Pathogenesis of edema md pathology exam question Robbins based answer
Reading File
Searching the Web
Robbins pathology edema pathogenesis mechanisms increased hydrostatic pressure reduced plasma osmotic pressure lymphatic obstruction sodium retention inflammation
Finding Sources
Finding Sources
Searching PubMed
edema pathogenesis
Reading File
Reading File
Reading File
Pathogenesis of Edema
Edema is an abnormal accumulation of fluid in the interstitial tissue spaces. Fluid in serous cavities is called an effusion. Severe generalized edema with swelling of subcutaneous tissues and serous cavity effusions is termed anasarca.
Normally, capillary fluid exchange is determined mainly by:
- Capillary hydrostatic pressure, which drives fluid out of vessels
- Plasma colloid osmotic pressure, mainly due to albumin, which draws fluid into vessels
- Lymphatic drainage, which returns excess interstitial fluid and proteins to the circulation
Edema develops when excess fluid filtration exceeds reabsorption and lymphatic drainage capacity. - Robbins & Kumar Basic Pathology, pp. 77-79
Major mechanisms of edema
| Mechanism | Pathogenesis | Important examples |
|---|---|---|
| 1. Increased hydrostatic pressure | Raised venous or capillary pressure increases movement of fluid from capillaries into interstitium. | Congestive heart failure, venous thrombosis, venous compression by mass, constrictive pericarditis, prolonged dependency |
| 2. Reduced plasma colloid osmotic pressure | Hypoalbuminemia lowers plasma oncotic pressure, reducing reabsorption of interstitial fluid at the venous end of capillaries. | Nephrotic syndrome, advanced liver disease, malnutrition, protein-losing enteropathy |
| 3. Lymphatic obstruction | Failure of lymphatic drainage causes accumulation of protein-rich interstitial fluid. | Malignancy, inflammation/fibrosis, surgery, radiotherapy, filariasis |
| 4. Sodium and water retention | Retained sodium raises total body water and increases hydrostatic pressure. It may also dilute plasma proteins, reducing colloid osmotic pressure. | Renal failure, excessive salt intake with impaired renal function, hyperaldosteronism |
| 5. Increased vascular permeability | Inflammation causes endothelial gaps or injury, allowing fluid and plasma proteins to escape into tissues. | Acute and chronic inflammation, angiogenesis, burns, allergic reactions |
1. Increased hydrostatic pressure
This is most often due to impaired venous return.
- In deep-vein thrombosis, venous obstruction causes localized edema of the affected limb.
- In congestive cardiac failure, reduced cardiac output produces systemic venous congestion, increasing capillary hydrostatic pressure and causing dependent edema.
- Reduced renal perfusion in heart failure activates the renin-angiotensin-aldosterone system (RAAS). Sodium and water retention then further increases blood volume, venous pressure, and edema, creating a vicious cycle.
2. Reduced plasma colloid osmotic pressure
Albumin is the most important plasma protein contributing to colloid osmotic pressure.
Hypoalbuminemia occurs due to:
- Increased loss of albumin: nephrotic syndrome, protein-losing enteropathy
- Decreased synthesis: advanced liver disease
- Inadequate intake: severe malnutrition
The fall in plasma oncotic pressure causes fluid to remain in, or move into, the interstitium. The consequent fall in effective circulating volume may stimulate RAAS, worsening sodium and water retention.
3. Lymphatic obstruction
Lymphatics normally return excess interstitial fluid and proteins to the bloodstream. Obstruction causes lymphedema, which is often localized and protein-rich.
Causes include:
- Surgical removal of lymph nodes, such as after breast carcinoma surgery
- Radiation-induced fibrosis
- Tumor infiltration or compression of lymphatics
- Inflammatory fibrosis
- Parasitic infection such as filariasis
4. Sodium retention
Primary renal sodium retention increases intravascular volume and thereby increases capillary hydrostatic pressure. It also causes dilution of plasma proteins, lowering colloid osmotic pressure.
It occurs in:
- Acute or chronic renal failure
- Glomerulonephritis
- Excess salt intake in renal insufficiency
- Secondary hyperaldosteronism due to renal hypoperfusion
5. Increased vascular permeability
During inflammation, mediators such as histamine cause endothelial contraction or endothelial injury. This permits escape of protein-rich fluid into the interstitium.
This edema is characteristic of:
- Acute inflammation
- Chronic inflammation
- Angiogenesis
- Allergic reactions and burns
Transudate versus exudate
| Feature | Transudate | Exudate |
|---|---|---|
| Protein content | Low | High |
| Mechanism | Increased hydrostatic pressure or reduced plasma oncotic pressure | Increased vascular permeability due to inflammation |
| Typical examples | Heart failure, cirrhosis, nephrotic syndrome | Acute inflammation, infection, malignancy-associated inflammation |
Exam conclusion: Edema results from disturbance of capillary fluid balance or impaired lymphatic drainage. The principal mechanisms are increased hydrostatic pressure, reduced plasma colloid osmotic pressure, lymphatic obstruction, sodium retention, and increased vascular permeability. - Robbins & Kumar Basic Pathology, Table 3.1, p. 78
This is a shared conversation. Sign in to Orris to start your own chat.