Edema formation involves multiple mechanisms that disrupt normal fluid balance between capillaries and tissues. The key mechanisms include increased capillary hydrostatic pressure, decreased plasma colloid osmotic pressure, increased capillary permeability, sodium and water retention, and lymphatic drainage obstruction. Understanding these mechanisms helps identify the incorrect option in our question.
Increased capillary hydrostatic pressure is a major mechanism of edema formation. Normal capillary pressure ranges from 25 to 30 millimeters of mercury. When this pressure increases above 35 millimeters of mercury due to conditions like heart failure or venous obstruction, excessive fluid filtration occurs, leading to tissue edema. This demonstrates why decreased capillary hydrostatic pressure would not cause edema.
Plasma colloid osmotic pressure is created by proteins in the blood, particularly albumin. This pressure normally ranges from 25 to 28 millimeters of mercury and acts to pull fluid back into capillaries. When protein levels decrease due to liver disease, malnutrition, or protein loss, the osmotic pressure drops below 20 millimeters of mercury, reducing fluid reabsorption and causing edema formation.
Sodium and water retention is another major mechanism of edema formation. When kidney function is impaired, glomerular filtration decreases, leading to reduced sodium and water excretion. Additionally, increased aldosterone and antidiuretic hormone secretion promote further retention. This results in expanded blood volume, increased capillary hydrostatic pressure, and subsequent edema development.