Although every body organ serves some purpose, many can be artificially supported or even replaced in the event of failure or damage. Kidney function can be replicated temporarily or permanently by hemodialysis. A failing heart can be supported by an intra-aortic balloon pump or a left ventricular assist device. The heart can be replaced by mechanical version until a suitable donor heart becomes available for transplant. The function of the pancreas can be simulated by oral enzyme replacement and insulin injections. Poorly functioning lungs can be supported by mechanical ventilation almost indefinitely. Limbs can be replaced with prostheses; skin grafts made of biological textiles can temporarily cover burns and wounds; and metallic joints can serve as sturdy replacements for worn-out knees and hips. Through increasingly sophisticated biomedical engineering, sight can be restored to the blind, hearing to the deaf, and movement to the paralyzed.

Some organs, however, have functions so complex that modern technology has been unable to replicate the basic services they provide. The brain is an obvious example. Could its function be replaced without compromising the fundamental element that defines you as human? And with someone else's brain in your skull, if such a transplantation were possible, would you still be you?

Second only to the brain in organ complexity, and not as obviously so to most people, is the liver. Although it was revered in ancient times as the center of the soul, we tend to immediately associate liver with the homely food item served up in roadside diners under a pile of onions. Even if we had the opportunity to look at a specimen of human liver, we would probably see little beyond a mass of homogeneous dark red tissue.

Despite its less than impressive appearance, no one has been able to devise a replacement part for the liver. Why this should be so can be best appreciated by examining the physiological ramifications of liver failure--caused for example, by long-standing cirrhosis, in which the liver is irreversibly scarred and all of its functions eventually depressed.

When old or damaged red blood cells are broken down, free bilirubin (part of the hemoglobin molecule) is a by-product that cannot be eliminated without chemical modification by the liver. The most visible sign of liver failure is jaundice, or yellow skin, which is caused by an excess of bilirubin in the blood and skin. Elevated bilirubin levels (often as high as 40 times normal) also cause yellowing of the sclera of the eye and insatiable itching of the skin.

End-stage liver failure is characterized by low protein levels in the blood, since the liver is the chief manufacturer of serum proteins. The clinical picture of malnutrition occurs despite normal dietary intake. As the oncotic pressure exerted by proteins to hold fluid in the vessels drops, interstitial fluid collects in the soft tissues of the extremities. This results in edema, primarily in the legs. There also may be abnormal pooling of liquid in the abdomen--a condition know as ascites. The main dangers of ascites are spontaneous infection and physical restriction of the diaphragm as a result of fluid mass.

Glucose is stored in the liver in the form of glycogen, and then converted back to glucose as dictated by the body's metabolic demands. When cirrhosis replaces the cells responsible for storage and reconstitution of usable glucose with fibrous tissue, fatigue and weakness are the obvious outcomes.

One of the most striking difficulties encountered in treating patients with alcoholic cirrhosis is the complication of severe bleeding. This is due to a deficit of certain blood clotting factors produced by the healthy liver. Any surgical procedure or traumatic injury, if accompanied by excessive bleeding, can be life threatening.

Liver cirrhosis dramatically elevates the pressure in the hepatic portal system. The congested blood causes the vessels surrounding the stomach and esophagus to dilate and possibly break open and bleed into the upper GI tract. This condition, added to the clotting deficiencies described above, can trigger massive bleeding--a common cause of rapid demise in patients with long-standing cirrhosis.

The huge array of molecules of varying size and structure that can be chemically metabolized by the liver is nearly incomprehensible. Potent drugs and potentially dangerous chemicals can be rendered inert by one pass through a healthy liver. Cirrhotic livers lose the ability to detoxify these substances; consequently, they remain active in the body for prolonged periods of time. Physicians must be cautious in administering certain drugs to patients with cirrhosis to avoid overdosage.

Considering the consequences of cirrhosis, it is clear that an artificial liver could not be just a pump or filter, as for the heart and kidneys. It would have to store energy molecules, chemically alter bile products, produce proteins and clotting factors, and clear the blood of toxins, drugs, and old or damaged red blood cells. No wonder, then, that the only option for a failed liver is a transplant--a healthy living organ replacing a dying one. The liver is truly an inimitable organ. Its functions simply cannot be duplicated, even by the most elaborately engineered spare parts.