With this chapter we begin discussing the
blood cells and cells of the
macrophage system and
lymphatic system. Functions of red blood cells, which are the most abundant cells of the blood and are
necessary for the delivery of oxygen to the tissues.
Red Blood Cells (Erythrocytes)
A major function of red blood cells, also known as
erythrocytes, is to transport
hemoglobin, which in turn carries oxygen from the lungs to the tissues. In
some lower animals, hemoglobin circulates as free protein in the plasma, not enclosed in red blood
cells. When it is free in the plasma of the human being, about 3 percent of it leaks through the capillary
membrane into the tissue spaces or through the glomerular membrane of the kidney into the
glomerular filtrate each time the blood passes through the capillaries. Therefore, hemoglobin must
remain inside red blood cells to effectively perform its functions in humans. The red blood cells have
other functions besides transport of hemoglobin. For instance, they contain a large quantity of
carbonic
anhydrase, an enzyme that catalyzes the reversible reaction between carbon dioxide (CO2) and water
to form carbonic acid (H2CO3), increasing the rate of this reaction several thousandfold. The rapidity
of this reaction makes it possible for the water of the blood to transport enormous quantities of CO2 in
the form of bicarbonate ion (HCO3−) from the tissues to the lungs, where it is reconverted to CO2 and
expelled into the atmosphere as a body waste product. The hemoglobin in the cells is an excellent
acid-
base buffer (as is true of most proteins), so the red blood cells are responsible for most of the acidbase
buffering power of whole blood.
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