Lecture 8: Vascular Disorders and Thrombosis: Mechanisms of Edema

What are Starling's forces and what do they describe?

Starling's forces describe water distribution between plasma and interstitium, determined mainly by osmotic (oncotic) and hydrostatic pressure differentials between the compartments, represented by an equation attributed to Ernest Starling.

What is hydrostatic pressure in the context of Starling's forces?

The pressure difference between the inside of the capillaries and the interstitial space. Combined with oncotic pressure, this net difference determines the direction of water movement.

What is oncotic pressure and how does it differ from osmotic pressure?

Oncotic pressure is the osmotic pressure specifically due to plasma proteins, which cannot freely cross the capillary endothelium. It creates a concentration gradient between plasma and interstitium, drawing water into the vascular space.

Which type of molecules account for most of plasma osmolality, and why do they contribute little to oncotic pressure?

Electrolytes (~84% of plasma osmolality) contribute little to oncotic pressure because they move relatively freely between intravascular and interstitial compartments, maintaining similar concentrations in both.

Which molecules account for most of the oncotic pressure gradient between plasma and interstitium?

Plasma proteins (~1% of plasma osmolality) account for most of the oncotic pressure gradient because their concentrations are much higher in plasma than in interstitium.

What is the major plasma protein responsible for maintaining oncotic pressure?

Albumin.

What is the major protein in the interstitium responsible for maintaining oncotic pressure?

Collagen.

What process results from alterations in factors regulating normal fluid distribution?

Edema—defined as excessive interstitial fluid accumulation.

What are the four main mechanisms of edema?

1) Increased microvascular permeability

2) Increased intravascular hydrostatic pressure

3) Decreased oncotic pressure

4) Decreased lymphatic drainage

Describe the principle of increased microvascular permeability as a mechanism of edema.

Inflammation triggers release of mediators causing vasodilation, widening interendothelial gaps, and allowing water to move from intravascular to interstitial space—producing tissue swelling.

Give a common example of edema caused by increased microvascular permeability.

Localized swelling at sites of acute inflammation, such as around an infected wound.

Describe the principle of increased intravascular hydrostatic pressure as a mechanism of edema.

Increased capillary hydrostatic pressure pushes more fluid into the interstitium. This can be localized (affecting specific tissue) or generalized (affecting whole organs, body cavities, or tissues).

What are possible causes of increased intravascular hydrostatic pressure?

Increased blood flow (hyperemia), passive blood accumulation (congestion), and increased blood volume.

Describe the principle of decreased oncotic pressure as a mechanism of edema.

Reduction in plasma protein concentration (especially albumin) decreases oncotic pull, reducing fluid return to the vasculature and promoting interstitial fluid accumulation.

What is the most common cause of decreased oncotic pressure?

Hypoalbuminemia.

List mechanisms of hypoalbuminemia that can lead to edema.

Severe blood loss, protein-losing enteropathy, protein-losing nephropathy, severe burns, loss of hepatic functional mass, and profound malnutrition (emaciation).

Describe the principle of decreased lymphatic drainage as a mechanism of edema.

Impaired lymphatic function prevents removal of excess interstitial fluid, leading to localized swelling.

What are possible causes of decreased lymphatic drainage?

Lymphatic vessel compression (e.g., neoplasia, inflammation), constriction from fibrosis, or blockage by thrombus/embolus.

What is an effusion?

Accumulation of fluid in a body cavity such as the abdomen, thorax, or pericardial sac.

What is abdominal effusion called?

Ascites.

What is thoracic effusion called?

Pleural effusion.

What is pericardial effusion?

Fluid accumulation within the pericardial sac.

What are the three broad categories of effusions?

Pure transudate, modified transudate, and exudate.

How is a pure transudate classified based on protein, cellularity, and appearance?

Protein <2.5 g/dL, cellularity <1500 cells/µL, low fibrinogen, and generally translucent, clear to straw-colored.

How is a modified transudate classified based on protein and cellularity?

Protein >2.5 g/dL, cellularity <5000 cells/µL; falls between pure transudate and exudate, often having low protein or low cellularity (or both).

How is an exudate classified based on protein, cellularity, and appearance?

Protein >2.5 g/dL, cellularity >5000 cells/µL, high fibrinogen due to inflammation, and generally cloudy or opaque.

Which type of effusion typically indicates inflammation?

Exudate.

What are the two main mechanisms responsible for transudative effusions?

Increased intravascular hydrostatic pressure and decreased oncotic (osmotic) pressure.

What is the alternative name for a thoracic transudative effusion?

Hydrothorax.

What is the alternative name for a pericardial transudative effusion?

Hydropericardium.

What is the alternative name for an abdominal transudative effusion?

Hydroperitoneum or ascites.

How do transudative effusions differ from exudative effusions in pathophysiology?

Transudative effusions are due to pressure changes (hydrostatic or oncotic), while exudative effusions are due to inflammation.

How can transudative and exudative effusions be distinguished grossly?

Transudates are usually clear and low in protein/cells, whereas exudates are cloudy/opaque and high in protein/cells.

What is a chylous effusion?

A specific type of effusion (typically thoracic) caused by thoracic duct obstruction, characterized by opaque white to pink-tinged fluid.

What are the protein and cellularity characteristics of a chylous effusion?

Protein >2.5 g/dL (may be artifactually high due to large lipoproteins), and variable cellularity, usually >3000 cells/µL.