Anemias

Anemia means deficiency of hemoglobin in the blood,
which can be caused by either too few red blood cells
or too little hemoglobin in the cells. Some types of
anemia and their physiologic causes are the following.
Blood Loss Anemia. After rapid hemorrhage, the body
replaces the fluid portion of the plasma in 1 to 3 days,
but this leaves a low concentration of red blood cells.
If a second hemorrhage does not occur, the red blood
cell concentration usually returns to normal within 3
to 6 weeks.
In chronic blood loss, a person frequently cannot
absorb enough iron from the intestines to form hemoglobin
as rapidly as it is lost. Red cells are then produced
that are much smaller than normal and have too
little hemoglobin inside them, giving rise to microcytic,
hypochromic anemia, which is shown in Figure 32–3.
Aplastic Anemia. Bone marrow aplasia means lack of
functioning bone marrow. For instance, a person
exposed to gamma ray radiation from a nuclear bomb
blast can sustain complete destruction of bone
marrow, followed in a few weeks by lethal anemia.
Likewise, excessive x-ray treatment, certain industrial
chemicals, and even drugs to which the person might
be sensitive can cause the same effect.

Megaloblastic Anemia. Based on the earlier discussions
of vitamin B12, folic acid, and intrinsic factor from the
stomach mucosa, one can readily understand that loss
of any one of these can lead to slow reproduction of
erythroblasts in the bone marrow. As a result, the red
cells grow too large, with odd shapes, and are called
megaloblasts. Thus, atrophy of the stomach mucosa,
as occurs in pernicious anemia, or loss of the entire
stomach after surgical total gastrectomy can lead to
megaloblastic anemia. Also, patients who have intestinal
sprue, in which folic acid, vitamin B12, and other
vitamin B compounds are poorly absorbed, often
develop megaloblastic anemia. Because in these states
the erythroblasts cannot proliferate rapidly enough to
form normal numbers of red blood cells, those red cells
that are formed are mostly oversized, have bizarre
shapes, and have fragile membranes. These cells
rupture easily, leaving the person in dire need of an
adequate number of red cells.