40.1. Types of Tissues (p. 718)
A. Levels of Organization
- Levels of organization are cells, tissues, organs, and organ systems. (Fig.
- Structure and function of each level depends on structure and function of
organ, tissue, and cell type.
B. Four Major Types of Tissue
- Epithelial tissue covers body surfaces and lines body cavities.
- Connective tissue binds and supports body parts.
- Muscular tissue causes body parts to move.
- Nervous tissue responds to stimuli and transmits impulses.
C. Epithelial Tissue
- Epithelial tissue forms a continuous layer over body surfaces including
- There are three types of epithelial tissue. (Fig. 40.2)
- Squamous epithelium is composed of flat cells (e.g. air sac linings
of lungs, walls of capillaries).
- Cuboidal epithelium has cube-shaped cells.
- Columnar epithelium has elongated cells that resemble pillars or
columns (e.g. small intestine).
- Epithelium varies in number of cell layers. (Fig. 40.2)
- Simple epithelium has one cell layer; all cells contact basement
- Pseudo stratified epithelium appears layered; actually, all cells
contact basement membrane.
- Stratified epithelium is composed of more than one layer of cells.
- Epithelial cells can have cilia. (Fig. 40.2)
- Ciliated epithelium cells are covered with cilia (e.g., lining
of human respiratory tract).
- Cilia can bend and move material over the surface of the epithelium.
- Secretory epithelia can be unicellular or have multicellular glands.
- Glands are a single cell or a group of cells that secrete products
into the lumen of or onto the lining of a tube or cavity, into blood, or
to outside of the body; they are classified in two types:
- Exocrine glands secrete their products into ducts or directly
into a tube or cavity.
- Endocrine glands secrete their product directly into the bloodstream.
- Epithelium forms outer layer of skin of animals; it may produce an outer,
nonliving protective cuticle.
- Epithelial tissue cells are packed tightly; they join to one another in
one of three ways:
- Tight junctions have plasma proteins extending between neighboring
cells; bind cells tightly.
- Adhesion junctions have cytoskeletal elements joining internal
plaques in neighboring cells.
- Gap junctions form when two identical plasma membrane channels
of neighboring cells join; ions and small molecules pass between cells.
D. Connective Tissue
- Connective tissue binds structures together, fills spaces, stores
fat, and forms blood cells.
- Connective tissue provides source cells for muscle and skeletal cells in
animals that regenerate parts.
- Connective tissue cells are separated widely by a matrix, a noncellular
material between cells.
- Loose Fibrous and Dense Fibrous Connective Tissue
- Cells of loose and fibrous connective tissues are fibroblasts.
- Fibroblasts space apart separated by jelly matrix of white collagen fibers
and yellow elastic fibers.
- Collagen fibers provide flexibility and strength; elastic fibers provide
- Loose fibrous connective tissue supports epithelium and provides
support, flexibility, and protective covering encasing internal organs.
- Dense fibrous connective tissue contains closely packed collagenous
fibers; found in tendons, which attach muscles to bone, and ligaments,
which bind bones to other bones at joints.
- Adipose Tissue and Reticular Connective Tissue
- Adipose Tissue (Fig. 40.3b)
- This is loose connective tissue that insulates, provides protective padding,
and stores fat.
- In mammals, it is beneath skin, around kidneys, and on surface of heart.
- Reticular Connective Tissue
- It is present in lymph nodes, spleen, and bone marrow.
- Reticular fibers associated with reticular cells resembling fibroblasts
supports free blood cells.
- Cartilage and Bone
- Cartilage and bone are rigid connective tissues.
- Structural proteins (cartilage) or calcium salts (bone) are deposited in
- Cartilage cells lie in small chambers or lacunae embedded
in a strong, flexible matrix. (Fig. 40.3c)
- In some animals, such as sharks and rays, entire skeleton is cartilage.
- Human fetal skeleton is entirely cartilage but is gradually replaced by bone.
- Various types of cartilage are classified by type of collagen and elastic
fiber found in matrix.
- Cartilage is at end of long bones, human nose, framework of human ear, in
walls of respiratory ducts, and within intervertebral discs.
- In bone, matrix of calcium salts is deposited around protein fibers.
- Calcium salts give bone rigidity; protein fibers provide elasticity and
- Compact bone cells (osteocytes) lie within lacunae arranged
in concentric circles within osteons (Haversian systems) around
tiny tubes called central canals.
- Canals contain nerve fibers and blood vessels.
- Nutrients brought by blood reach all of cells via minute canals (canaculi)
containing thin processes of osteocytes that connect them with one another
and with central canals.
- Spongy bone at end of long bones is designed for strength, has many
long bony bars and plates.
- Blood transports nutrients and oxygen to cells and removes CO2
and wastes; also has role in fluid, ion and pH balance.
- Blood is connective tissue with cells separated by liquid plasma.
- In vertebrates, the blood cells are mainly of two types. (Fig. 40.4)
- Red blood cells (erythrocytes) carry oxygen.
- White blood cells (leukocytes) aid in fighting infection.
- Platelets present in plasma are fragments of giant cells found
in bone marrow; and play a role in blood clotting.
- Unlike other connective tissues, intercellular matrix of blood (i.e., plasma)
is not made by cells; instead, plasma is a mixture of molecules that enter
blood at various locations. (Table 40.1)
F. Muscular Tissue (Fig. 40.5a)
- Muscular (contractile) tissue is composed of cells called muscle
- Muscle fibers contain actin and myosin filaments; interactions result
in animal movement.
- Three types of vertebrate muscle tissue are: skeletal, cardiac, and smooth.
- Skeletal muscle attaches by tendons to bones of skeleton.
- Skeletal muscle moves body parts, is under voluntary control, and contracts
faster than other types.
- Skeletal muscle fibers are long, cylindrical, multinucleate cells arising
from fusion of several cells.
- Skeletal fibers are striated due to light and dark bands of overlapping
actin and myosin filaments.
- Smooth (visceral) muscle is not striated.
- Spindle-shaped fibers form layers with the thick middle portion of one
fiber opposite the thin ends of adjacent fibers. (Fig. 40.5b)
- Nuclei form an irregular pattern in the tissue.
- Smooth muscle is not under voluntary control; it is therefore involuntary.
- Smooth muscle is found in walls of viscera (e.g. intestine, stomach, etc.)
and blood vessels.
- Smooth muscles drive intestinal contractions and blood vessel constrictions.
- Cardiac muscle is found only in heart wall and powers the heartbeat
that pumps blood.
- Cardiac muscle combines features of smooth and skeletal muscle. (Fig.
- Unlike skeletal muscles with many nuclei, cardiac muscles have one centrally
- Although it appears to be one mass of muscle fibers, cardiac muscle fibers
are individual cells.
- Cardiac muscle cells are bound end-to-end at intercalated disks where
folded membranes between two fibers contain desmosomes and gap junctions
- Impulses move from cell to cell so heartbeat is coordinated.
G. Nervous Tissue
- Nervous tissue is composed of neurons in the brain, spinal
cord and nerves.
- Neurons have three parts. (Fig. 40.6)
- Dendrites receive a stimulus and conduct signals to cell body.
- Cell body contains most cytoplasm and nucleus of the neuron.
- Axon conducts nerve impulses away from cell body.
- Long axons and dendrites form neuron fibers; bound together by connective
tissue, they form nerves.
- Neurons detect stimuli and conduct signals to brain or spinal cord; nerves
lead to muscles or glands.
- There are several types of neuroglial cells in CNS. (Fig. 40.6)
- Neuroglial cells outnumber neurons nine to one; once thought to only support
or nourish neurons.
- Oligodendrocytes form myelin around an axon.
- Microglial cells also phagocytize bacterial and cellular debris.
- Astrocytes provide nutrients and produce a growth factor known as glial-derived
growth factor (GDGF) that may be used to cure diseases of neural degeneration.
- They lack long processes but communicate among themselves and with neurons.
40.2. Organs and Organ Systems (p. 724)
A. Organs are combinations of two or more different tissues performing
- Organ systems are many different organs performing common functions.
- Skin is considered an integumentary system since it cannot be placed
in another system; it is a system composed of skin and accessory organs (i.e.,
nails, hair, glands, and sensory receptors).
B. Skin as an Organ
- Human skin protects underlying tissues from trauma, desiccation, radiation
damage, and microbial invasion.
- Skin produces a precursor molecule that is converted to vitamin D after
exposure to UV light.
- Skin helps regulate body temperature.
- Laden with sensory receptors, skin collects information about external environment.
- Skin has an outer epidermal layer (epidermis) and a deeper layer
(dermis). (Fig. 40.7)
C. Regions of Skin
- Epidermis is the outer, thinner layer of skin.
- Epidermis is composed of stratified squamous epithelium.
- Cells are derived from a basal layer of stem cells that undergo continuous
- Newly formed cells push to the surface away from their blood supply; they
flatten and harden as they accumulate keratin, a hard, waterproof
- Eventually, keratinized cells die and are sloughed off.
- Melanocytes located in basal layer produce melanin pigment
that absorbs UV light, protecting cells from radiation damage.
- Nails grow from special epidermal cells at base of the nail in a region
called the nail root.
- Visible portion of a nail is the nail body.
- Cells become keratinized as they grow out over the nail bed.
- Vascular dermal tissue under nail provides pink color; white half-moon
area is thicker germinal area.
- Dermis is the deeper and thicker layer of skin.
- Dermis contains elastic fibers and collagen fibers; these run parallel
with skin surface.
- Hair follicle contains a nonliving hair shaft and living
hair root that produced it.
- Hair shaft is formed of dead, keratinized epidermal cells that protect
the surface of skin.
- Arrector pili muscle is smooth muscle attached to hair follicle;
contracting causes hair to erect.
- Follicles have sebaceous glands producing sebum, an oil
secreted to lubricate hair and skin.
- Sweat (sudoriferous) glands are coiled tubules present in most
regions of skin that secrete a fluid (sweat) onto the surface of skin.
- Many small receptors are present in dermis.
- There are separate receptors for pressure, touch, temperature, and pain.
- Pressure receptors have onion-like sense organs buried deep in dermis
and around joints.
- In cats, Pacinian corpuscles are concentrated in paws, leg joints, and
- Closely related sensors in tongue of woodpeckers help them find insects
in tree bark.
- Touch receptors are flat and oval shaped: concentrated in fingertips,
palms, lips, tongue, nipples, penis, and clitoris.
- Heat and cold sense organs are encapsulated inside sheaths of connective
- Nerve fibers branch out through skin; free nerve endings are pain receptors.
- Dermis contains blood vessels that constrict (you turn pale) and dilate
- Subcutaneous layer lies below dermis.
- It is composed of loose connective tissue, including adipose tissue.
- Adipose tissue helps insulate the body by minimizing both heat gain and
- This layer gives a rounded appearance to the body.
- Excessive development of adipose tissue occurs with obesity.
- Skin Cancer
- There has been an increase in persons with skin cancer due to sunbathing
and use of tanning machines.
- Excessive exposure to UV radiation can convert cells in basal layer of epidermis
to cancer cells.
D. Organ Systems (p. 726).
- Individual organs function as part of organ systems.
- Organ systems carry out life processes..
- Body Cavities
- Human body has two main cavities: dorsal cavity with brain and spinal
cord, and larger ventral cavity. (Fig. 40.8)
- Ventral cavity located on front side of body develops from coelom
and is divided by muscular diaphragm in humans and other mammals.
- Thoracic (chest) cavity is located in upper part of the ventral cavity,
above a muscular
diaphragm, and contains heart and lungs.
- Abdominal cavity is located in lower part of ventral cavity, below
a muscular diaphragm, and
contains major portions of digestive and excretory systems, and much of reproductive
40.3. Homeostasis (p. 728)
A. Cells of the body live in an internal environment, tissue fluid that
bathes the cells of an animal's body.
- This concept was first proposed by Claude Bernard, a famous French physiologist
- Quality of internal environment (e.g., composition and temperature) must
stay within normal range.
- Relative internal stability allows animals to tolerate considerable external
- American physiologist Walter Cannon first used term: homeostasis.
B. Homeostasis is maintenance of internal conditions in a cell or organism
by means of self-regulating mechanisms that curtail fluctuations above and below
a normal range.
- Most organ systems of human body contribute to homeostasis.
- Respiratory system adds oxygen, removes carbon dioxide; amounts are increased
to meet needs.
- Liver removes and stores glucose as glycogen, then replaces blood glucose
levels when low.
- Hormone insulin is secreted by pancreas to regulate glucose levels.
- Kidneys are under hormonal control to excrete wastes and salts and to
maintain blood pH.
- Although homeostasis is controlled by hormones, it is ultimately controlled
by nervous system.
- Brain contains centers that regulate temperature and blood pressure.
- Regulation requires a receptor that detects unacceptable levels and signals
a regulator center that can direct an adaptive response; once normalcy is
obtained, receptor is no longer stimulated.
- Negative feedback mechanism involves a response in which output is
counter to and cancels input, decreasing likelihood of a response. (Fig. 40.9)
- A house thermostat is an analogy.
- Negative feedback causes heater or air conditioner to maintain temperature
within narrow limits.
- Positive feedback mechanism involves output that intensifies and
increases the input, thereby increasing likelihood of a response.
- Once childbirth begins, each event makes the process continue until completion.
- Sequences in blood clotting likewise progress to form a blood clot.
C. Regulation of Body Temperature (Fig. 40.10)
- Regulator center for body temperature is located in hypothalamus, a part
of the brain.
- When body temperature of blood falls below normal, a regulator center directs
smooth muscles of the blood vessels in skin to constrict, which reduces blood
flow to peripheral tissues, and thereby reduces loss of heat to external environment.
- In hairy animals, arrector pili muscles pull hairs erect forming a thicker
- If temperature falls even lower, regulator center sends nerve impulses to
skeletal muscles, initiating shivering to generate heat.
- If body temperature is too warm, regulator center directs skin blood vessels
to dilate, which increases blood flow to peripheral tissues and increases
- Regulator center activates sweat glands, increasing sweat production and
increasing evaporative cooling.