Chemistry Reference
In-Depth Information
The composition of the diet is of concern, because
components like phytate, ascorbic acid, other metals
may have great infl uence on absorption (see Chapter 7).
The transfer of metals from the intestinal lumen into
the mucosal cells is not always associated with further
transport into the organism (systemic uptake). The
binding of metals by the low molecular weight protein,
metallothionein, in the mucosa and subsequent loss
by mucosal shed has been suggested as an important
mechanism in homeostatic regulation of the systemic
uptake of an essential metal-like zinc (Nordberg and
Kojima, 1979; Richards and Cousins, 1976). High oral
doses of zinc induce metallothionein synthesis in the
mucosal cells and block the uptake of copper by the
intestinal mucosa. This is used clinically to avoid tis-
sue accumulation of copper in patients with Wilson's
disease (see Chapter 26). A homeostatic mechanism for
iron implies that iron is stored in intestinal cells bound
to ferritin (see Chapter 30).
in the lung or intestinal compartment. Correspond-
ing mathematical expressions for total absorption or
uptake (I), and concentration (amount) of metal in
lung (E), or intestine (E2) are also given.
As mentioned in the previous sections, aerosols
deposited on the mucociliary escalator will be trans-
ferred to the gastrointestinal tract, and the uptake there
will be governed by the factors discussed in Section 3.2.
It is important to remember that for the effects of met-
als that occur as a result of systemic uptake, both the
part of metal that is absorbed directly through the lung
and the part that is absorbed after gastrointestinal
translocation of inhaled particles will have to be taken
into account, as well as those amounts of metal that are
absorbed from the gastrointestinal tract from exposure
through food and drink.
4 TRANSPORT, BIOTRANSFORMATION,
AND DISTRIBUTION
3.3 Total Absorption
Figure 3 describes the pathways of metals after inha-
lation or gastrointestinal intake and includes retention
The transport and distribution of metals will, to a
large extent, depend on the form in which the metal
occurs in the blood, which is the main transporting
Gastro-
intestinal
intake
Inhaled
amount
A
G
Exhaled
amount
Mucociliary
C
1
A
Unabsorbed
fecal
output
Deposition &
transport
C
2
A
C
4
E
C
5
( G + C
1
A + C
4
E
1
)
Lung
E
1
Intestine
E
2
C
3
E
1
C
6
E
2
Uptake
1
FIGURE 3
The pathways of metals after inhalation or gastrointestinal intake including retention in the lung or
intestinal compartment. Corresponding mathematic expression for total absorption or uptake (1) and concentra-
tion (amount) of metal in lung (E
1
) or intestine (E
2
) are also given.
