Biology of Disease

Lymphocytes are key components of the immune system. Monocytes circulate
in the blood for about 72 h and then enter the tissues and transform into
macrophages, which play a key role in inflammation and defense (Chapter 4).

Platelets are vesicle-like structures about 3 Mm in diameter with a volume of
7 fdm^3 (femtodecimeters^3 or femtoliters). They are formed by the fragmentation
of large precursor cells called megakaryocytes and, like erythrocytes, are not
complete cells although they contain numerous granules, some organelles and
a tubular system (Figure 13.5). Their lifespan is 10 12 days and they function
in blood clotting or hemostasis (Section 13.4).

13.3 Hemoglobins

Hemoglobin is the red-colored, oxygen-transporting protein in erythrocytes.
Its Mris about 64 000 and it is made up of four subunits, each containing an
iron-containing heme group (Figure 13.6). Each molecule can carry up to four
O 2 molecules. Oxygen is taken up as the blood passes through the lungs and
is transported to all parts of the body allowing respiration, the oxidation of
fuels, to occur in the mitochondria. The iron in the heme group of Hb remains
in the ferrous (Fe(II)) state throughout. Should the iron become oxidized to
Fe(III), methemoglobin is formed, which is incapable of carrying oxygen. This
oxidation happens to a small extent continuously, so that normal blood always
contains a few percent of methemoglobin. However, methemoglobin reductase,
present in erythrocytes, constantly catalyzes its reduction back to Hb. The rare
individuals with a genetic deficiency of this enzyme have severe problems and
tend to be cyanosed unless they are treated with a reducing agent.

Hemoglobin was one of the first proteins to have its complete structure
determined. Indeed, because Hb is so important medically, it is fair to say
that more is probably known about Hb than any other protein. It is vital to life
because the low solubility of oxygen in water means that insufficient amounts
can be carried by blood in solution. The binding of O 2 to Hb in erythrocytes

HEMOGLOBINS

CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY ().

Mitochondrion

A granule containing for

example, growth factors,

fibrinogen, Factor V, fibronectin

Dense tubular system

Open canalicular system

Glycogen granules

Submembranous filaments

(platelet contractile protein)

Electron dense granule containing,

for example ADP, Ca2+, serotonin

Figure 13.5 Schematic of a blood platelet. The platelet is packed with granules that have a major role in blood clotting.

Margin Note 13.1 Platelet

concentrates

Platelet concentrates can be

prepared by centrifugation and

may be stored for up to five days

(Chapter 6). Such concentrates are

used to treat patients suffering from

thrombocytopenia (Section 13.9)

and who have insufficient platelets

and to prevent bleeding in patients

with bone marrow failure.

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Margin Note 13.2 Cyanosis –

going blue in the face

Cyanosis (from Greek cyan, blue)

is the bluish complexion resulting

from lack of oxygen in the circulating

blood. It is most frequently observed

under the nails, lips as well as the

skin. Cyanosis occurs following an

inadequate oxygen intake in the

lungs or from many other reasons,

for example the stagnation of blood

in the circulation during heart failure.

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