Download Section 1 - Angelfire

7.1
Figure 1
David, “the boy in the plastic bubble,” had
severe combined immunodeficiency syndrome.
Figure 2
Diatom bathed in seawater
mesoderm: the middle tissue layer of an
animal embryo. In more complex animals, this
layer will become the muscles and other connective tissues, the blood vessels and blood
cells, and various other organs, structures,
and systems, depending on the organism.
ectoderm: the outermost tissue layer of
an animal embryo. In more complex animals,
this layer will become the skin, the outermost
parts of the nervous system, and various
other outer organs, structures, and systems,
depending on the organism.
endoderm: the innermost tissue layer of
an animal embryo. In more complex animals,
this layer will become the digestive tract, the
respiratory tract, and various other inner
organs, structures, and systems, depending
on the organism.
242 Chapter 7
The Importance of a
Circulatory System
Your circulatory system carries nutrients to cells, wastes away from cells, and
chemical messages from cells in one part of the body to distant target tissues. It
distributes heat throughout the body and, along with the kidneys, maintains
acceptable levels of body fluid.
No cell is further than two cells away from a blood vessel that carries nutrients. Your circulatory system has 96 000 km of blood vessels to sustain your 100
trillion cells. No larger than the size of your fist and with a mass of about 300 g,
the heart beats about 70 times/min from the beginning of life until death. During
an average lifetime, the heart pumps enough blood to fill two large ocean tankers.
Every minute, 5 L of blood cycles from the heart to the lungs, picks up
oxygen, and returns to the heart. Next, the heart pumps the oxygen-rich blood
and nutrients to the tissues of the body. The oxygen aids in breaking down highenergy glucose into low-energy compounds and releases energy within the tissue
cells. The cells use the energy to build new materials, repair existing structures,
and for a variety of other energy-consuming reactions. Oxygen is necessary for
these processes to occur, and the circulatory system plays a central role in providing that oxygen.
The circulatory system is also vital to human survival because it transports
wastes and helps defend against invading organisms. It permits the transport of
immune cells throughout the body. To appreciate the importance of the immune
system, consider the story of David, “the boy in the plastic bubble”(Figure 1).
David was born without an immune system, which meant that his body was
unable to produce the cells necessary to protect him from disease. As a result,
David had to live in a virtually germ-free environment. People who came in contact with him had to wear plastic gloves. Eventually, David received a bone
marrow transplant from his sister. Unfortunately, a virus was hidden in the bone
marrow. The sister, who had a functioning immune system, was able to protect
herself from the virus, but David was not.
The Challenge of Transporting Oxygen and Nutrients
Single-celled organisms, such as the diatom in Figure 2, have no need for a circulatory system because oxygen can diffuse directly into them from the surrounding seawater and wastes can diffuse out.
Even simple multicellular organisms, such as the sponge in Figure 3, have no
need for a circulatory system in an aquatic environment. Because the sponge has
only two cell layers, all cells remain in contact with water. The flagella pull water
into the sponge, bringing oxygen and nutrients into the body. Water and wastes
are expelled through the large pore, or osculum, at the top of the body.
Relying on diffusion to deliver oxygen and nutrients is too limiting for more
complex multicellular animals, which have three cell layers. The middle cell layer,
or mesoderm layer, is sandwiched between the ectoderm and endoderm and does
not come into direct contact with circulating fluids or water. Cells of the mesoderm
need a circulatory system to bring them into contact with oxygen and nutrients.
Open and Closed Circulatory Systems
In an open circulatory system, blood carrying oxygen and nutrients is pumped into
body cavities where it bathes the cells directly. This low-pressure system is commonly found in snails (Figure 4), insects, and crustaceans. There is no distinction
7.1
osculum
central cavity
collar cells
flagellum
endoderm
ectoderm
between the blood and the interstitial fluid in this system. Interstitial fluid is a fluid
that occupies the spaces between cells. The contraction of one or more hearts
pushes blood from one body cavity, or sinus, to another. Muscular movements by
the animal during locomotion can assist blood movement, but diverting blood
flow from one area to another is limited. When the heart relaxes, blood is drawn
back toward the heart through open-ended pores.
In a closed circulatory system, the blood is always contained within blood
vessels. This system, commonly found in earthworms (Figure 5), squids, octopuses, and vertebrates, separates blood from the interstitial fluid by enclosing
blood inside tubes or vessels. The earthworm has five heartlike vessels that pump
blood through three major blood vessels. Larger blood vessels branch into
smaller vessels that supply blood to the various tissues.
Figure 3
The sponge is composed of two cell layers:
the ectoderm, on the outside of the sponge,
and the endoderm, on the inside. Seawater
acts as a transport system, carrying nutrients
and removing wastes. Nutrients and wastes
diffuse across cell membranes.
interstitial: refers to the space between cells
sinus: a body cavity or air space surrounding an internal organ
heart
Practice
Understanding Concepts
1. Describe the main functions of the circulatory system.
2. Differentiate between an open and a closed circulatory system.
Figure 4
The snail has an open circulatory system.
3. Identify one advantage and one disadvantage of an open circulatory
system.
capillaries
4. Describe the similarities and differences between the circulatory systems of sponges, snails, and earthworms.
5. Why do multicellular animals need a circulatory system?
The Importance of a Circulatory System
1. The circulatory system
• brings oxygen and nutrients to cells;
• takes wastes away from cells;
hearts
blood vessels
Figure 5
The worm has a closed circulatory system.
Circulation and Blood 243