Body Tissues and Membranes
Tissue
is a group of cells that have similar structure and that function together as a
unit. A nonliving material, called the intercellular matrix, fills the spaces
between the cells. This may be abundant in some tissues and minimal in others.
The intercellular matrix may contain special substances such as salts and
fibers that are unique to a specific tissue and gives that tissue distinctive
characteristics. There are four main tissue types in the body: epithelial,
connective, muscle, and nervous. Each is designed for specific functions.
Epithelial tissue has cells that are tightly
packed, with little space between them. This tissue protects the body from
drying out, injury, and bacterial invasion. In the resperatory tract,
epithelial tissue sweeps up impurities by means of cilia. Along the digestive
tract, it secretes mucus, which protects the lining from enzymes. Epithelial
cells readily divide to produce new cells that replace lost or damaged ones. Skin
cells as well as those that line the stomach and intestines are being replaced.
Epithelial tissues are classified by the shape of the cells and the number of
cell layers. Simple epithelial tissue is composed of a single layer, and
stratified epithelial tissue is composed of two or more layers. Squamous
epithelium has flattened cells; cuboidal epithelium has cube-shaped cells and
columnar epithelium has elongated cells.
Squamous
Epithelium
Simple squamous epithelium is composed of a single layer of
flattened cells, and that means that its protective function is not as
significant as that of other epithelial tissues. It is found in areas where
secretion absorption and filtration occur. An example is when simple squamous
epithelium lines the lungs where oxygen and carbon dioxide are exchanged, and
it lines the walls of capillaries, where nutrients and wastes are exchanged.
Stratified
squamous
epithelium has many cell layers and does play a protective role. While the
deeper cells may be cuboidal, the outer layer is composed of squamous-shaped
cells. The outer part of skin is stratified squamous epithelium. New cells
produced in a basal state layer become reinforced by keratin, which is a
protein that provides strength as they move to the skins surface. Stratified
squamous is found in the lining of the various orifices of the body.
Cuboidal
Epithelium
Simple
Columnar epithelium
has a single layer of cube-shaped cells attached to a basement membrane. This
type of epithelium is commonly found in glands, such as salivary glands, the
thyroid gland, and the pancreas, where its function is secretion. It also
covers the ovaries and lines most of the kidney tubules. In one part of the
kidney tubule, it absorbs substances from the tubule, and in another part it
secretes substances into the tubule. They are most likely found lining the
larger ducts of certain glands, such as the mammary glands and the salivary
glands. Often this tissue has only two layers.
Columnar
Epithelium
Simple
columnar
epithelium has cells that are longer than they are wide. They are made to
perform particular functions. Some of these cells are goblet cells that secrete
mucus onto the free surface of the epithelium. This tissue is known for lining
digestive organs, including the small intestine. It also lines the uterine
tubes. Stratified columnar epithelium is not very common but does exist in
parts of the pharynx and the male urethra.
Pseudostratified
Columnar Epithelium
Pseudostratified
columnar
epithelium is named like that because it appears to be layered. However, true
layers do not exist because each cell touched the basement membrane.
Pseudostratified ciliated columnar lines parts of the reproductive tract as
well as the air passages of the respiratory system, including the nasal
cavities and the trachea and its branches.
Transitional
Epithelium
The
term epithelium implies changeability, and this tissue changes in response to
tension. It forms the lining of the urinary bladder, the ureters, and part of
the urethra-organs that may need to stretch. The cells in the bladder are able
to slide in relation to one another while at the same time forming a barrier
that prevents any part of urine for diffusing.
Connective
Tissue
Connective
tissue binds structures together, provides support and protection, fill spaces,
produces blood cells, and stores fat. Connective tissue cells are widely
separated by an extracellular matrix composed of an organic ground substance that contains fibers and
varies in consistency from solid to semifluid to fluid. The fibers within the
matrix are of three types. They are collagen, elastin, and reticular fibers.
Fibrous
Connective Tissue
Fibrous
connective tissue includes loose connective tissue and dense connective tissue.
The bodys membranes are composed of an epithelium and fibrous connective
tissue. Loose connective tissue commonly lies between other tissues or between
organs, binding them together. Dense connective tissue has a matrix produced by
fobroblasts that have bundles of white collagenous fibers.
Cartilage
Cartilage
have cells which lie in small chambers called lacunae, which are separated by a
matrix that is solid yet flexible. The tissue lacks a direct blood supply,
therefore it heals very slowly. There are three types of cartilage that are
classified according to the type of fiber in the matrix. Hyline cartilage is the most common, and
contains fibers,and has a glossy appearence.
Bone
Bone
is the most rigid of the connective tissues. It has a very hard matrix of
mineral salts. The minerals give the bone its ridgity texture. There are two
different kinds of bones. Theres the compact bone, which consists of many
cylindrical-shaped units. Then, theres the spongy bone, which contains numerous
bony bars and plates called trabeculae.
Blood
Blood
is a connective tissue composed of cells suspended in a liquid matrix called
plasma. Blood cells have two types. The red blood cells, and the white blood
cells. Platlets also are important, because they keep the blood from clotting.
Muscular
tissue is composed of cells called muscle fibers. Muscle fibers contain actin
and myosin,which are protein filaments whose interaction accounts for movement.
There are also three types of vertebrate muscles. 
Skeletal
Muscle
Skeletal
muscle is also called voluntary muscle. It is also attached by tendons to the
bones of the skeleton. The skeletal muscle is found everythere in the body.
Smooth
Muscle
Smooth
muscle is called this because the arangement of actin and myosin does not give
the appearance of cross-striations. Smooth muscles are found in the intestines,
stomach, uterus, urinary bladder, and the blood vessels.
Cardiac
Muscle
Cardiac
muscle is only found in the walls of the heart. Its contraction pumps blood and
accounts for the heartbeat. They also have a single centrally placed nucleus.
Nervous
Tissue
Nervous
tissue is found in the brain and spinal cord, which contains specialized cells
called neurons that conduct nerve impulses. A neuron has three parts. First,
theres the dendrite, which collects signals that may result in a nerve impulse.
Second, theres the cell body, which contains the nucleus and most of the
cytoplasm. Last, theres the axon, which conducts nerve impulses. 
á Some of the risks and dangers could be if you were
building a roof for a house, and you step on a nail with your foot. You would
have to go to a doctor and get a tetnis shot to prevent infection.
á If you were playing basketball for a sport, or just
for fun and you tear a ligament, then you would have to undergo surgery to try
to fix or heal your ligament.
á If you would go and get a nipple ring, it could cause
a serious infection, and you could have to go and get some medication for it to
help it heal.