Download The Task of Transportation

S E C T I O N
9.1
The Task of Transportation
E X P E C TAT I O N S
Explain the functions of a
transport system in
multicellular organisms.
Identify the main types of
transport system found in
animals.
Distinguish between an open
and closed transport system.
Explain how adaptations such
as double circulation alter the
efficiency of a transport system.
Figure 9.1 A jellyfish does not
require a specialized internal
transport system.
Organisms like those shown in Figure 9.1 can rely
on their external environments to supply what they
need for all their life activities. In most animals,
however, the transport system plays a key role in
keeping the body in a state of general physical
well-being. Its functions include delivering
nutrients and oxygen to each and every cell of the
body, removing the waste products of cellular
processes, and serving as the pathway from one
part of the body to another for disease-fighting
agents, hormones, and other chemical messengers.
In warm-blooded animals, the transport system also
assists in the important function of controlling
body temperature. The transport system is therefore
an essential link among the cells and organs within
the body, and between individual cells and the
external environment.
In Chapter 8 you learned that while all aerobic
organisms must exchange gases with their
environment, different organisms have evolved
different kinds of respiratory systems to meet this
challenge. The same is true of transport systems.
Almost all multicellular organisms have a vascular
system, a system of fluid tissue that plays a role in
transporting nutrients and other materials to the
cells of the organism. Most animals have a
circulatory system, which is a vascular system in
which the progress of fluid is controlled by muscle
movements so that it follows a specific pattern. A
cardiovascular system, found in the higher animals,
282
MHR • Internal Systems and Regulation
is a circulatory system in which the vascular fluid
is pumped around the body by the action of a
specialized organ, the heart. The following
paragraphs describe the transport mechanisms
found in different kinds of organisms.
Internal Transport at Its Simplest
A self-sufficient unicellular organism such as
Amoeba has no need for an organized transport
system. Respiratory gases and nutrients enter the
cell through diffusion or active transport across the
cell membrane. Once inside the cell, these
substances are distributed to the cell’s organelles
by the streaming or flowing of cytoplasm. Waste
materials diffuse or are carried back across the cell
membrane to be released into the environment. As
you can see in Figure 9.2, such an organism
exchanges the materials it needs directly with the
external environment.
Some multicellular animals, such as those in the
family Cnidaria (which includes hydra and
jellyfish), also lack an organized transport system.
In these animals, fluid taken in through the mouth
enters a body cavity that extends through most of
the organism, including the tentacles. Materials are
exchanged directly between the fluid in the body
cavity and the individual cells of the organism,
none of which are more than a few cells away from
the body cavity.
REWIND
For more information about diffusion, turn to Chapter 1,
Section 1.3.
Although planaria have a higher metabolic rate
than cnidaria (and therefore need a faster exchange
of materials), planaria can still rely on diffusion to
meet their metabolic needs. Nutrients dissolve
directly into individual cells from a digestive
cavity. This cavity branches and rebranches within
the organism to provide a surface for the exchange
of materials with individual cells (see Figure 9.3).
A separate cavity absorbs waste materials that are
then expelled from the animal.
They also lack a specialized mechanism for
pumping fluid from one body part to another.
Instead, fluid containing dissolved respiratory
gases, nutrients, and other substances is ingested
directly from the external environment. This fluid
flows freely within the organism’s body cavities,
sometimes assisted by the contraction of cells and
muscle fibres as the animal moves.
right side of
excretory
system
left side of
digestive
system
substances enter
cell by diffusion
substances move
within cell
Figure 9.2 The movement, or streaming, of cytoplasmic fluid
helps to distribute materials to the organelles within a cell.
None of these organisms have blood, which is a
vascular tissue adapted to carry particular substances.
MINI
Figure 9.3 In the multicellular planarian, the movements of
the animal help to stream fluid through a central cavity, from
which material can diffuse into and out of its cells.
The Specialized Transport System
Most multicellular organisms cannot rely on
diffusion or active transport to bring materials from
the external environment to cells of the internal
tissues quickly enough to meet the cells’ metabolic
requirements. For such organisms to survive, some
means of speeding the exchange of materials
between the inside of the organism and the outside
world is required. Before organisms could evolve to
LAB
How Effective Is Diffusion as
a Transport Mechanism?
In this lab, you will compare the rate of diffusion of a
solution into tissues of different sizes. Carefully cut a potato
into sections of different shapes and sizes. Select four
sections for this experiment. Your thinnest section should
be no less than 1 mm wide, and your thickest section no
more than 3 cm wide. The sections should be of varying
dimensions and volumes. Immerse the potato sections into
a solution of potassium permanganate, and leave them for
30 min. At the end of this period, use forceps to remove all
the sections. Cut each one in half and measure how far the
purple solution penetrated the potato flesh. Record your
results in a table.
Analyze
Calculate the rate of diffusion in each section by dividing
the diffusion distance by the time the potato was immersed
in the solution. Was the diffusion rate constant for each
section? Provide an explanation for any variation. Assuming
that the same diffusion rate you measured here could be
applied to your body. Use this information to calculate
approximately how long it would take for an oxygen
molecule absorbed in your lung to reach the fingers of your
writing hand (if you had no specialized transport system).
Transport and Circulation • MHR
283
sizes more than a few cells thick, adaptations
providing for an organized transport system were
necessary. The evolution of a transport system also
made it possible for larger animals to dedicate
specially-adapted organs and structures to specific
tasks, and then distribute the products of these
tasks to other organs and structures. This, in turn,
allowed for the emergence of multicellular
organisms capable of undertaking complex activities.
The Open Transport System
If you dissect a grasshopper, you will find only one
blood vessel. This vessel, the aorta, carries blood
into the animal’s body cavity. The body cavity is
subdivided into a number of sinuses (or chambers)
that bring the blood and other body fluids into
contact with the internal cells of the animal. The
exchange of materials takes place within the
sinuses. Co-ordinated movements of body muscles
help to move the blood around inside the insect
and then back to a sinus surrounding the long,
tubular heart. From there, the blood is again
pumped through the aorta to the sinuses of the
body cavity (see Figure 9.4). In this circulatory
system, the blood bathes the cells directly. A
transport system in which the blood does not
always stay contained within blood vessels is
called an open transport system.
In an open transport system, fluid can slosh back
and forth; therefore it circulates relatively slowly.
The result is a system that cannot provide for the
rapid delivery of materials around the body. This is
one reason why the open transport system is best
suited to insects and other arthropods that have
relatively small body cavities. Another reason is
that (as you saw in Chapter 8) insects have a
tracheal respiratory system that delivers oxygen
directly from the environment to the animals’ cells.
The respiratory and circulatory systems are
separate in these animals, so even a very active
insect can meet its oxygen requirements despite its
relatively slow circulation.
BIO
FACT
Because there is no reason for the blood of grasshoppers
and other insects to carry oxygen, it lacks an oxygencarrying respiratory pigment and thus appears clear and
watery.
The Closed Transport System
Animals that rely on the circulatory system to carry
respiratory gases need a faster flow of blood than
can be provided by an open transport system.
These organisms, including the annelids
(segmented worms such as earthworms) and
vertebrates, have a closed transport system. In this
type of system, the blood does not bathe the cells
directly, but rather is pumped around the body
within a network of vessels. This network includes
larger vessels that collect blood for pumping,
smaller vessels that distribute the blood throughout
the body, and tiny capillaries that provide a surface
for the exchange of materials with individual cells.
In a closed transport system, blood circulates in
only one direction, passing through the animal’s
gas exchange system in its cycle through the body.
Although all closed transport systems share
these features, they differ in ways that result in
varying levels of efficiency. The efficiency of a
circulatory system is measured by the rate at which
heart
aorta
brain
anus
mouth
intestine
nerve cord
stomach
Figure 9.4 The transport system of a grasshopper. The long, tubular heart extends
most of the length of the abdomen. Some insects have additional hearts located at
the base of their wings or antennae to help pump blood into these body parts.
284
MHR • Internal Systems and Regulation
brain
crop
gizzard
clitellum
mouth
segments
anus
aortic
arches (hearts)
dorsal
blood vessel
intestine
bristles
ventral
blood vessel
Figure 9.5 The earthworm has a relatively simple closed circulatory system. The
five aortic arches located near the head link the dorsal and ventral blood vessels.
it can transport substances around the body.
Factors that affect this efficiency include the
composition of the blood, the path of circulation,
and the speed of blood flow (usually measured in
terms of blood pressure). The circulatory systems
of different animals show adaptations related to
their unique metabolic requirements.
Circulation in Annelids
The simplest closed circulatory system is found in
annelids. This system consists of two main blood
vessels connected by a series of five pairs of heartlike pumps called aortic arches. Blood enters the
aortic arches from the dorsal vessel, which (like the
arches) contracts to pump the blood forward. From
the aortic arches the blood is pumped into the
ventral vessel located under the intestinal tract.
The blood then flows through a branching series of
smaller and smaller vessels to the internal organs
and tissues, where the exchange of materials occurs
across the thin walls of the capillaries. Some
vessels also lead to the animal’s body walls, where
the exchange of gases takes place between the
capillaries and the external environment. The
capillaries lead into larger vessels, which in turn
direct the blood into the dorsal vessel and aortic
arches to complete the cycle.
Dissolved in the blood of the earthworm is a
respiratory pigment, a molecule that binds to
oxygen. The presence of this pigment allows the
blood to pick up and transport more oxygen than
would otherwise be possible. The respiratory
pigment found in earthworms is the same pigment
that is found in the blood of vertebrates.
Figure 9.6 Unlike the earthworm, the bird breathes through
lungs. It needs a different kind of circulatory system to
deliver oxygen around its body.
As you saw in Chapter 8, the earthworm uses its
entire skin surface to exchange gases. This means
that the blood is being oxygenated as it circulates
around the animal. For animals with specialized
respiratory structures such as gills or lungs,
however, a more complex circulatory system is
required. This is necessary to ensure that the blood
is first passed through the respiratory surface
where the exchange of gases takes place, and then
is delivered to the rest of the body.
In contrast to the circulatory system of the
earthworm, which lacks a true heart, all vertebrates
have a closed circulatory system in which the
regular contractions of a muscular heart force blood
past the respiratory surface, through a network of
vessels, and into the capillaries where the exchange
of materials takes place. Blood travels from the
Transport and Circulation • MHR
285
heart to the capillaries through a distinct set of
vessels called arteries, and returns from the
capillaries to the heart through another set of vessels
called veins. The path of circulation and the
structure of the heart both play a role in determining
the efficiency of the circulatory system in the
different animals discussed in the following pages.
See Figure 9.7 for a comparison of these systems.
Circulation in Fish
The heart of a fish has two main chambers and two
lesser cavities organized in a row. Blood flows
through this series of chambers and is pumped out
through a ventral artery to the network of
capillaries in the gills, where the exchange of
respiratory gases takes place. From the gills the
blood travels to a dorsal artery that branches
through the body to deliver oxygenated blood. This
kind of circulation, in which the blood travels
through the heart only once during each complete
circuit around the body, is called single circulation.
The circulatory system of the fish offers the
advantage that all the blood travelling from the
heart to the body has been oxygenated in the gills.
In this circulation pathway, however, much of the
blood pressure generated by the heart is lost as the
blood passes through the capillaries in the gills.
This happens because the finer tubes of the
capillaries offer more resistance to the blood,
slowing it down. The relatively slow passage of
blood through the rest of the body limits the rate of
sinus
venosus
conus
arteriosus
oxygen delivery and therefore limits the metabolic
rate of the animal. Since fish are cold-blooded, they
do not need to generate as much energy as warmblooded animals such as birds and mammals.
Circulation in Amphibians
Animals in the class Amphibia (which includes
frogs, toads, and salamanders) have a heart with a
third chamber. The blood is pumped from the heart
to the lungs, and then flows back to the heart
where it is pumped again before flowing into the
arteries that carry blood to the rest of the body. The
structure of the heart means that there is some
mixing of oxygenated and deoxygenated blood —
that is, some of the blood that returns to the heart
from the body then flows back to the body without
first passing to the lungs.
The extra chamber of the heart allows blood to
be pumped through the heart twice during every
complete cycle of the circulatory system, creating a
double circulation. Since only some of the blood is
pumped twice in each circuit, the amphibian
circulatory system is called an incomplete double
circulatory system. The incomplete separation of
oxygenated and deoxygenated blood in the heart
reduces the efficiency of the respiratory system.
Remember, however, that amphibians can respire
through their skin as well as through their lungs, so
the blood does not have to pass by the lungs to
pick up oxygen.
lungs
body
lungs
atrium
right
atrium
deoxygenated
blood
ventral
aorta
left
atrium
oxygenated
blood
ventricle
right
atrium
right
ventricle
left
atrium
left
ventricle
ventricle
all deoxygenated
blood
A The fish has a two-chambered
heart and complete, single
circulation.
B The amphibian has a
three-chambered heart and
incomplete, double circulation.
Figure 9.7 The circulatory systems of a fish, an amphibian, and a mammal.
Variations in the structure of the heart and in the path of circulation affect the
efficiency of the circulatory system in these different animals.
286
body
MHR • Internal Systems and Regulation
C The mammal has a
four-chambered heart and
complete, double circulation.
BIO
FACT
Birds (about 9000 species) and mammals (about
4500 species) are alone among living things in being warmblooded, meaning that their internal temperatures are kept
constant. This is possible only because their muscles, which
generate heat as they contract, are constantly supplied with
richly oxygenated blood by their very efficient respiratory
and circulatory systems.
Figure 9.8 Adaptations in the circulatory system of
mammals provides them with a much higher metabolic rate
than is found in fish.
Circulation in Birds and Mammals
Active, warm-blooded vertebrates have very high
energy requirements. Their bodies demand lots of
oxygen, and need it delivered quickly. The high
level of cellular activity also generates wastes that
must be removed quickly from the fluid
surrounding individual cells in order to keep those
cells healthy. The circulatory systems of birds and
mammals show two important adaptations that
allow this more efficient circulation. The first is a
mechanism to keep oxygenated and deoxygenated
blood completely separate, so the blood that flows
from the heart to the body contains as much
oxygen as possible. The second is a mechanism to
maximize blood pressure in order to force blood
through the capillaries quickly.
SECTION
REVIEW
1.
K/U Describe the differences in the transport
mechanisms found in an amoeba and in a planarian.
2.
K/U Explain why an open transport system is well
suited to the metabolic needs of an insect.
3.
C Using landmarks that your classmates will be
familiar with, map out a walking or cycling route that
could illustrate a path of single circulation, and
another route that could illustrate a path of double
circulation. Trade maps with a partner and try to
explain how your partner’s two routes compare to
the circulatory systems of different animals.
4.
All birds and mammals have a heart consisting
of four chambers: two atria (singular atrium) and
two ventricles. Blood entering the right atrium
from the body is pumped into the right ventricle.
From there it moves through an artery to the lungs,
where the exchange of respiratory gases occurs.
The blood loses pressure as it flows through the
capillary networks of the lungs — but rather than
flowing out to the body, this oxygenated blood is
carried back to the heart. It flows into the left
atrium, enters the left ventricle, and from there is
pumped into the aorta and other arteries that
deliver blood to the body. This system is called a
closed, complete, double circulatory system. In the
sections that follow, you will examine the
mammalian transport systems in more detail, using
the human system as an example.
I The heart rate of mammals is inversely
proportional to their size. Accumulate, graph, and
analyze data that will either support or discredit this
hypothesis.
5.
K/U Reptiles have two aorta — one travels down
the right side of the body, and the other down the
left. In contrast, birds and mammals each have only
one aorta; in mammals it is on the left side of the
body, and in birds it is on the right. What does this
suggest about the evolution of the circulatory
system? What selective advantage is offered by the
loss of one aorta?
6.
MC Amphibian species are sometimes referred to as
“indicator species” because changes in their health
can provide information about damage to an
ecosystem. With reference to the transport of
respiratory gases, explain why a frog would be more
sensitive than a muskrat to chemical changes in the
water of the pond they share.
Transport and Circulation • MHR
287