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In humans, kidneys are situated one on each side of the spine, and are embedded in fatty tissue. They are bean-shaped, possessing a convex outer border and a concave inner border. The inner border presents an indentation, the hilum, at which the blood vessels enter and leave. In front is the renal vein carrying blood from the kidney; behind it lies the renal artery carrying blood to the kidney. Most posterior is the ureter, a tube that conveys urine to the bladder. The hilum arises from a deeper indentation, the sinus of the kidney, in which the ureter dilates to form a small sac, the renal pelvis. The kidney also embodies glomeruli, aggregations or loops of capillaries enclosed within thin envelopes of endothelial lining called Bowman's capsules, located at the blind ends of the renal tubules.

Function

Urine is produced in the glomeruli and renal tubules and carried to the renal pelvis by collecting tubules. The glomeruli act as simple filters, through which water, salts, and waste products from the blood pass into the spaces of Bowman's capsules and from there down into the renal tubules. Most of the water and salt is reabsorbed from these tubules; the remainder is excreted as urine. The renal tubules also secrete other salts and waste products from the blood into the urine. The average amount of urine excreted in 24 hours is about 1.4 litres, but the quantity varies considerably, depending on intake of fluid and loss from such sources as the skin in perspiration, or from vomiting.

The kidneys are also important in maintaining a balance of fluid and salt and a normal degree of acidity. When disorders upset these delicate balances, the kidneys act to restore them by excreting more or less water, salt, and hydrogen ions. The kidneys help maintain normal blood pressure by secreting the hormone renin and elaborate a hormone that stimulates the production of red blood cells.

Diseases of the Kidney

Nephritis, or inflammation of the kidney, is one of the commonest kidney diseases. Its chief characteristics are the appearance in the urine of such elements as albumin, a condition known as albuminuria; red and white blood cells; and hyaline or granular casts, all revealed by microscopic examination of the urine. It is much more common in childhood and adolescence than in middle age.

The commonest form of nephritis is glomerulonephritis; it often occurs within three to six weeks following a streptococcal infection. The patient complains of chills; fever; headache; backache; puffiness, or oedema, of the face, especially around the eyes; nausea; and vomiting. Urine may become scanty and smoky in appearance. Prognosis is generally good, and most patients recover completely. A few persons, however, develop chronic nephritis. In this form of nephritis, kidney damage progresses over many years, during which patients are symptom free. Eventually, however, they may develop uraemia (urine in the blood) and kidney failure.

The nephroses include a variety of types of nephritis marked by degenerative changes in the tubules of the kidney. Pure nephrosis is rare; more common are those types associated with glomerulonephritis or other diseases affecting the kidney. Nevertheless, the term nephrosis is still employed for a syndrome characterized by the presence of generalized oedema, by large amounts of albumin in the urine, by excessive cholesterol in the blood, and by relatively normal urinary output.

Nephrosclerosis, or hardening of the small arteries supplying the kidney, is a disorder characterized by the presence of albumin, casts, and occasionally white or red blood cells in the urine (haematuria); it usually accompanies hypertensive vascular disease. Its fundamental lesion is a sclerosis of the small arteries of the kidney, with secondary atrophy of the glomeruli and pathological changes in the interstitial tissue.

Renal calculi, or kidney stones, may form in the kidney or renal pelvis from crystals deposited from the urine. They are composed mostly of calcium oxalate. Infection or obstruction may play a part in their formation. Sometimes they occur when the level of blood calcium is abnormally high, as may be the case when the parathyroid glands overproduce urine. Occasionally, stones may develop when the blood level of uric acid is too high, usually from overconsumption of meat. Excessive dietary intake of calcium and oxalate and low fluid intake have also been associated with formation of stones. In most cases, however, the cause is not known. Stones may cause bleeding, secondary infection, or obstruction. Small kidney stones tend to travel down the ureter towards the bladder; their movement is usually accompanied by severe pain. Colic caused by stones usually requires one or more injections of narcotics for relief. The pain may develop suddenly after muscular exercise. Once a stone drops into the bladder, it may be passed with the urine unnoticed, and the pain ceases. If the stone is too large to pass, treatment is necessary, either with surgery or with lithotripsy, a procedure that uses shock waves generated outside the body to disintegrate the stones.

Uraemia is poisoning caused by accumulation in the blood of waste products normally excreted by the kidney. It occurs most often as the end stage of chronic kidney disease and is characterized by drowsiness, headache, nausea, inability to sleep, spasms, seizures, and coma. Prognosis is poor. By the 1980s, however, such techniques as repeated periodic dialysis to clear the blood of accumulated waste products and toxins, and kidney-transplant operations, offered new hope to patients.

Pyelonephritis is an infection of the kidney with bacteria. Acute pyelonephritis is often accompanied by fever, chills, pain on the affected side, frequent passing of urine and burning on urination. Chronic pyelonephritis is a progressive, usually symptom-free disease that may eventually lead to destruction of the kidney and to uraemia. Pyelonephritis is more common in women than men, and more usual in diabetics.

Wilms's tumour, a highly malignant form of kidney tumour, is most frequent in young children. Recently devised treatment has brought about a cure in many children with this disease. In systemic lupus erythematosus, which tends to strike women in their thirties more than other groups, the body makes antibodies that damage the kidney.