Pathopedia-India.com: Hemodynamic Disorders 2

Normally capillary walls are permeable to water and electrolytes but impermeable to plasma proteins which maintain the colloid osmotic pressure of the blood. When capillaries are damaged by toxic or anoxic causes, it becomes permeable to proteins. Plasma proteins move into tissue space lowering the colloid osmotic pressure of blood and raising that of the tissue. As a result there is escape of more fluids which is retained in the tissue by the protein molecules. Example: Acute inflammation, burns, severe anemia, allergic reactions etc

2. Increased capillary hydrostatic pressure of blood:

- Capillary hydrostatic pressure depends upon venous pressure and not upon arterial pressure. So, when there is obstruction in venous return, capillary hydrostatic pressure rises at both ends of the capillaries, resulting in more output of fluid at arterial end and no absorption at venous end ( more over some fluid may come out at venous end). This causes retention of fluid in tissue leading to edema e.g.

- Congestive cardiac failure

- Constrictive pericarditis

- Cirrhosis of liver

- Venous thrombosis etc

3. Decreased colloid osmotic pressure of blood:

Colloid osmotic pressure of blood is the force that retains fluid in the capillaries against the driving force of capillary hydrostatic pressure. Colloid osmotic pressure of blood depends partly on total amount of plasma proteins and partly on their relative proportion. Colloid osmotic pressure depends upon the number of molecules per unit volume of solution. Hence, of the protein, albumin having the smallest molecular weight exerts greater influence than globulins (albumin exerts 4 times stronger effect than globulins). Loss of proteins particularly albumin results in edema. When total proteins go below 4 gm% or there is loss of albumin with alteration of Albumin : Globulin ratio, from normal 2-3:1 to 1:1, edema will result.

Examples:

Cirrhosis of liver

Severe protein malnutrition as in Kwashiorkor (reduced synthesis),

Protein losing glomerulopathies (Nephrotic syndrome)

Protein losing gastroenteropathies

4. Lymphatic obstruction:

Some of the intercellular fluid is drained out by lymph vessels and any obstruction in its outflow will usually cause local edema e.g.

Filariasis : It often causes massive lymphatic obstruction in inguinal region. This edema extends to external genitalia and lower limbs producing elephantiasis. Long standing edema causes proliferation of connective tissue causing solid edema.

Breast carcinoma: After removal and/or irradiation of breast, there may be severe edema of the arm and the overlying skin may show accentuation of the depression of hair follicles -called “peau d’orange” (orange peel).

5. Increased Tissue Osmotic Pressure:

In acute inflammation there is increased tissue colloid osmotic pressure due to break down of tissue proteins into osmotically active small particles and leakage of serum proteins through damaged capillary walls raising colloid osmotic pressure of tissue.

6. Primary Sodium retention:

Retention of Na+ & water causes both increased hydrostatic pressure and reduced osmotic pressure. This occurs in excessive salt intake with renal insufficiency, such as acute renal failure. Retention of Na+ also causes increased renin-angiotensin-aldosterone secretion.