Chapter 7
Defense Responses to Toxicants
7.1 INTRODUCTION
As seen from the foregoing chapters, living organisms are subjected to the
influence of a large number of environmental toxicants in addition to the
essential nutrients that are absorbed. This chapter examines how organisms
may be able to respond to the impact of many of those toxicants. The
consequences that may result when such defense mechanisms fail will also be
discussed.
7.2 RESPONSES OF HUMANS AND ANIMALS
This section focuses on five body systems, including the respiratory tract,
gastrointestinal tract, membranes, liver, and kidneys in humans and, in some
instances, in animals.
7.2.1 THE RESPIRATORY TRACT
An adult breathes more than 13,000 liters of air a day. This is not only the
body’s largest intake of any substance but also the most immediately important
to life. Humans can go without food for many days and without water for
many hours without serious health effects, but life without air terminates in a
very few minutes. Air is inhaled through the nasal cavity, nasopharynx, and
trachea. The trachea divides into the main bronchi, which go to the right and
left lungs (Figure 7.1). The right lung consists of three lobes, and the left lung,
two. The bronchi divide into finer and finer tubes, called bronchioles. Located
at the ends of the bronchioles are many tiny air sacs called alveoli, these are
where the exchange of gases takes place. At the alveoli, a thin sheet of moving
blood picks up molecular oxygen (O 2 ) from the inhaled air and unloads carbon
dioxide (CO 2 ) for exhalation.
The respiratory tract is one of the principal ports of entry for air pollutants
and is remarkably well equipped to cope with harmful invaders. There are three
main processes that operate in their defense against the invasion of foreign
agents: filtration, inactivation, and removal.
[16:53 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-007.3d]
Ref: 4365 MING-HO YU Chap-007 Page: 99 99-110
99
Defense Responses to Toxicants
7.1 INTRODUCTION
As seen from the foregoing chapters, living organisms are subjected to the
influence of a large number of environmental toxicants in addition to the
essential nutrients that are absorbed. This chapter examines how organisms
may be able to respond to the impact of many of those toxicants. The
consequences that may result when such defense mechanisms fail will also be
discussed.
7.2 RESPONSES OF HUMANS AND ANIMALS
This section focuses on five body systems, including the respiratory tract,
gastrointestinal tract, membranes, liver, and kidneys in humans and, in some
instances, in animals.
7.2.1 THE RESPIRATORY TRACT
An adult breathes more than 13,000 liters of air a day. This is not only the
body’s largest intake of any substance but also the most immediately important
to life. Humans can go without food for many days and without water for
many hours without serious health effects, but life without air terminates in a
very few minutes. Air is inhaled through the nasal cavity, nasopharynx, and
trachea. The trachea divides into the main bronchi, which go to the right and
left lungs (Figure 7.1). The right lung consists of three lobes, and the left lung,
two. The bronchi divide into finer and finer tubes, called bronchioles. Located
at the ends of the bronchioles are many tiny air sacs called alveoli, these are
where the exchange of gases takes place. At the alveoli, a thin sheet of moving
blood picks up molecular oxygen (O 2 ) from the inhaled air and unloads carbon
dioxide (CO 2 ) for exhalation.
The respiratory tract is one of the principal ports of entry for air pollutants
and is remarkably well equipped to cope with harmful invaders. There are three
main processes that operate in their defense against the invasion of foreign
agents: filtration, inactivation, and removal.
[16:53 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-007.3d]
Ref: 4365 MING-HO YU Chap-007 Page: 99 99-110
99