expLore on your oWn
section 4.8 Different tissues combine
to form an organ. Body organs are located
in five major cavities: the cranial cavity
(brain); spinal cavity (spinal cord); thoracic
cavity (heart and lungs); abdominal cavity
(stomach, liver, most of the intestine, other
organs); and pelvic cavity (reproductive organs, bladder,
rectum). The various organs in the body are arranged into
eleven organ systems. In an organ system, two or more organs
interact in ways that contribute to the body’s survival. Each
system performs a specific function, such as transporting blood
(cardiovascular system) or reproduction.
section 4.9 An example of an
organ system is the integument, or
skin. Skin has an outer epidermis and
an underlying dermis. Most epidermal
cells are keratinocytes, which make
the protein keratin. Keratin makes the
skin’s outer layer tough and waterproof.
Melanocytes in the epidermis produce
pigment that gives skin its color. Hair, nails, sweat glands, and
oil glands are derived from the epidermis.
Skin protects the rest of the body from abrasion, invading
bacteria, ultraviolet radiation, and dehydration. It helps con-
trol internal temperature, contains cells that synthesize vitamin
D, and serves as a blood reservoir for the rest of the body.
Receptors in skin are essential for detecting environmental
stimuli.
section 4.10 Extracellular fluid (blood
and tissue fluid) is the body’s internal
environment. Tissues, organs, and organ
systems work together to maintain the stable
state of homeo stasis in this environment.
Maintaining homeostasis requires sensory
receptors, which can detect a stimulus,
integrators that interpret it, and effectors that carry out a
response. In negative feedback, a change in a condition triggers
a response that reverses the change. Negative feedback is the
main control mechanism of homeostasis.
section 4.11 Physiological responses
that govern temperature rely on negative
feedback controls that respond to heat stress
and cold stress.suMMary
section 4.1 A tissue is a group of
similar cells that perform the same function
(Table 4.4). Epithelial tissue covers body
surfaces and lines internal cavities. Each
kind of epithelium has one surface exposed
to body fluids or the outside environment;
the opposite surface rests on a basement
membrane between it and underlying tissue.
Glands are derived from epithelium. Exocrine glands
release substances (such as saliva and tears) onto the surface
of an epithelium through ducts or tubes. Endocrine glands
secrete hormones directly into extracellular fluid.
section 4.2 Connective tissues bind,
support, strengthen, and protect other
tissues. Most have fibers of structural
proteins (especially collagen), fibroblasts,
and other cells within a matrix. They include
fibrous connective tissue and specialized
connective tissues such as cartilage, bone,
adipose tissue, and blood.
section 4.3 Muscle tissue contracts. It
helps move the body or its parts. The three
types of muscle tissue are skeletal muscle,
smooth muscle, and cardiac muscle.
section 4.4 Nervous tissue receives
and integrates information from inside
and outside the body and sends signals for
responses. Neurons and the support cells
called neuroglia are the main cells in nervous
tissue.
section 4.6 Tight junctions help prevent
substances from leaking across a tissue.
Adhering junctions bind cells together in
tissues. Gap junctions link the cytoplasm of
neighboring cells.
section 4.7 Membranes cover all body
surfaces and cavities. Those made of
epithelium include mucous and serous
membranes. Connective tissue membranes
include the synovial membranes of certain
joints. The skin is a cutaneous membrane.CorbisAs epithelium, your skin contains fibers of collagen and elastin. These structural proteins
have different properties that you can see in action when you pull on a patch of skin. Notice that even
if you pull firmly, the skin doesn’t tear. Which type of protein fiber gives the skin that tensile strength?
Which type returns the skin to its original shape when you let go?
84 Chapter 4
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