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Elimination of Toxicants (Gerald A. Leblanc)

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CHAPTER 10

Elimination of Toxicants
GERALD A. LEBLANC

10.1

INTRODUCTION

The ability to efficiently eliminate toxic materials is critical to the survival of a species.
The complexity of toxicant elimination processes has increased commensurate with the
increased complexity associated with animal form. For unicellular organisms, passive
diffusion can suffice for the elimination of toxic metabolic wastes produced by the
organism. Similarly, as exogenous toxic materials derived from the environment diffuse
into a unicellular organism, they can also readily diffuse out of the organism. The
large surface area to mass ratio of these organisms ensures that a toxic chemical within
the cell is never significantly distanced from a surface membrane across which it
can diffuse.
As organisms evolved in complexity, several consequences of increased complexity
compromised the efficiency of the passive diffusion of toxic chemicals:
1.
2.
3.
4.
5.

They increased in size.
Their surface area to body mass decreased.
Their bodies compartmentalized (i.e., cells, tissues, organs).
They generally increased in lipid content.
They developed barriers to the external environment.

Size. With increased size of an organism, a toxic chemical has greater distance to
traverse before reaching a membrane across which it can diffuse to the external environment. Thus overall retention of the chemical will increase as will propensity for the
chemical to elicit toxicity.
Surface Area to Body Mass Ratio. Increased size of an organism is associated with a
decrease in the surface area to body mass ratio. Accordingly, the availability of surface
membranes across which a chemical can passively diffuse to the external environment
decreases and propensity for retention of the chemical increases.

A Textbook of Modern Toxicology, Third Edition, edited by Ernest Hodgson
ISBN 0-471-26508-X Copyright  2004 John Wiley & Sons, Inc.

203

204

ELIMINATION OF TOXICANTS

Compartmentalization. With increased complexity comes increased compartmentalization. Cells associate to form tissues and tissues associate to form organs. Compartmentalization increases the number of barriers across which chemicals must traverse
before sites of elimination are reached. As different compartments often have different physicochemical characteristics (i.e., adipose tissue is largely fat while blood is
largely aqueous), chemicals are faced with the challenge to be mobile in these various
environments.
Lipid Content. As a general...
CHAPTER 10
Elimination of Toxicants
GERALD A. LEBLANC
10.1 INTRODUCTION
The ability to efficiently eliminate toxic materials is critical to the survival of a species.
The complexity of toxicant elimination processes has increased commensurate with the
increased complexity associated with animal form. For unicellular organisms, passive
diffusion can suffice for the elimination of toxic metabolic wastes produced by the
organism. Similarly, as exogenous toxic materials derived from the environment diffuse
into a unicellular organism, they can also readily diffuse out of the organism. The
large surface area to mass ratio of these organisms ensures that a toxic chemical within
the cell is never significantly distanced from a surface membrane across which it
can diffuse.
As organisms evolved in complexity, several consequences of increased complexity
compromised the efficiency of the passive diffusion of toxic chemicals:
1. They increased in size.
2. Their surface area to body mass decreased.
3. Their bodies compartmentalized (i.e., cells, tissues, organs).
4. They generally increased in lipid content.
5. They developed barriers to the external environment.
Size. With increased size of an organism, a toxic chemical has greater distance to
traverse before reaching a membrane across which it can diffuse to the external envi-
ronment. Thus overall retention of the chemical will increase as will propensity for the
chemical to elicit toxicity.
Surface Area to Body Mass Ratio. Increased size of an organism is associated with a
decrease in the surface area to body mass ratio. Accordingly, the availability of surface
membranes across which a chemical can passively diffuse to the external environment
decreases and propensity for retention of the chemical increases.
A Textbook of Modern Toxicology, Third Edition, edited by Ernest Hodgson
ISBN 0-471-26508-X Copyright
2004 John Wiley & Sons, Inc.
203
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