Essentials of Anatomy and Physiology

RBCs is actually recycled over and over again. The
globin or protein portion of the hemoglobin molecule
is also recycled. It is digested to its amino acids, which
may then be used for the synthesis of new proteins.
Another part of the hemoglobin molecule is the
heme portion, which cannot be recycled and is a waste
product. The heme is converted to bilirubin by

macrophages. The liver removes bilirubin from circu-

lation and excretes it into bile; bilirubin is a bile pig-

ment. Bile is secreted by the liver into the duodenum

and passes through the small intestine and colon, so

bilirubin is eliminated in feces, and gives feces their

characteristic brown color. In the colon some bilirubin

is changed to urobilinogen by the colon bacteria.

258 Blood

New RBCs

formed in

red bone marrow

Used to make

new RBCs

Iron Heme Globin

Stored in

liver

Bilirubin

Small

intestine

Large intestine

Bilirubin

Colon

bacteria

Urobilin

Urobilin

Urine

Amino acids

Protein synthesis

Macrophages in

liver, spleen, and

red bone marrow

phagocytize old RBCs

Kidney

RBCs 120 daysCirculate

Figure 11–4. Life cycle of red blood cells. See text for description.

QUESTION:Which components of old RBCs are recycled? Which is excreted? (Go to the

macrophage and follow the arrows.)