydration (thirst, neurological symptoms)

Consequences of hyperhydration

In general: higher ECT volume → pulmonary edema ; Higher ICT volume → brain edema

• Changes in volume and osmolarity of compartments: edema (pulmonary, cerebral, systemic)
• Cardiovascular: hypertension, heart failure
• Neurological: confusion, convulsions, coma (mainly in hypotonic)
• Electrolyte imbalances: hyponatremia (hypotonic), hypernatremia (hypertonic)

Therapy

• Treatment of cause of hyperhydration
• Fluid restriction
• Diuretics
• Dialysis

Edema

Definition: abnormal accumulation of fluid in the interstitial space (tissues)

Pathogenesis

• ↑ Hydrostatic pressure in capillaries (↑ P_c)
  • Venous obstruction (thrombosis), chronic venous insufficiency
  • Heart failure (right-sided - systemic edema; left-sided - pulmonary edema)
  • Renal failure (Na⁺ and water retention)
• ↓ Oncotic plasma pressure (↓ π_c)
  • Protein loss (nephrotic syndrome, burns)
  • Reduced synthesis (liver failure, malnutrition)
  • Excessive hydration
• ↑ Capillary permeability (↑ K_f)
  • Inflammation (histamine, bradykinin)
  • Allergy (angioedema)
  • Burns, trauma, ischemia
• Impaired lymphatic drainage (lymphedema)
  • Congenital defects, inflammation, lymph node surgery, tumors, radiation, infections(filariasis),non-pitting (typically firm, painless, pale)
• ↑ Oncotic interstitial pressure (↑ π_i)
  • less common, i.e., myxedema in hypothyroidism
• ↓ Hydrostatic pressure of the interstitium (↓ P_i)
  • More theoretical

Types of edema

1. Localized: inflammation, allergy, local obstruction
2. Generalized (anasarca): heart failure, nephrotic syndrome, liver failure
3. According to localization: pulmonary, cerebral, ascites (pathological accumulation of free fluid in the peritoneal cavity), hydrothorax (pathological accumulation of free fluid in the pleural cavity), hydropericardium
4. By composition: transudate (low protein, ↑ Pc or↓ πc), exudate (high protein,↑ Kf)

Consequences of edema

• Acute: life-threatening (pulmonary, cerebral, laryngeal), hypovolemic shock (moving to third compartments)
• Chronic: tissue hypoxia, poorer healing, atrophy, risk of infection

Sources

• Amboss
• Guyton & Hall. Textbook of Medical Physiology.
• Linda S. Costanzo. Textbook of Physiology.
• Stefan Silbernagl, Florian Lang. Color Atlas of Pathophysiology