Jaundice-Excess Bilirubin in the
Jaundice refers to a yellowish tint to
the body tissues,including a yellowness of the skin as well as the deep
tissues. The usual cause of jaundice is large quantities of bilirubin in the
extracellular fluids, either free bilirubin or conjugated bilirubin. The normal
plasma concentra-tion of bilirubin, which is almost entirely the free form,
averages 0.5 mg/dl of plasma. In certain abnormal con-ditions, this can rise to
as high as 40 mg/dl, and much of it can become the conjugated type. The skin
usually begins to appear jaundiced when the concentration rises to about three
times normal—that is, above 1.5 mg/dl.
The common causes of jaundice are (1) increased destruction of red
blood cells, with rapid release of bilirubin into the blood, and (2)
obstruction of the bile ducts or damage to the liver cells so that even the
usual amounts of bilirubin cannot be excreted into the gas-trointestinal tract.
These two types of jaundice are called, respectively, hemolytic jaundice and obstructivejaundice.
They differ from each other in the followingways.
Jaundice Is Caused by Hemolysis of Red Blood Cells.
In hemolytic jaundice, the excretory function of the liver is not
impaired, but red blood cells are hemolyzed so rapidly that the hepatic cells
simply cannot excrete the bilirubin as quickly as it is formed. Therefore, the
plasma concentration of free bilirubin rises to above-normal levels. Likewise,
the rate of formation of uro-bilinogen in
the intestine is greatly increased, and muchof this is absorbed into the blood
and later excreted in the urine.
Jaundice Is Caused by Obstruction of Bile Ducts or Liver
Disease. In obstructive jaundice, caused either byobstruction of the bile
ducts (which most often occurs when a gallstone or cancer blocks the common
bile duct) or by damage to the hepatic cells (which occurs in hepatitis), the rate of bilirubin
formation is normal, butthe bilirubin formed cannot pass from the blood into
the intestines. The free bilirubin still enters the liver cells and becomes
conjugated in the usual way. This conju-gated bilirubin is then returned to the
blood, probably by rupture of the congested bile canaliculi and direct emptying
of the bile into the lymph leaving the liver. Thus, most of the bilirubin in the plasma becomes theconjugated type rather
than the free type.
Jaundice. Chemical laboratory tests can be used to
dif-ferentiate between free and conjugated bilirubin in the plasma. In
hemolytic jaundice, almost all the bilirubin is in the “free” form; in
obstructive jaundice, it is mainly in the “conjugated” form. A test called the van denBergh reaction can be used to
differentiate between thetwo.
When there is total obstruction of bile flow, no biliru-bin can
reach the intestines to be converted into uro-bilinogen by bacteria. Therefore,
no urobilinogen is reabsorbed into the blood, and none can be excreted by the
kidneys into the urine. Consequently, in total
obstructive jaundice, tests for urobilinogen in the urine are completely
negative. Also, the stools become clay colored owing to a lack of stercobilin
and other bile pigments.
Another major difference
between free and conju-gated bilirubin is that the kidneys can excrete small
quantities of the highly soluble conjugated bilirubin but not the albumin-bound
free bilirubin. Therefore, in severe obstructive jaundice, significant
quantities of conjugated bilirubin appear in the urine. This can be
demonstrated simply by shaking the urine and observ-ing the foam, which turns
an intense yellow. Thus, by understanding the physiology of bilirubin excretion
by the liver and by the use of a few simple tests, it is often possible to differentiate
among multiple types of hemolytic diseases and liver diseases, as well as to
deter-mine the severity of the disease.