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biology
ANIMAL STRUCTURE AND FUNCTION
thirteenth Ed iti on
Cec i e Sta r r
Ralph Taggart
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Animal Structure and Function
Biology: The Unity and Diversity of Life,
Thirteenth Edition
Cecie Starr, Ralph Taggart, Christine Evers,
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31 Animal Tissues and Organ Systems
Learning Roadmap
Where you have been With this chapter, we begin our
survey of the tissues and organ systems (Section 1.2) in animals.
The chapter expands on the nature of animal body plans (24.2)
and trends in vertebrate evolution (25.2). We also revisit the topic
of cancer (8.6,11.6) and look again at the evolution of human skin
color (14.1).
Where you are now
Animal Organization
Most animals have cells organized as
tissues, organs, and organ systems.
The components function in concert
to maintain conditions in the body’s
internal environment.
Epithelial and Connective Tissues
Epithelial tissue covers the body’s
surface and lines its internal tubes.
Connective tissue underlies epithelial
tissue and supports and connects
body parts.
Muscle and Nervous Tissue
Muscle tissue consists of cells that
contract in response to signals
from nervous tissue. Nervous tissue
receives and integrates information
from inside and outside the body.
Organ Systems
Vertebrates have a coelom, and
many organs reside in body cavities
derived from it. Interactions among
organ systems sustain life.
Example of an Organ System
Skin is an organ system that protects
the body, conserves water, produces
vitamin D, and helps maintain body
temperature. Temperature control is
an example of negative feedback.
Where you are going The nervous system is the focus of Chapters 32 and 33. Chapter 34
describes the function of endocrine glands. Chapter 35 explains how muscles contract and interact
with the skeleton. Chapters 36 and 37 consider the transport and immune functions of blood. Chapters
38, 39, and 40 describe how you take in essential substances and eliminate metabolic wastes. Finally,
Chapters 41 and 42 describe organs involved in reproduction and how a body develops.
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31.1 Stem Cells—It’s All About Potential
All of the cells of your body “stem” from stem cells
(Figure 31.1). Stem cells are self-renewing cells that can
either divide and produce more stem cells 1 , or differentiate into specialized cells that characterize specific
body parts 2 .
Stem cells vary in their potential to form a new
individual or new tissues. A fertilized human egg and
the cells produced by its first four divisions are “totipotent,” meaning they can develop into a new individual if placed in a womb. Later divisions produce
embryonic cells that are “pluripotent.” A pluripotent
cell does not have the ability to develop into a new
individual, but can give rise to into any of the cell
types in a body.
Most stem cells in an adult are “unipotent,” meaning they yield only one specific type of cells. Some
adult stem cells produce new skin cells; others produce new blood cells. However, adults have few stem
cells that can make new muscle cells or nerve cells.
Thus, heart muscle lost to a heart attack, leg muscles
destroyed by muscular dystrophy, or nerves severed in
an injured spinal cord are not replaced.
Embryonic stem cells hold great potential as a treatment to repair tissues that are normally not regenerated in the adult body. In the United States, the first
clinical trials of such treatments began in 2010. These
trials involve stem cells initially harvested from
human embryos, and then grown in the laboratory.
One trial is testing whether the stem cells can repair
nervous tissue in the spinal cord of people with recent
injuries. In another, stem cells are being used to regenerate eye tissue in age-related macular degeneration,
a common condition that causes blindness. One day,
embryonic stem cell treatments might also help people
with other nerve and muscle disorders such as heart
disease, muscular dystrophy, multiple sclerosis, and
Parkinson’s disease.
The use of embryonic stem cells remains controversial despite their potential as a universal toolkit for
repairing damaged tissues. Many people oppose the
use of human embryos for any purpose. Unipotent
adult stem cells may offer an alternative to embryonic
cells if researchers can find a way to make them dedifferentiate—to turn back their developmental clock so
the cells again become pluripotent. Harvesting and
culturing the few pluripotent stem cells in adults also
offers promise.
Researchers are also investigating methods of
controlling cell differentiation. To be of use in clinical treatments, pluripotent stem cells from any source
must be induced to differentiate into a desired cell
type. By analogy, pluripotent cells are like college
freshmen who need to be directed toward a specific
major. Investigations into mechanisms of directing
pluripotent cell differentiation have the additional
benefit of informing us about how normal development gives rise to the many specialized cells that
constitute the human body.
stem cell Cell capable of replication or of differentiation into some
or all cell types.
cell
type 1
stem
cell
or
stem
cell
1
stem
cell
cell
type 2
2
or
stem
cell
cell
type 3
stem
cell
mitosis
differentiation
Figure 31.1 Stem cells. Each stem cell can divide to form new
stem cells or differentiate to form specialized cell types.
50 µm
The photo at the left shows a colony of human embryonic stem
cells growing in a laboratory at the University of Pittsburgh.
523
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31.2 Organization of Animal Bodies
Most animal bodies have cells organized as tissues, organs,
and organ systems.
■ Physical constraints and evolutionary history influence the
structure and function of body parts.
■ Links to Life’s levels of organization 1.2, Homeostasis 1.3,
Cell junctions 4.11, Diffusion 5.8, Adapting to life on land 25.6
■
Levels of Organization
In all animals, development produces a body with
several to many types of cells (Figure 31.2A). An adult
human has about 200 different kinds of cells. In most
animals, cells of different types are organized in tissues often anchored by extracellular matrix (Figure
31.2B). Cell junctions of the types described in Section
4.11 typically connect the cells of a tissue. They hold
cells in place and allow them to cooperate in a specific
task or tasks.
Four types of tissue occur in all vertebrate bodies:
1. Epithelial tissue covers body surfaces and lines the
internal cavities such as the gut.
2. Connective tissue holds body parts together and
provides structural support.
3. Muscle tissue moves the body or its parts.
4. Nervous tissue detects stimuli and relays signals.
Different types of cells characterize different tissues. For example, muscle tissue includes contractile
cells not found in nervous tissue or epithelial tissue.
Typically, animal tissues are organized into organs.
A Cell
B Tissue
C Organ
(cardiac
muscle cells)
(cardiac muscle)
(heart)
Figure 31.2 Levels of organization in a vertebrate (human) body.
524 UNIT VI
An organ is a structural unit of two or more tissues
organized in a specific way and capable of carrying
out specific tasks. For example, a human heart is an
organ that includes all four tissue types (Figure 31.2C).
The heart’s wall is made up mostly of cardiac muscle
tissue. A sheath of connective tissue covers the muscle,
and internal chambers are lined with epithelial tissue.
The heart receives signals via nervous tissue.
In organ systems, two or more organs and other
components interact physically, chemically, or both in
a common task. For example, in the vertebrate circulatory system, the force generated by a beating heart (an
organ) moves blood (a tissue) through blood vessels
(organs), thereby transporting gases and solutes to and
from all body cells (Figure 31.2D). Multiple organ systems sustain the organism (Figure 31.2E).
The Internal Environment
By weight, an animal body is mainly fluid: a waterbased solution of salts, proteins, and other solutes.
The bulk of this body fluid is intracellular, which
means it is inside cells. The remainder is extracellular.
Extracellular fluid is the environment in which body
cells live. It bathes cells and provides them with the
substances they require to stay alive. It also functions
as a dumping ground for cellular waste. In vertebrates,
extracellular fluid consists mainly of interstitial fluid
(the fluid in spaces between cells) and plasma, the
fluid portion of the blood (Figure 31.3).
D Organ System
e Organism
(circulatory system)
(human)
How animals work
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Cells survive only if solute concentrations and temperature of the fluid surrounding them remain within
a narrow range. Maintaining conditions of the cell’s
environment within this range is an important aspect
of homeostasis (Section 1.3).
Evolution of Animal Structure
An animal’s structural traits (its anatomy) evolve in
concert with its functional traits (its physiology). Both
types of traits are genetically determined and vary
among individuals. In each generation, genes for those
traits that best help individuals survive and reproduce in their environment are preferentially passed
on. Over many generations, anatomical and structural
traits become optimized in ways that reflect their function in a specific environment.
Physical constraints affect evolution of body structure. For example, dissolved substances travel through
extracellular fluid by diffusion. Diffusion alone could
not sustain a large or thick body because gases, nutrients, and wastes would not move quickly enough
through the body to keep up with cellular metabolism.
Thus, mechanisms that speed the distribution of materials evolved along with increases in body size. In vertebrates, a circulatory system serves this purpose. The
system includes a network of extensively branched
blood vessels that extends through the body. Every living cell is close enough to a blood vessel to exchange
substances with it by diffusion (Figure 31.4).
As another example, vertebrates faced new physical challenges as they left their aquatic habitat for the
land (Section 25.6). Gases can only enter or leave an
animal’s body by diffusing across a moist surface.
In an aquatic organism, the surrounding water both
delivers oxygen and moistens the respiratory surface.
By contrast, a land animal must extract oxygen from
air, which can dry a respiratory surface. Evolution of
lungs allowed vertebrates to maintain a moist respira-
tory surface inside their body. Cells inside the lung
secrete the fluid that keeps this surface moist.
Lungs are not modified fish gills. Rather, lungs
evolved from outpouchings of the gut in fishes ancestral to land vertebrates. As this example illustrates,
evolution by natural selection often modifies existing
tissues or organs. There is evidence of evolutionary
compromise in the anatomy and physiology of many
animals. For example, as a legacy of the lungs’ ancestral connection to the gut, the human throat connects
to both the digestive tract and respiratory tract. As a
result of this dual connection, food sometimes goes
where air should, and a person chokes. It would be
safer if food and air entered the body through separate
passageways. However, because evolution modifies
existing structures, it often does not produce the most
optimal body plan.
extracellular fluid Of a multicelled organism, body fluid that is not
inside cells; serves as the body’s internal environment.
interstitial fluid Of a multicelled organism, body fluid in spaces
between cells.
Take-Home Message
plasma
interstitial
fluid
lymph, cerebrospinal fluid,
mucus, and other fluids
Intracellular
Fluid
(28 liters)
Figure 31.4 Branching blood vessels. The vessels deliver
oxygen to within close proximity of all cells in a human body.
Extracellular
Fluid (ECF)
(15 liters)
Human Body Fluids
(43 liters)
Figure 31.3 Distribution of fluids in a human body.
How are animal bodies organized?
» In most animals, cells are organized as tissues. Each tissue consists of cells
of a specific type that cooperate in carrying out a particular task. Tissues are
organized into organs, which in turn are components of organ systems.
» The animal body consists largely of fluid. The bulk of this fluid is in cells.
The fluid outside cells (extracellular fluid) is the body’s internal environment.
Maintaining the solute concentration and temperature of this fluid is an
important facet of homeostasis.
» Many anatomical traits evolved as solutions to physical challenges.
However, these solutions are sometimes imperfect because evolution modifies existing structures, rather than building a body plan from the ground up.
CHAPTER 31
animal tissues and organ systems 525
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31.3 Epithelial Tissue
Epithelial tissue covers the body’s external surfaces and lines
internal tubes and cavities.
■ Links to Fibrous proteins 3.6, Cilia 4.10, Cell junctions 4.11,
DNA replication errors 8.6
■
Variations in Structure and Function
General Characteristics
Epithelial tissue, or an epithelium (plural, epithelia), is
a sheetlike layer of cells with little extracellular matrix
between them. One surface of the epithelium, referred
to as its apical surface, faces the outside world or the
interior of a body cavity or tube. The opposite surface,
referred to as the basal surface, secretes a noncellular
basement membrane that attaches the epithelium to an
underlying tissue. Blood vessels do not run through an
epithelium, so nutrients reach cells by diffusing from
vessels in an adjacent tissue.
Most of what you see when you look in a mirror—
your skin, hair, and nails—is epithelial tissue or structures derived from it. Hair, fur, nails, hooves, beaks,
and feathers all form when specialized epithelial cells
Simple squamous epithelium
•L
ines blood vessels, the heart,
and air sacs of lungs
•A
llows substances to cross
by diffusion
Simple cuboidal epithelium
•L
ines kidney tubules, ducts of
some glands, reproductive tract
•F
unctions in absorption and
secretion, movement of
materials
mucus-secreting gland cell
Simple columnar epithelium
•L
ines some airways, parts
of the gut
•F
unctions in absorption and
secretion, protection
Figure 31.5 Micrographs and drawings of three types of simple epithelia, with
examples of their functions and locations.
526 UNIT VI
produce large amounts of the protein keratin. The
visible part of a hoof, hair, or feather consists of the
remains of such cells.
Epithelial cells may be arranged as a single layer or
multiple layers. A simple epithelium is one cell thick,
whereas a stratified epithelium includes multiple layers of cells.
Cells of an epithelium are typically described by
their shape. Cells in squamous epithelium are flattened or scalelike. (Squama is the Latin word for scale.)
Cells of cuboidal epithelium are short cylinders that
look like cubes when viewed in cross-section. Cells
in columnar epithelium are taller than they are wide.
Figure 31.5 shows the three types of simple epithelium
and describes their functions.
Simple squamous epithelium facilitates the
exchange of materials. It is the thinnest type of epithelium, and gases and nutrients diffuse across it easily.
This type of epithelium lines blood vessels and the
inner surface of the lungs. By contrast, stratified squamous epithelium has a protective function. It makes
up the outermost layer of human skin.
Cells of cuboidal and columnar epithelium function
in movement, absorption, or secretion of substances.
Those that move substances along the surface of an
epithelium have cilia at their apical surface. For example, ciliated epithelial cells in the oviducts propel an
egg from an ovary toward the uterus (the womb).
In some epithelia, cells have fingerlike extensions
called microvilli at their free surface. Microvilli are
typically shorter than cilia, do not move, and have
an internal framework of actin filaments rather than
microtubules. Microvilli increase the surface area
across which substances can be detected by, absorbed
into, or secreted from a cell.
Three types of intercellular junctions connect cells
in animal tissues (Section 4.11). One type, the tight
junction, occurs only in epithelial tissue. Tight junctions connect the plasma membranes of adjacent cells
so securely that fluids cannot seep between the cells.
An epithelium with cells connected by tight junctions keeps fluid contained within a particular body
compartment from seeping into underlying tissue. For
example, tight junctions join the epithelial cells in the
lining of the gut. The junctions allow the gut epithelium to function as a selective barrier. Substances in
the gut can enter the body’s internal environment only
by controlled movement into and across cells of the
gut epithelium.
How animals work
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Figure 31.6 Examples of an
exocrine and an endocrine gland.
Figure It Out: Which type of gland
is ductless?
Answer: Endocrine glands are
ductless and secrete hormones
into the blood.
Endocrine gland
cell that secretes
capillary hormone
Exocrine gland
parotid gland
(secretes saliva)
parotid duct (delivers
saliva to mouth)
thyroid gland (secretes
hormones into blood)
Epithelial tissues subject to mechanical stress such
as skin epithelium have many adhering junctions.
These junctions function like buttons that hold a shirt
closed. They connect the plasma membranes of cells at
distinct points but do not form a seal between them.
Epithelial Cell Secretions
Specialized epithelial cells called gland cells secrete
substances that function outside the cell. In most animals, some gland cells cluster as multicelled glands
that release substances onto the skin, or into a body
cavity or fluid. There are two main types of glands
(Figure 31.6). Exocrine glands have ducts or tubes that
deliver their secretions onto an internal or external
surface. Exocrine secretions include mucus, saliva,
tears, digestive enzymes, earwax, and breast milk.
Endocrine glands do not have ducts. They release signaling molecules called hormones into a body fluid.
Most commonly, hormones enter small blood vessels
(capillaries). We discuss the function of endocrine
glands in detail in Chapter 34.
Carcinomas—Epithelial Cell Cancers
Adult animals make few new muscle cells or nerve
cells, but they constantly renew their epithelial cells.
For example, each day you lose skin cells and grow
new ones to replace them. An adult sheds about 0.7
kilogram (1.5 pounds) of skin each year. Similarly, the
lining of your intestine is replaced every four to six
days. All those cell divisions provide lots of opportunities for DNA replication errors that can lead to cancer. As a result, epithelium is the animal tissue most
likely to become cancerous.
An epithelial cell cancer is called a carcinoma.
About 95 percent of skin cancers are carcinomas.
Breast cancers are usually carcinomas of epithelial cells
that line the milk ducts or of the breast’s glandular
epithelium. Similarly, most lung cancers arise in cells
of the lung’s epithelial lining.
Take-Home Message
What are the functions of epithelial tissue?
basement membrane Secreted material that attaches epithelium
to an underlying tissue.
endocrine gland Ductless gland that secretes hormones into a
body fluid.
epithelial tissue Sheetlike animal tissue that covers outer body
surfaces and lines internal tubes and cavities.
exocrine gland Gland that secretes milk, sweat, saliva, or some
other substance through a duct.
gland cell Secretory epithelial cell.
microvilli Thin projections from the plasma membrane of some
epithelial cells; increase the cell’s surface area.
» Epithelia are sheetlike tissues that line the body’s surface and its cavities,
ducts, and tubes. They function in protection, absorption, and secretion.
Some epithelia have cilia or microvilli at their surface.
» Glands are secretory organs derived from epithelium. Exocrine glands
secrete material through a duct onto a body surface or into a body cavity.
Endocrine glands secrete hormones into the blood.
» Specialized epithelial cells that produce large amounts of the protein
keratin are the source of hair, nails, hooves, and feathers.
» Epithelial tissues undergo continual turnover and are the most frequent
site for cancers.
CHAPTER 31
animal tissues and organ systems 527
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31.4 Connective Tissues
■ Connective tissues connect body parts and provide
structural and functional support to other body tissues.
■ Links to Hemoglobin 3.2, Lipids 3.5, Extracellular matrix
4.11, Alternative energy sources in food 7.7
Connective tissues consist of cells in an abundant
extracellular matrix. Soft connective tissues hold
other tissues in place or connect them to one another.
Cartilage, bone tissue, adipose tissue, and blood are
specialized connective tissues. We provide a brief
overview of the specialized connective tissues here,
and describe their function in more detail in the chapters that follow.
Soft Connective Tissues
Fibroblasts, the most common cells in soft connective
tissues, secrete a matrix of complex carbohydrates and
long fibers of the proteins collagen and elastin. The
most abundant soft connective tissue, loose connective
tissue, has fibroblasts and fibers dispersed widely in
its matrix (Figure 31.7A). Loose connective tissue keeps
internal organs in place and underlies epithelia. It is
densely supplied with blood vessels.
Dense, irregular connective tissue makes up deep
skin layers, supports intestinal muscles, and forms
capsules around organs that do not stretch, such as
collagen fiber
Specialized Connective Tissues
All vertebrate skeletons include cartilage, which is
mainly a rubbery matrix composed of collagen fibers
and glycoproteins. Cartilage cells (chondrocytes)
secrete the material that surrounds them (Figure 31.7D).
When you were an embryo, the first skeleton that
formed consisted of cartilage. As development continued, bone replaced most of it. Cartilage still supports
your nose, throat, and outer ears. It covers the ends of
bones at joints and acts as a shock absorber between
vertebrae. Blood vessels do not extend through
glycoprotein-rich
matrix with fine
collagen fibers
collagen fibers
fibroblast
collagen
fibers
elastic fiber
a Loose connective tissue
kidneys. Its matrix has fibroblasts and collagen fibers
oriented every which way, as in Figure 31.7B.
By contrast, dense, regular connective tissue has
fibroblasts in orderly rows between parallel, tightly
packed bundles of fibers (Figure 31.7C). This organization helps prevent tears when the tissue is subject to
mechanical stress. Dense, regular connective tissue
is the main tissue in tendons and ligaments. Tendons
connect skeletal muscle to bones and do not stretch.
Ligaments attach one bone to another and are elastic.
Like a rubber band, they can be stretched out, then
spring back to their original shape. Tendons and ligaments are not well supplied with blood. If they are
torn, they are very slow to heal.
fibroblast
• Underlies most epithelia
b Dense, irregular
connective tissue
c Dense, regular
connective tissue
•P
rovides elastic support and
serves as a fluid reservoir
• In deep skin layers, around
intestine, and in kidney capsule
• In tendons connecting muscle
to bone and ligaments that
attach bone to bone
•B
inds parts together, provides
support and protection
•P
rovides stretchable attachment between body parts
cartilage cell
(chondrocyte)
d Cartilage
• Internal framework of nose, ears,
airways; covers the ends of bones
•S
upports soft tissues, cushions bone
ends at joints, provides a low-friction
surface for joint movements
Figure 31.7 Animated Connective tissue structure and function.
528 UNIT VI
How animals work
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cartilage, and little or no cell division occurs in adult
cartilage, so torn cartilage does not heal. In addition,
production of matrix declines with age.
Adipose tissue stores fats. It consists of cells (adipocytes) with little matrix between them, and it is richly
supplied with blood vessels. Most adipose tissue in
a human adult is white adipose tissue. Its cells bulge
with so much stored fat that their nucleus is typically
pushed to one side and flattened (Figure 31.7E). In addition to its role as the body’s main energy reservoir,
white adipose tissue acts as insulation and cushions
body parts. A less abundant tissue, called brown adipose tissue, specializes in producing heat. In human
adults, brown adipose tissue is concentrated in the
neck and upper chest. Cells of brown adipose tissue
store less fat than cells of white adipose tissue and
have many specialized mitochondria. Compared to
typical mitochondria, those of brown adipose tissue
produce less ATP and release more energy as heat.
Bone tissue consists of living cells (osteocytes) in a
matrix hardened by calcium and phosphorus (Figure
31.7F). Blood vessels run through channels in the tissue.
Bone tissue is the main component of bones, which are
organs that interact with skeletal muscles to move a
body. Bones also support and protect internal organs.
Blood cells form in the spongy interior of some bones.
Blood is considered a connective tissue because its
cells and platelets descend from stem cells in bone
(Figure 31.7G). Red blood cells filled with hemoglobin transport oxygen (Section 3.2). White blood cells
defend the body against pathogens. Platelets are cell
fragments that function in clot formation. Cells and
platelets drift in plasma, a fluid extracellular matrix
consisting mostly of water and dissolved proteins.
adipose tissue Connective tissue that specializes in fat storage.
blood Circulatory fluid; in vertebrates it is a fluid connective
tissue consisting of plasma, red blood cells, white blood cells, and platelets.
bone tissue Connective tissue consisting of cells surrounded by a
mineral-hardened matrix of their own secretions.
cartilage Connective tissue consisting of cells surrounded by a
rubbery matrix of their own secretions.
connective tissue Animal tissue with an extensive extracellular
matrix; structurally and functionally supports other tissues.
Take-Home Message
What are connective tissues?
» Various soft connective tissues underlie epithelia, form capsules around
organs, and connect muscle to bones or bones to one another.
» A vertebrate skeleton consists of two connective tissues: rubbery cartilage
and mineral-hardened bone. Blood is a connective tissue because blood cells
form in bone. The cells are carried by plasma, the fluid portion of the blood.
» Adipose tissue is a specialized connective tissue that stores fat.
Plasma (fluid portion of the blood)
white
blood
cell
red
blood
cell
compact
bone tissue
nucleus
fat cell
(adipocyte)
bulging with
stored fat
blood vessel
platelet
bone cell
(osteocyte)
e Adipose tissue
f Bone tissue
•U
nderlies skin and occurs
around heart and kidneys
•M
akes up the bulk of
most vertebrate skeletons
•S
erves in energy storage,
provides insulation, cushions and protects some
body parts
•P
rovides rigid support,
attachment site for muscles,
protects internal organs,
stores minerals, produces
blood cells
G Blood • Flows through blood vessels,
heart
• Distributes essential gases,
nutrients to cells; removes
wastes from them
CHAPTER 31
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31.5 Muscle Tissues
■ Muscle
■ Links
moves bodies or propels materials through them.
to Glycogen 3.4, Cell junctions 4.11
Cells of muscle tissues contract (shorten) in response
to signals from nervous tissue. ATP provides the
energy that fuels muscle contractions. Muscle tissue
occurs in most animals, but we focus here on the kinds
found in vertebrates. We discuss the mechanism of
muscle contraction in detail in Section 35.7.
Skeletal Muscle Tissue
Skeletal muscle tissue interacts with bones to move
contract pass swiftly from cell to cell at gap junctions
that connect the cells along their length. The rapid
flow of signals ensures that all cells in cardiac muscle
tissue contract as a unit.
Compared to other muscle tissues, cardiac muscle
has far more mitochondria. They provide the continually beating heart with a dependable supply of ATP.
Cardiac muscle and smooth muscle tissue are said
to be “involuntary” because people cannot deliberately
make these tissues contract.
Smooth Muscle Tissue
body parts. It consists of parallel arrays of long, cylindrical cells called muscle fibers, which have a striated,
or striped, appearance (Figure 31.8A). Muscle fibers are
multinucleated and form by cell fusion during embryonic development. Skeletal muscle contracts reflexively, as when you pull your hand away after touching
a hot object. More often, its contraction is deliberate, as
when you reach for something. Thus, skeletal muscle
is commonly described as “voluntary” muscle.
Along with the liver, skeletal muscle is a major site
for glycogen storage. Metabolic activity in skeletal
muscles is the major source of body heat.
Many tubular organs, such as the stomach, uterus,
and bladder, have smooth muscle tissue in their wall.
Smooth muscle cells are unbranched, with tapered
ends and a single nucleus at their center (Figure 31.8C).
Smooth muscle tissue is not striated. It contracts more
slowly than skeletal muscle, but its contractions can
be sustained longer. Contraction of smooth muscle
propels material through the gut, reduces the diameter
of blood vessels and airways, and closes sphincters (a
sphincter is a ring of muscle in a tubular organ).
Cardiac Muscle Tissue
smooth muscle tissue Muscle that lines blood vessels and forms
cardiac muscle tissue Muscle of the heart wall.
skeletal muscle tissue Muscle that pulls on bones and moves body
parts; under voluntary control.
the wall of hollow organs.
Cardiac muscle tissue occurs only in the heart wall
(Figure 31.8B). Like skeletal muscle tissue, it has a striated appearance. Cardiac muscle consists of branching cells, each with a single nucleus, attached end to
end by adhering junctions. The junctions hold the
cells together during forceful contractions. Signals to
Take-Home Message
What is muscle tissue?
» Muscle tissue consists of cells that contract in response to
nervous signals. Contraction requires ATP.
nucleus
nucleus
adjoining
ends of
cells
a Skeletal muscle
b Cardiac muscle
c Smooth muscle
•L
ong, multinucleated, cylindrical cells
with conspicuous striping (striations)
•S
triated, branching cells (each with a
single nucleus) attached end to end
•C
ells with a single nucleus, tapered
ends, and no striations
•P
ulls on bones to bring about movement, maintain posture
• Found only in the heart wall
•F
ound in the walls of arteries, the
digestive tract, the reproductive
tract, the bladder, and other organs
•R
eflex activated, but also under
voluntary control
•C
ontraction is not under voluntary
control
Figure 31.8 Animated Three types of muscle tissue.
530 UNIT VI
•C
ontraction is not under voluntary
control
How animals work
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31.6 Nervous Tissue
Nervous tissue detects changes in the internal or external
environment, integrates information, and controls the activity
of muscle and glands.
signalreceiving
extensions
■
Nervous tissue makes up the communication lines of a
body. Neurons are the signaling cells in nervous tissue.
Each neuron has a cell body, a region that contains the
nucleus and other organelles. Long cytoplasmic extensions that project from the cell body allow the cell to
receive and send electrochemical signals (Figure 31.9).
When a neuron receives sufficient stimulation, an
electrical signal travels along its plasma membrane to
the ends of specialized cytoplasmic extensions. The
electrical signal causes release of chemical signaling
molecules from these endings. These molecules diffuse
across a small gap to an adjacent neuron, muscle fiber,
or gland cell, and alter that cell’s behavior.
Your nervous system has more than 100 billion
neurons. There are three types. Sensory neurons are
excited by specific stimuli, such as light or pressure.
Interneurons receive and integrate sensory information. They store information and coordinate responses
to stimuli. In vertebrates, interneurons occur mainly in
the brain and spinal cord. Motor neurons relay commands from the brain and spinal cord to glands and
muscle cells (Figure 31.10).
Neuroglial cells, also called neuroglia, keep neurons
positioned where they should be, and provide them
with nutrients. Neuroglial cells also wrap around the
signal-sending cytoplasmic extensions of most motor
neurons. They act as insulation and speed the rate at
which signals travel.
nervous tissue Animal tissue composed of neurons and support-
ing cells; detects stimuli and controls responses to them.
neuroglial cell Cell that supports and assists neurons.
neuron One of the cells that make up communication lines of
a nervous system; transmits electrical signals along its plasma
membrane and communicates with other cells through chemical
messages.
cell body
of neuron
signal-sending
extension
neuroglial cell
wrapped around
a signal-sending
cytoplasmic
extension of
the neuron
Figure 31.9 Animated Micrograph and graphic of a motor
neuron. The neuron has a cell body
with a nucleus (visible as a dark spot),
and cytoplasmic extensions. A neuroglial cell wraps around and insulates
the signal-sending extension.
Take-Home Message
What is nervous tissue?
» Nervous tissue consists of neurons and the cells that support them.
Different kinds of neurons detect specific stimuli, integrate information,
and issue or relay commands to other tissues.
» The supporting cells in nervous tissue are referred to as neuroglial cells,
or neuroglia.
Figure 31.10 Example of a coordinated
interaction between skeletal muscle tissue
and nervous tissue.
Sensory neurons in the lizard’s eyes relay
information about the position of a fly to
interneurons in the lizard’s brain.
Signals from interneurons in the lizard’s
brain flow to motor neurons, which in turn
send stimulatory signals to the muscle fibers
of the lizard’s long, coiled-up tongue. The
tongue uncoils swiftly and precisely to reach
the very spot where the fly is perched.
CHAPTER 31
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31.7 Organ Systems
Organs typically include all four types of tissues and are
components of an organ system.
■ Link to Animal body plans 24.2
■
cranial cavity
spinal cavity
Organs in Body Cavities
thoracic cavity
Vertebrate Organ Systems
Figure 31.12 introduces the eleven organ systems of the
human body. Other vertebrates have the same systems.
diaphragm
abdominal cavity
pelvic cavity
Figure 31.11 Animated Main body cavities that hold
human organs. Figure It Out: Which organs lie in body
cavities that are not part of the coelom?
Answer: The spinal cord and brain
Like other vertebrates, humans are bilateral and have a
lined body cavity known as a coelom (Section 24.2). A
sheet of smooth muscle called the diaphragm divides
the human coelom into an upper thoracic cavity and a
lower cavity with abdominal and pelvic regions ( Figure
31.11). The heart and lungs reside in the thoracic cavity. The abdominal cavity holds digestive organs such
as the stomach, intestines, and liver. The bladder and
reproductive organs are in the pelvic cavity.
The cavities that hold the brain (the cranial cavity)
and spinal cord (the cranial cavity) are not derived
from the coelom.
Organ systems work cooperatively to carry out
specific tasks. For example, organ systems interact to
provide cells with essential raw materials and remove
wastes (Figure 31.13). Food and water enter the body
by way of the digestive system, which includes all
Figure 31.12 Animated Below, human organ systems and their functions.
Integumentary System
Nervous System
Muscular System
Skeletal System
Protects body from injury,
dehydration, and pathogens; controls its temperature; excretes certain
wastes; receives some
external stimuli.
Detects external and
internal stimuli; controls and coordinates
the responses to stimuli; integrates all organ
system activities.
Moves body and
its internal parts;
maintains posture;
generates heat by
increases in metabolic activity.
Supports and protects
body parts; provides
muscle attachment
sites; produces red
blood cells; stores calcium, phosphorus.
532 UNIT VI
Circulatory System
Endocrine System
Rapidly transports
many materials to
and from interstitial
fluid and cells; helps
stabilize internal pH
and temperature.
Hormonally controls
body functioning;
with nervous system
integrates short- and
long-term activities.
(Male testes added.)
How animals work
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components of the tubular gut such as the stomach
and intestine, as well as organs that aid digestion such
as the pancreas and gallbladder. The digestive system
also eliminates undigested wastes.
The respiratory system, which includes lungs and
airways that lead to them, takes in oxygen. The heart
and blood vessels of the circulatory system deliver
nutrients and oxygen to cells, and remove waste carbon dioxide and solutes from them. The circulatory
system delivers carbon dioxide to the respiratory system for expulsion in exhalations. The circulatory system also moves excess water, salts, and soluble wastes
to the urinary system. Organs of the urinary system
include kidneys that filter wastes from the blood.
Urine produced by the kidneys is stored in a bladder
until it can be eliminated from the body.
Figure 31.13 does not show the nervous, endocrine,
muscular, and skeletal systems, but these too help
vertebrates obtain essential substances and eliminate
wastes. For example, the nervous system detects
changes in internal levels of water, solutes, and nutrients. Signals from the nervous and endocrine systems
to the kidneys encourage conservation or elimination
of water. They also stimulate the muscle contractions
that allow you to eat or drink.
food, water intake
oxygen inhaled
Digestive
System
nutrients,
water,
solutes
Respiratory
System
oxygen
carbon
dioxide
exhaled
carbon
dioxide
Urinary
System
Circulatory System
water,
solutes
excretion
of food
residues
transport of
materials to
and from cells
elimination of soluble
wastes, excess water,
and salts
Figure 31.13 Some of the ways that organ systems interact to keep the
body supplied with essential substances and eliminate unwanted wastes.
Other organ systems that are not shown also take part in these tasks.
Take-Home Message
What are organs and organ systems?
» Organs consist of multiple tissues and are themselves components of
organ systems. Cooperative action of organ systems sustains the body.
Lymphatic System
Respiratory System
Digestive System
Urinary System
Reproductive System
Collects and returns
some tissue fluid to
the bloodstream;
defends the body
against infection and
tissue damage.
Rapidly delivers oxygen to the tissue fluid
that bathes all living
cells; removes carbon
dioxide wastes of cells;
helps regulate pH.
Ingests food and water;
mechanically, chemically
breaks down food and
absorbs small molecules
into internal environment;
eliminates food residues.
Maintains the volume
and composition of
internal environment;
excretes excess
fluid and bloodborne
wastes.
Female: Produces eggs; provides a
protected, nutritive environment for the
development of new individuals. Male:
Produces and transfers sperm to the
female. Hormones of both systems
also influence other organ systems.
CHAPTER 31
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31.8 Human Integumentary System
In vertebrates, the integumentary system consists of
skin, structures derived from skin, and an underlying layer
of connective and adipose tissue.
■ Links to Human skin color 14.1, Homo erectus 26.5
■
Of all vertebrate organs, the outer body covering called
skin has the largest surface area. Skin consists of two
layers, a thin upper epidermis and the dermis beneath
it (Figure 31.14). The dermis connects to the hypodermis,
an underlying layer of connective and adipose tissue. The depth of the hypodermis varies among body
regions. The hypodermis beneath the skin of eyelids is
thin, with few adipose cells. By contrast, the hypodermis of the buttocks is thickened by many adipose cells.
Vertebrate skin has many functions. It contains sensory receptors that keep the brain informed of external
conditions. It serves as a barrier to keep out pathogens
and it helps control internal temperature. In land vertebrates, skin also helps conserve water. In humans,
reactions that produce vitamin D occur in the skin.
Structure of Human Skin
Epidermis is a stratified squamous epithelium with an
abundance of adhering junctions and no extracellular
matrix. Human epidermis consists mainly of keratinocytes, epithelial cells that synthesize the waterproofing
protein keratin.
Figure 31.15 Vitiligo. Lee Thomas, an African American television
reporter, has vitiligo. The death of melanocytes has turned his
hands white and produced white blotches on his face and arms.
Mitotic cell divisions in deep epidermal layers continually produce new keratinocytes that displace older
cells upward toward the skin’s surface. As cells move
upward, they become flattened, lose their nucleus, and
die. Dead keratinocytes at the skin surface form an
abrasion-resistant layer that helps prevent water loss.
Melanocytes, another type of epidermal cell, make
pigments called melanins and donate them to keratinocytes. Variations in skin color arise from differences
in the distribution and activity of melanocytes, and in
hair
epidermis
stratified
squamous
epithelium
duct of
sweat
gland
blood
vessel
dermis
mainly
dense
connective
tissue
pressuresensitive
sensory
receptor
smooth
muscle
sweat
gland
Figure 31.14 Animated Structure of human skin
and underlying tissue. The
photo is a cross-section of
thickened human skin.
534 UNIT VI
hypodermis
mainly adipose
tissue and loose
connective tissue
hair
follicle
sebaceous
gland
How animals work
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the type of melanin they produce (Section 14.1). One
melanin is brown to black. Another is red to yellow.
The effect of melanocytes can be seen with vitiligo, a
skin disorder in which the destruction of these cells
results in light patches of skin (Figure 31.15).
Melanin functions as a sunscreen, absorbing ultraviolet (UV) radiation that could damage DNA and
other biological molecules. When skin is exposed to
sunlight, melanocytes produce more of the brownishblack melanin, resulting in a protective “tan.”
Dermis consists primarily of dense connective tissue with stretchy elastin fibers and supportive collagen fibers. Blood vessels, lymph vessels, and sensory
receptors weave through the dermis. Dermis is much
thicker than epidermis, and more resistant to tearing.
Leather is animal dermis that has been treated with
chemicals to preserve it.
Sweat glands, sebaceous glands, and hair follicles
are pockets of epidermal cells that migrated into the
dermis during early development. Sweat is mostly
water. As it evaporates, it cools the skin surface and
helps keep the body from overheating. Sebaceous
glands produce sebum, an oily mix of triglycerides,
fatty acids, and other lipids. Sebum helps keep skin
and hair soft. It also has antimicrobial properties.
The part of a hair that you see is the keratin-rich
remains of dead cells that originated in the follicle at
the hair’s base. Hair cells divide every 24 to 72 hours,
making them among the fastest-dividing cells in the
body. As cells at the base of a hair follicle replicate,
they push cells above them upward, lengthening the
hair. Smooth muscle attaches to each hair, and when
this muscle reflexively contracts in response to cold or
fright, the hair is pulled upright.
Evolution of Human Skin
Compared to other primates, humans have far more
sweat glands and shorter, finer body hairs (Figure
31.16). According to one hypothesis, an increase in
sweat glands and a loss of body hair occurred in concert with the evolution of bipedalism. During brisk
walking or running, the metabolic activity in skeletal
muscles produces heat that raises the body temperature. Presumably, when our bipedal ancestors began to
run under the hot African sun, individuals with finer
hair and more sweat glands were at an advantage
because they were less likely to overheat.
dermis Deep layer of skin that consists of connective tissue with
nerves and blood vessels running through it.
epidermis Outermost tissue layer; in animals, the epithelial layer
of skin.
Figure 31.16 Primate skin. Humans have less body hair and
more sweat glands than other primates such as chimpanzees.
When young, our closest primate relatives, chimpanzees and bonobos, have pink skin and a covering
of long, black body hair. Our early ancestors probably
had similarly pink skin and dark hair. Thus, loss of
body hair that facilitated cooling would have created
a new selective challenge—an increased exposure to
potentially damaging sunlight.
The dark skin now observed in all African populations is considered an adaptation to this challenge.
With this in mind, researchers reasoned that determining when dark skin evolved would provide an
estimate of when humans lost their body hair. To
determine when skin first darkened, the researchers looked at sequence variations in the human
MC1R gene, which governs melanin deposition. The
sequence comparisons indicated that dark skin color
evolved by as early as 1.2 million years ago, presumably in concert with hair loss. This date supports the
hypothesis that bipedalism selected for hair loss; the
loss occurred during the time of Homo erectus, the first
primate for which we have evidence of long-distance
bipedal travel.
Later, as Section 14.1 explained, some populations
of humans dispersed from Africa to higher latitudes
where sunlight was less intense and their skin color
reverted to a more chimpanzeelike pinkness.
Take-Home Message
What are the functions of the integumentary system?
» The integumentary system consists of skin, derivatives of skin such as hair,
and underlying connective tissue.
» Skin has sensory receptors that inform the brain about the environment. It
also serves as a barrier against pathogens, produces vitamin D, and functions
in temperature regulation.
CHAPTER 31
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31.9 Negative Feedback in Homeostasis
A negative feedback system involving multiple organ systems
allows the body to maintain its internal temperature.
■ Link to Metabolic heat 7.5
■
In vertebrates, homeostasis involves interactions
among sensory receptors, the brain, and muscles and
glands. A sensory receptor is a cell or cell component
that detects a specific stimulus. Sensory receptors
involved in homeostasis function like internal watchmen that monitor the body for changes. Information
from sensory receptors throughout the body flows to
the brain. The brain evaluates incoming information,
then signals effectors—muscles and glands—to take
the necessary actions to keep the body functioning.
Homeostasis often involves negative feedback, a process in which a change causes a response that reverses
the change. An air conditioner with a thermostat is a
familiar nonbiological example of a negative feedback
system. A person sets the air conditioner to a desired
temperature. When the temperature rises above this
preset point, a sensor in the air conditioner detects the
change and turns the unit on. When the temperature
declines to the desired level, the thermostat detects
this change and turns off the air conditioner.
A negative feedback mechanism also keeps your
internal temperature near 37°C (98.6°F). Consider
what happens when you exercise on a hot day (Figure
31.17). Muscle activity generates heat, and your internal
temperature rises. Sensory receptors in the skin detect
negative feedback A change causes a response that reverses the
change; important mechanism of homeostasis.
sensory receptor Cell or cell component that detects a specific
type of stimulus.
Stimulus
Exertion on a hot day raises
internal body temperature
What is the role of negative feedback in homeostasis?
»Negative feedback prevents dramatic changes in internal
conditions. Sensory receptors detect changes and send signals to the brain, which sends signals to muscles and glands.
The signals cause a response that reverses the initial change.
Brain
Receptors monitor internal
temperature and signal the
brain when it increases.
Brain receives signals from
sensory receptors and signals
muscles and glands.
Response
Muscles and Glands
Skeletal muscles
in the chest
wall contract
more frequently,
increasing the
rate of breathing.
536 UNIT VI
Take-Home Message
Sensory Receptors
Body temperature declines
Figure 31.17 Animated Negative feedback mechanism
that reduces body temperature when it rises.
the increase and signal the brain, which sends signals
that bring about a response. Blood flow shifts, so more
blood from the body’s hot interior flows to the skin.
The shift maximizes the amount of heat given off to
the surrounding air. At the same time, sweat glands
increase their output. Evaporation of sweat helps cool
the body surface. Breathing quickens and deepens,
speeding the transfer of heat from the blood in your
lungs to the air. Hormonal changes make you feel
more sluggish. As your activity level slows and your
rate of heat loss increases, your temperature falls.
Sensory receptors also notify the brain when body
temperature declines. The brain responds by sending signals that divert blood flow away from the skin
and tighten smooth muscles attached to hairs so hairs
stand up. With prolonged cold, the brain commands
skeletal muscles to contract ten to twenty times a second. This shivering increases heat production by muscles. When you warm up, blood returns to your skin,
you stop shivering, and your goose bumps subside.
Through the process of negative feedback, the body
can prevent large variations in external temperature
from causing similarly large changes inside the body.
Negative feedback smooths out variations in body
temperature, ensuring that cells can function properly.
Smooth muscle
in blood vessels
supplying the skin
relax and widen;
more blood flows
to skin, and more
heat radiates to
surrounding air.
Sweat glands
secrete more
sweat, which
cools the body
as it evaporates.
Endocrine glands
that affect general
activity levels
slow secretion
of hormones that
stimulate activity.
How animals work
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Stem Cells—It’s All About Potential (revisited)
Skin cells are relatively easy to grow in culture and are
already in wide use for medical treatments. One currently available product is produced using infant foreskins that were removed during routine circumcisions.
The foreskin (a tissue that covers the tip of the penis)
provides a rich source of keratinocytes and fibroblasts.
These cells are grown in culture with other biological
materials (Figure 31.18). The resulting product is used
to close chronic wounds, help burns heal, and cover
sores on patients with epidermolysis bullosa, a genetic
disorder that causes skin to slough off.
Researchers also have much more ambitious
hopes for cultured fibroblasts. They are working on
ways to make these cells behave like embryonic stem
cells. Nonembryonic cells that have been altered so
they have the properties of embryonic stem cells are
referred to as induced pluripotent stem cells (IPSCs).
The first IPSCs were produced using a virus to
insert genes into fibroblasts. When the inserted genes
were expressed, the resulting proteins caused the
cells to dedifferentiate. However, IPSCs produced in
this manner are considered unsuitable for clinical use
because their genome has been permanently altered
by the gene insertion. The insertion could cause the
cells to behave in unexpected ways and perhaps even
become cancerous.
Researchers needed a way to bring about dedifferentiation without permanently altering the genome.
In late 2010, Derrick Rossi of the Harvard Stem Cell
Institute reported that he and his team had devised
and successfully tested such a method. They had converted fibroblasts to IPSCs by introducing synthetic
modified RNAs (rather than genes). The fibroblasts
took up the RNAs by endocytosis, then translated
them into the proteins that caused dedifferentiation.
Summary
Section 31.1 All cells in an animal body
are derived from stem cells, cells that can
divide or differentiate into a specialized
cell type. The first divisions of a fertilized egg yield totipotent cells that can
form any tissue or develop into a new individual. Later
embryos have pluripotent cells that can still form any
tissue. After birth, cells are less versatile. Researchers
hope to use embryonic stem cells to produce new cells
of types that are not normally replaced in adults. They
are also trying to making adult cells behave like embryonic stem cells.
B When placed over a
wound, the cells produce
growth factors and other
proteins that aid healing.
a Apligraf, a living cellular construct with a two-layered
structure. The top layer is keratinocytes, and the lower
layer is fibroblasts.
Figure 31.18 Cultured skin cells.
Rossi next used the same method to introduce RNAs
that caused the former fibroblasts to begin to differentiate as muscle cells.
Rossi’s method of creating IPSCs removes one
potential barrier to clinical use of these cells, but it
remains too early to know if IPSCs are functionally
equivalent to embryonic stem cells. For now, most
stem cell researchers advocate keeping all options
open by continuing to study both embryonic stem
cells and IPSCs.
How would you vote? An estimated 500,000 preimplantation embryos are now stored in fertility clinics in the
United States. Many will never be implanted in their mother.
They are a potential source of stem cells, or a potential child
for a woman who is willing to carry the embryo to term.
Should parents of stored embryos be able to donate them
for use in embryonic stem cell research?
Section 31.2 Most animals have four types
of tissues organized as organs and organ
systems. Extracellular fluid serves as the
body’s internal environment. In humans,
it consists mainly of interstitial fluid and
plasma. Animal structure has been influenced both by
physical constraints and evolutionary history.
Section 31.3 Epithelial tissue covers the
body surface and lines its internal tubes
and cavities. Epithelial cells have little
extracellular matrix between them. An
epithelium has a free apical surface.
Its basal surface secretes a basement membrane that
CHAPTER 31
animal tissues and organ systems 537
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attaches it to underlying connective tissue. Tight junctions occur only in epithelial cells. The gut is lined by
an epithelium with tight junctions. Gut epithelium
functions as a selective barrier by controlling which
substances move into the body’s internal environment.
Some ciliated epithelial cells move materials across their
surface. Others have microvilli that increase their surface
area for absorption or secretion. Hair, fur, and nails are
keratin-rich remains of specialized epithelial cells.
Gland cells are epithelial cells whose secretions act
outside the cell. Ductless endocrine glands secrete hormones into blood. Exocrine glands secrete products such
as milk or saliva through ducts.
Section 31.4 Connective tissues “connect”
tissues to one another, both functionally
and structurally. Different types bind,
organize, support, strengthen, protect,
and insulate other tissues. All consist of
cells in a secreted matrix. Soft connective tissue underlies skin, holds internal organs in place, and connects
muscle to bone, or bones to one another. All soft connective tissues have the same components (fibroblasts and
a matrix with elastin and collagen fibers) but in different proportions. Loose connective tissue holds internal
organs in place. Ligaments and tendons consist of dense
connective tissue.
Rubbery cartilage and mineral-hardened bone tissue
are components of the skeleton. Fat stored in adipose
tissue is the body’s main energy reservoir. Blood consists of fluid plasma, cells, and platelets. It is considered
a connective tissue because blood cells and platelets
arise from stem cells in bone.
Section 31.5 Muscle tissues contract and
move a body or its parts. Muscle contraction is a response to signals from the nervous system and is fueled by ATP.
Skeletal muscle consists of long fibers
with multiple nuclei and has a striated (striped) appearance. Skeletal muscles, which pull on bones, are under
voluntary control. They have large stores of glycogen.
The metabolic reactions carried out by skeletal muscle
are the main source of body heat.
Cardiac muscle, found only in the heart wall, consists
of branching cells and has a striated appearance. Gap
junctions allow signals to travel fast between the cells.
Smooth muscle tissue is found in the walls of tubular
organs and some blood vessels. Its cells taper at both
ends and are not striated.
Section 31.6 Nervous tissue makes up the
communication lines through the body. It
consists of neurons that send and receive
electrochemical signals, and neuroglial
cells that support them. A neuron has a
central cell body and long cytoplasmic extensions that
send and receive signals. Sensory neurons detect information, interneurons integrate and assess information
about internal and external conditions, and motor neurons command muscles and glands.
538 UNIT VI
Section 31.7 Vertebrates are bilateral and
coelomate and many of their internal
organs reside inside a body cavity derived
from the coelom. An organ system consists of two or more organs that interact
chemically, physically, or both in tasks
that help keep individual cells as well as the whole body
functioning smoothly. All vertebrates have the same set
of organ systems.
Section 31.8 The integumentary system
consists of skin and structures such as
hair that are derived from it. It functions in protection, temperature control,
detection of shifts in external conditions,
vitamin production, and defense against pathogens. The
outermost layer of skin, the epidermis, is a stratified
squamous epithelium consisting mainly of keratinocytes. Melanocytes produce the melanin that gives skin
its color and serves as a natural sunblock. The deeper
dermis consists mainly of dense connective tissue and
contains blood vessels, nerves, and muscles. Underlying
the dermis is the hypodermis, a layer of connective tissue and adipose cells.
Sweat glands and hair follicles are collections of
epidermal cells that descended into the dermis during
development. Compared to our closest related primate
relatives, we have more sweat glands and finer, shorter
body hair. These traits helped our early ancestors in
Africa disperse heat generated by walking and running
under hot conditions. The reduction in body hair was
accompanied by an increase in melanin deposition that
protected the skin against sunlight. Later, when some
humans moved to regions with less sunlight, their skin
color reverted to a lighter state.
Section 31.9 Homeostasis requires sensory receptors
that detect changes, an integrating center (the brain),
and effectors (muscles and glands) that bring about
responses. Negative feedback often plays a role in
homeostasis: A change causes the body to respond in a
way that reverses the change.
Self-Quiz
Answers in Appendix III
1.
tissues are sheetlike with one free surface.
a.Epithelial
c. Nervous
b.Muscle
d.Connective
2.
are found only in epithelial tissue.
a.Tight junctions
c. Gap junctions
b.Adhering junctions
d.all of the above
3. Glands are specialized
a.epithelial
b.muscle
tissue.
c. nervous
d.connective
4. A rubbery secreted matrix of glycoproteins and collagen
surrounds the cells in
.
a.bone
c. adipose tissue
b.cartilage
d.blood
5. Blood cells develop from stem cells in
a.epidermis
c. cartilage
b.dermis
d.bone
.
How animals work
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
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data analysis activities
Cultured skin for healing wounds Diabetes is a disorder
1. What percentage of wounds had healed at 8 weeks when
treated the standard way? When treated with cultured skin?
2. What percentage of wounds had healed at 12 weeks when
treated the standard way? When treated with cultured skin?
3. How early was the healing difference between the control
and treatment groups obvious?
6. Your body’s main energy reservoir is
a.glycogen stored in cardiac muscle
b.lipids stored in adipose tissue
c. starch stored in skeletal muscle
d.phosphorus stored in bone
7. Cytoplasmic extensions of
chemical messages.
a.neuroglial cells
b.neurons
.
send and receive
c. fibroblasts
d.melanocytes
8.
muscle pulls on bones and
muscle
regulates the diameter of blood vessels.
c. Skeletal/smooth
a.Skeletal/cardiac
b.Smooth/cardiac
d.Smooth/skeletal
9. Straps of dense, regular connective tissue
a.connect muscles to bones b. underlie the skin
b.produce blood cells
d.lack fibroblasts
.
10.Microvilli
some epithelial cells.
a.provide pigment to
c. wrap tightly around
b.increase surface area of d.receive signals from
11.Tears are an
secretion released by specialized
tissue cells.
a.endocrine/epithelial
c. exocrine/epithelial
b.endocrine/connective
d.exocrine/connective
12.Cancers most commonly arise in
tissue.
c. nervous
a.epithelial
b.muscle
d.connective
13.Adhering junctions attach cells of
end to end.
c. loose connective tissue
a.cartilage
b.cardiac muscle
d.nervous tissue
14.With negative feedback, detection of a change brings
about a response that
the change.
a.reverses
c. has no effect on
b.accelerates
d.mimics
60
Percent of wounds healed
in which the blood sugar level is not properly controlled.
Among other effects, this disorder reduces blood flow to
the lower legs and feet. As a result, about 3 million diabetes patients have ulcers, or open wounds that do not heal,
on their feet. Each year, about 80,000 require amputations.
Several companies provide cultured cell products
designed to promote the healing of diabetic foot ulcers.
Figure 31.19 shows the results of a clinical experiment
that tested the effect of the cultured skin product shown
in Figure 31.18 versus standard treatment for diabetic foot
wounds. Patients were randomly assigned to either the
experimental treatment group or the control group and
their progress was monitored for 12 weeks.
50
40
standard
treatment
cultured skin
treatment
30
20
10
4 weeks
8 weeks
12 weeks
Figure 31.19 Results of a multicenter study of the effects of standard treatment versus use of a cultured cell product for diabetic foot ulcers. Bars
show the percentage of foot ulcers that had completely healed.
15.Match each term with the most suitable description.
exocrine gland a.signaling cell in nervous tissue
endocrine gland b.secretion through duct
fibroblast
c. collagen-producing cell
melanocyte
d.contraction is involuntary
neuron
e.pigment-producing cell
smooth muscle f. main source of metabolic heat
skeletal muscle g.main cell type in epidermis
blood
h.fluid connective tissue
keratinocyte
i. includes interstitial fluid, lymph
extracelluar fluid j. secretes hormones
Critical Thinking
1. Many people oppose the use of animals for testing the
safety of cosmetics. They argue that alternative test methods
are available, such as the use of lab-grown tissues in some
cases. Given what you learned in this chapter, speculate on
the advantages and disadvantages of tests that use lab-grown
tissues as opposed to living animals.
2. Radiation and chemotherapy drugs preferentially kill
cells that divide frequently, most notably cancer cells. These
cancer treatments also cause hair to fall out. Why?
3. Each level of biological organization has emergent properties that arise from the interaction of its component parts.
For example, cells have a capacity for inheritance that molecules making up the cell do not. What are some emergent
properties of specific types of tissues?
4. The micrograph to the right shows
cells from the lining of an airway leading to the lungs. The gold cells are ciliated and the darker brown ones secrete
mucus. What type of tissue is this? How
can you tell?
CHAPTER 31
animal tissues and organ systems 539
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Appendix III. Answers to Self-Quizzes
Italicized numbers refer to relevant section numbers
Chapter 31
1. a
2. a
3. a
4. b
5. d
6. b
7. b
8. c
9. a
10.b
11.c
12.a
13.b
14.a
15.b
j
c
e
a
d
f
h
g
i
31.3
31.3
31.3
31.4
31.4
31.4
31.6
31.5
31.4
31.3
31.3
31.3
31.5
31.9
31.3
31.3
31.4
31.8
31.6
31.5
31.5
31.4
31.8
31.2
This page contains answers for this chapter only
Appendix III
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Art Credits and Acknowledgments
Fawcett. Page 538 Section 31.4, John Cunningham/ Visuals Unlimited; Section 31.5, Ed Reschke;
Section 31.6, Triarch/ Visuals Unlimited; Section
31.8, © Michael Shore Photography. Page 539
CNRI/ Photo Researchers, Inc.
This page contains credits for this chapter only
CHAPTER 31 Page 522, John D. Cunningham/
Visuals Unlimited; Learning Roadmap, from
top, © Gary Roberts/ worldwidefeatures.com;
© Don W. Fawcett; Ed Reschke; © Michael Shore
Photography. 31.1 left, Used with permission of
University of Wisconsin Board of Regents. 31.2
(B) Ed Reschke; (E) © Yuri Arcurs/ Shutterstock.
com. 31.4 Biophoto Associates/ Photo Researchers, Inc. 31.5 left, From RUSSELL/WOLFE/
HERTZ/STARR. Biology, 1E. © 2008 Brooks/
Cole, a part of Cengage Learning, Inc. Reproduced
by permission. www.cengage.com/permissions;
right from top, Ray Simmons/ Photo Researchers,
Inc.; Ed Reschke/ Peter Arnold, Inc.; © Don W.
Fawcett. 31.6 left, © iStockphoto.com/ Flashon
Studio; right, From RUSSELL/WOLFE/HERTZ/
STARR. Biology, 1E. © 2008 Brooks/Cole, a part
of Cengage Learning, Inc. Reproduced by permission. www.cengage.com/permissions. 31.7
above, (A) John Cunningham/ Visuals Unlimited;
(B–C) Ed Reschke; (D) Photo Researchers, Inc.;
(E) © University of Cincinnati, Raymond Walters College, Biology; (F) Michael Abbey/ Photo
Researchers, Inc.; (G) right, Photo Researchers,
Inc. 31.8 (A–B) Ed Reschke; (C) Biophoto Associates/ Photo Researchers, Inc. 31.9 above, Triarch/
Visuals Unlimited. 31.10 Kim Taylor/ Bruce
Coleman, Ltd. 31.14 left, John D. Cunningham/
Visuals Unlimited. 31.15 © Michael Shore Photography. 31.16 © iStockphoto.com/ Warwick ListerKaye. 31.18 (A–B) Courtesy of © Organogensis,
Inc., www.organogenesis.com. Page 537 Section
31.1, Used with permission of University of Wisconsin Board of Regents; Section 31.3, © Don W.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.