Disorders and Diseases of the Excretory System

The excretory system is subject to a number of well-recognised disorders, each traceable to a specific point of failure in filtration, reabsorption, or the tubes and organs that store and carry urine.

Abnormal urine findings are themselves diagnostic clues. Excess albumin in urine (albuminuria) points to injury of the endothelial-capsular membrane — from raised blood pressure, or from irritation of kidney cells by toxins or heavy metals. Excess ketone bodies in urine can result from diabetes mellitus, starvation, or a diet too low in carbohydrates. Leucocytes in urine suggest infection somewhere in the kidney or urinary tract.

1. Kidney stones (renal calculi) can form anywhere along the urinary tract, from the kidney tubules to the external opening, and are classified by composition: calcium stones (usually calcium oxalate or calcium phosphate — this is why kidney-stone patients are often told to limit tomatoes, which are rich in oxalic acid); struvite stones, which form in response to infection by urea-splitting bacteria and can grow quickly to a large size; uric acid stones, more common in people who drink too little water or eat a very high-protein diet; and cystine stones, arising from a genetic disorder that causes the kidney to excrete too much of a particular amino acid. Typical symptoms include intermittent pain below the rib cage in the back and sides, hazy or brownish/reddish/pinkish urine, a frequent urge to urinate, and pain during micturition; diagnosis draws on blood uric acid levels, urine colour, kidney X-ray and kidney sonography.

2. Uremia. Blood normally contains about 0.01-0.03% urea; if this rises above 0.05%, the condition is called uremia, which is harmful and can progress to kidney failure.

3. Nephritis is inflammation of the kidneys marked by proteinuria, caused by increased permeability of the glomerular-capsular membrane that lets large amounts of protein leak from blood into urine. This lowers the blood's colloidal osmotic pressure, which drives fluid out of the blood into interstitial spaces, producing edema.

4. Renal failure is a decrease or complete stop in glomerular filtration, and comes in two forms. Acute renal failure (ARF) is a sudden worsening of kidney function, most often following severe bleeding; it shows up as a marked drop in urine output (oliguria — under 400 ml/day in adults, or under 0.5 ml/kg/hour in children), can also follow acute obstruction of both ureters or exposure to nephrotoxic drugs, and is detected biochemically by an elevated serum creatinine level. Chronic kidney disease (CKD) is a progressive, generally irreversible decline in glomerular filtration rate, often caused by chronic glomerulonephritis, and is detected through a reduced kidney size on imaging together with the possibility of anaemia. …