PRODUCTION AND ELIMINATION OF BLOOD CELLS
Blood is produced in the bone marrow, a tissue in the
central cavity inside almost all of the bones in the body.
In infants, the marrow in most of the bones is actively
involved in blood cell formation. By later adult life,
active blood cell formation gradually ceases in the bones of
the arms and legs and concentrates in the skull, spine,
ribs, and pelvis.
Red blood cells, white blood cells, and platelets grow from
a single precursor cell, known as a hematopoietic stem cell.
Remarkably, experiments have suggested that as few as 10
stem cells can, in four weeks, multiply into 30 trillion red
blood cells, 30 billion white blood cells, and 1.2 trillion
platelets—enough to replace every blood cell in the body.
Red blood cells have the longest average life span of any of
the cellular elements of blood. A red blood cell lives 100
to 120 days after being released from the marrow into the
blood. Over that period of time, red blood cells gradually
age. Spent cells are removed by the spleen and, to a lesser
extent, by the liver. The spleen and the liver also remove
any red blood cells that become damaged, regardless of their
age. The body efficiently recycles many components of the
damaged cells, including parts of the hemoglobin molecule,
especially the iron contained within it.
The majority of white blood cells have a relatively short
life span. They may survive only 18 to 36 hours after being
released from the marrow. However, some of the white blood
cells are responsible for maintaining what is called
immunologic memory. These memory cells retain knowledge of
what infectious organisms the body has previously been
exposed to. If one of those organisms returns, the memory
cells initiate an extremely rapid response designed to kill
the foreign invader. Memory cells may live for years or even
decades before dying.
Memory cells make immunizations possible. An immunization,
also called a vaccination or an inoculation, is a method of
using a vaccine to make the human body immune to certain
diseases. A vaccine consists of an infectious agent that has
been weakened or killed in the laboratory so that it cannot
produce disease when injected into a person, but can spark
the immune system to generate memory cells and antibodies
specific for the infectious agent. If the infectious agent
should ever invade that vaccinated person in the future,
these memory cells will direct the cells of the immune
system to target the invader before it has the opportunity
to cause harm.
Platelets have a life span of seven to ten days in the
blood. They either participate in clot formation during that
time or, when they have reached the end of their lifetime,
are eliminated by the spleen and, to a lesser extent, by the
liver.
