Red blood cells (erythrocytes). Destruction of red
cells
Red cells have an average life span of 120 days. Although
they use glucose to produce energy necessary for their
survival, they cannot synthesize protein; therefore
reparative processes are not possible. As red cells
age, wear and tear leads to loss of some of the protein,
and the activity of some of the essential enzymes decreases.
Chemical reactions necessary for the survival of the
cell are consequently impaired. As a result, water passes
into the aging red cell, transforming its usual discoid
shape into a sphere. These spherocytes are inelastic,
and, as they sluggishly move through the circulation,
they are engulfed by phagocytes. Phagocytic cells form
a part of the lining of blood vessels, particularly
in the spleen, liver, and bone marrow. These cells,
called macrophages, are constituents of the reticuloendothelial
system and are found in the lymph nodes, in the intestinal
tract, and as free-wandering and fixed cells. As a group
they have the ability to ingest not only other cells
but also many other microscopic particles, including
certain dyes and colloids. Within the reticuloendothelial
cells erythrocytes are rapidly destroyed. Protein, including
that of the hemoglobin, is broken down, and the component
amino acids are transported through the plasma to be
used in the synthesis of new proteins. The iron removed
from hemoglobin passes back into the plasma and is transported
to the bone marrow, where it may be used in the synthesis
of hemoglobin in newly forming red cells. Iron not necessary
for this purpose is stored within the reticuloendothelial
cells but is available for release and reuse whenever
it is required. In the breakdown of red cells there
is no loss to the body of either protein or iron, virtually
all of which is conserved and reused. In contrast, the
porphyrin ring structure of hemoglobin, to which iron
was attached, undergoes a chemical change that enables
its excretion from the body. This reaction converts
porphyrin, a red pigment, into bilirubin, a yellow pigment.
Bilirubin released from reticuloendothelial cells after
the destruction of erythrocytes is conveyed through
the plasma to the liver, where it undergoes further
changes that prepare it for secretion into the bile.
The amount of bilirubin produced and secreted into the
bile is determined by the amount of hemoglobin destroyed.
When the rate of red cell destruction exceeds the liver's
capacity to handle bilirubin, the yellow pigment accumulates
in the blood, causing jaundice. Jaundice can also occur
if the liver is diseased (e.g., hepatitis) or if the
egress of bile is blocked (e.g., by a gallstone).
