single red blood cell contains about 250 million hemoglobin
molecules, so each of these tiny cells can scoop up about 1 bil-
lion molecules of oxygen!
Te fact that hemoglobin is contained in erythrocytes, rather
than existing free in plasma, prevents it (1) from breaking into
fragments that would leak out of the bloodstream (through po-
rous capillary walls) and (2) from making blood more viscous
and raising osmotic pressure.
Oxygen loading occurs in the lungs, and the direction of
transport is from lungs to tissue cells. As oxygen-deﬁcient blood
moves through the lungs, oxygen diﬀuses from the air sacs of
the lungs into the blood and then into the erythrocytes, where
it binds to hemoglobin. When oxygen binds to iron, the he-
moglobin, now called
, assumes a new three-
dimensional shape and becomes ruby red.
In body tissues, the process is reversed. Oxygen detaches
from iron, hemoglobin resumes its former shape, and the result-
, becomes dark
red. Te released oxygen diﬀuses from the blood into the tissue
ﬂuid and then into tissue cells.
About 20% of the carbon dioxide transported in the blood
combines with hemoglobin, but it binds to globin’s amino acids
rather than to the heme group. Tis formation of
bin) occurs more read-
ily when hemoglobin is in the reduced state (dissociated from
oxygen). Carbon dioxide loading occurs in the tissues, and the
direction of transport is from tissues to lungs, where carbon di-
oxide is eliminated from the body. We describe the loading and
unloading of these respiratory gases in Chapter 22.
Because erythrocytes lack mitochondria and generate A±P
by anaerobic mechanisms, they do not consume any of the
oxygen they carry, making them very eﬃcient oxygen trans-
Erythrocytes are the major factor contributing to blood vis-
cosity. Women typically have a lower red blood cell count than
men [4.2–5.4 million cells per microliter (1 μl
blood versus 4.7–6.1 million cells/μl respectively]. When the
number of red blood cells increases beyond the normal range,
blood becomes more viscous and ﬂows more slowly. Similarly,
as the number of red blood cells drops below the lower end of
the range, the blood thins and ﬂows more rapidly.
Functions of Erythrocytes
Erythrocytes are completely dedicated to their job of trans-
porting respiratory gases (oxygen and carbon dioxide).
, the protein that makes red blood cells red, binds easily
and reversibly with oxygen, and most oxygen carried in blood is
bound to hemoglobin. Normal values for hemoglobin are 13–18
grams per 100 milliliters of blood (g/100 ml) in adult males, and
12–16 g/100 ml in adult females.
Hemoglobin is made up of the red
pigment bound to
. Globin consists of four polypeptide chains—
two alpha (
) and two beta (β)—each binding a ringlike heme
. Each heme group bears an atom of iron
set like a jewel in its center (Figure 17.4b). A hemoglobin mol-
ecule can transport four molecules of oxygen because each iron
atom can combine reversibly with one molecule of oxygen. A
(a) Hemoglobin consists of globin (two alpha and two beta
polypeptide chains) and four heme groups.
Iron-containing heme pigment.
Structure of hemoglobin.
Hemoglobin’s structure makes it a highly efﬁcient