Nephrotic syndrome: loss of protein in the urine and sodium retention

One of the most important clinical causes of oedema is nephrotic syndrome. In nephrotic syndrome, glomerular capillaries leak a large amount of protein into the filtrate and urine due to increased glomerular capillary permeability. Thirty to 50 grams of plasma protein can be lost in the urine per day, sometimes causing the plasma protein concentration to drop below the normal one-third. Due to decreased plasma protein concentration, the plasma colloidal osmolality is reduced to a low level. This causes capillaries throughout the body to filter large amounts of fluid into different tissues, thereby causing oedema and a decrease in plasma volume.

Renal sodium retention in nephrotic syndrome occurs through a variety of mechanisms activated by leaking proteins and fluids from the plasma into the interstitial fluid, including stimulation of various sodium retention systems such as the renin-angiotensin system. , aldosterone and the sympathetic nervous system. The kidneys continue to hold sodium and water until the plasma volume is restored to almost normal. However, as large amounts of sodium and water are retained, plasma protein concentrations become thinner, causing more fluid to leak into the body's tissues. The net result is that the kidneys retain a lot of water until extracellular oedema develops unless treatment to restore plasma proteins is underway.