Proteinuria: Mechanisms, Pathogenesis and Classification | Protein Metabolism

The below mentioned article provides a short note on Proteinuria.

The glomerular membrane permits only a very small amount of the normal plasma proteins. The glomerular filtrate normally contains 10 to 25 mg protein per 100 ml. This is almost completely reabsorbed by the tubular epithelial cells. 150 mg is the upper limit of normal daily protein excretion in the urine. Hence, it is difficult to detect albumin in the urine normally.

In nephrotic syndrome, urinary albumin: Globulin ratios are very high. In chronic glomeru­lonephritis, the ratio is lower. In acute nephritis, the ratio is low. Low values are found in amyloid disease of the kidneys.

Certain foreign proteins entering the plasma are eliminated by the kidney, whereas normal plasma protein is retained in the blood. Thus, hemoglobin-when free in the plasma, egg albumin and other foreign proteins-if introduced into the blood-stream, appears in the urine.

Two major mechanisms operate to produce abnormal proteinuria:

1. Humoral Proteinuria:

Plasma contains ab­normal proteins of small molecular size which pass readily through normal glom­eruli.

2. Renal Proteinuria:

The glomerular mem­branes are so injured that they become more permeable to the normal plasma pro­teins or tubular epithelial cell damage re­sults in inadequate reabsorption of pro­teins from the glomerular filtrate.

3. Abnormal Proteinuria:

Abnormal proteinuria occurs when the quantities of proteins reaching the tubules exceed their re-absorptive capacity.

The pathogenesis of proteinuria may be sum­marized as:

1. In the majority of instances, glomerular abnormalities—structural or functional, primary or secondary—play a fundamen­tal role.

2. In many cases, associated tubular involve­ment is the primary factor.

3. A few cases are due to abnormal proteins in the plasma.

4. The urinary proteins originate in the lower urinary tract.

Proteinuria is classified into three classes:

1. Transitory Proteinuria:

It never indicates permanent renal disease. It may occur in fever, after intense physical exertion, and after minor emotional stimuli.

2. Continuous Proteinuria:

This is always pathological and is indicative of pyelo­nephritis.

3. Orthostatic Proteinuria:

This is caused by erect posture in lumbar lordosis and is common in adolescence.