& Function: Introduction
Blood is an extremely complex fluid, composed of both formed
elements (red cells, white cells, platelets) and plasma. Red blood
cells (erythrocytes) are the most common formed elements,
carrying oxygen to the cells of the body via their main component, hemoglobin. White
blood cells are generally present at about 1/700th the
number of erythrocytes and function as mediators of immune responses
to infection or other stimuli of inflammation. Platelets are the
formed elements that participate in coagulation. Plasma is largely
water, electrolytes, and plasma proteins, all of which are very
complex. The plasma proteins most important in blood clotting are
the coagulation factors. Because blood circulates throughout the
body, alterations in normal blood physiology—either formed
elements or plasma proteins—may have widespread adverse
Marrow and Hematopoiesis
Although the mature formed elements of blood are quite different
from each other in both structure and function, all of these cells
develop from a common progenitor cell, or stem cell, population,
which resides in the bone marrow. The developmental process is called hematopoiesis and
represents an enormous metabolic task for the body. More than 100
billion cells are produced every day. This makes the bone marrow
one of the most active organs in the body. In adults, most of the
active marrow resides in the vertebrae, sternum, and ribs. In children,
the marrow is more active in the long bones.
The process of differentiation from stem cell to mature erythrocyte,
granulocyte, lymphocyte, monocyte, or platelet is shown in Figure 6–1. It
is not clear exactly what early events lead dividing stem cells
down a particular path of development, but many different peptides,
called cytokines, are clearly involved (Table
6–1); see also Chapter 3. Perhaps
because mature white blood cells have a much shorter half-life in
the circulation, white blood cell precursors usually outnumber red
blood cell precursors by a ratio of 3:1 in the bone marrow.
Hematopoiesis: development of the formed elements of
blood from bone marrow stem cells. Cells below the horizontal line
are found in normal peripheral blood. The principal cytokines that
stimulate each cell lineage to differentiate are shown. (EPO, erythropoietin;
TPO, thrombopoietin; CSF, colony-stimulating factor; G, granulocyte;
M, macrophage; IL, interleukin; SCF, stem cell factor.) See Table 6–1 for details.
(Redrawn, with permission, from Ganong WF. Review
of Medical Physiology, 22nd ed. McGraw-Hill, 2005.)
6–1 Cytokines that Regulate Hematopoiesis. |Favorite Table|Download (.pdf)
6–1 Cytokines that Regulate Hematopoiesis.
|Cytokine||Cell Lines Stimulated||Cytokine Source|
|IL-1||Erythrocyte||Multiple cell types|
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