Each kidney contains about 1 million filtering units (glomeruli). The glomeruli are made up of many microscopic clusters of tiny blood vessels (capillaries) with small pores. These blood vessels are designed to leak fluid from the bloodstream into a system of miniature tubules that process the fluid to become urine. Normally this filtering system permits fluid and small molecules (but almost no protein or blood cells) to leak into the tubules. Diseases that affect the kidneys can be divided into three categories based on the way they affect different parts of the kidneys:

• Glomerulonephritis (or nephritic syndrome) is inflammation of the glomeruli, which causes blood cells and protein to escape from the glomerular capillaries into the urine.
• In nephrotic syndrome, damage to the capillaries of the glomeruli causes proteins to leak into the urine.
• Tubulointerstitial nephritis is inflammation of the tubules and the tissues surrounding the tubules (interstitium).

With glomerulonephritis, the inflammation is often the result of an abnormal immune reaction. Such a reaction can occur in two ways: (1) Antibodies (proteins made by the body to attack specific molecules called antigens) may attach directly to cells of the kidney or molecules trapped in them, causing inflammation. (2) Antibodies attach to antigens outside the kidney, and these antigen-antibody (or immune) complexes are carried to the kidney by the bloodstream and get trapped in the glomeruli, causing inflammation.

If enough glomeruli are damaged, kidney function is decreased. As a result, urine production falls and waste products build up in the blood. Also, when damage is severe, inflammatory cells and injured glomerular cells accumulate, compressing the capillaries within the glomerulus and interfering with filtration. Scarring may develop, which also impairs kidney function and reduces urine production. In some cases, tiny blood clots (microthrombi) may form in the small blood vessels, further decreasing kidney function. Rarely, glomerulonephritis can result from a hereditary condition. In other cases, glomerulonephritis is caused by inflammation of the blood vessels (vasculitis).

Nephrotic syndrome causes large amounts of protein to leak from blood into the urine. This leakage can be caused by damage to the glomeruli by inflammatory or noninflammatory processes. In inflammatory processes, red blood cells appear in the urine. Nephrotic syndrome caused by inflammation therefore has characteristics similar to those of glomerulonephritis. With noninflammatory processes, no red blood cells appear in the urine. Some forms of nephrotic syndrome can be severe. The glomeruli become scarred, and kidney failure develops. In less severe forms of nephrotic syndrome, kidney function decreases very little.

Tubulointerstitial nephritis often is caused by a toxic or allergic reaction to a drug. White blood cells or scar tissue appears in the kidney's tissues. Infection of the kidneys (pyelonephritis—see Urinary Tract Infections (UTI): Kidney Infection (Pyelonephritis)) can also cause tubulointerstitial nephritis. When inflammation damages the tubules and surrounding tissues, the kidneys may become unable to perform their normal functions, such as concentrating urine, eliminating (excreting) waste products from the body, or balancing the excretion of sodium and other electrolytes, such as potassium. If the damage is severe and affects both kidneys, the result is kidney failure.

Last full review/revision March 2007 by Seyed-Ali Sadjadi, MD