Click here if you are looking for a certified online tutor or home tutor
Homeostasis
A state of equilibrium or a tendency to reach equilibrium, either metabolically
within a cell or organism or socially and psychologically within an individual
or group, in short it is the maintenance of constant internal environment of the body. Homeostasis, in biology, overall
term for the tendency of biological systems to maintain a state of equilibrium.
This tendency ranges from systems of internal balance in individual organisms to
ecological patterns of balance in a community of organisms, as between numbers
of predators and prey. The concept was first advanced in the 19th century by the
French physiologist Claude Bernard, and was given its name by the
American physiologist Walter B. Cannon. Examples of homeostasis include the
body's self-regulation of hormone and acid-base levels, the
composition of body fluids, and cell growth and body temperatures.
On a broad scale, the world community of organisms tends to maintain some degree
of evolving balance when not seriously disturbed. The so-called Gaia hypothesis
of the earth as a living organism, which gained great popularity in
the 1980s, may in some ways be regarded simply as an extension of homeostasis
concepts.
Negative Feedback
Normally the levels of adrenocorticotropic hormone
(ACTH) in the bloodstream control the amount of glucocorticoids
secreted by the adrenal cortex. ACTH is secreted by the pituitary
gland in response to the body's demand for glucocorticoids.
Glucocorticoids already in the blood slow down the secretion of ACTH so that
when glucocorticoid concentrations are at a suitable level ACTH production
stops. This mechanism is known as a negative feedback loop.
The excess of glucocorticoids that causes Cushing's
syndrome can result in a number of ways. For example, it may result from a
tumor in the pituitary gland. A pituitary tumor is not affected by
negative feedback and secretes large amounts of ACTH even when there are already
high levels of glucocorticoids in the blood. The ACTH stimulates the adrenal
cortex to make glucocorticoids, resulting in an increase in glucocorticoid
concentrations. High levels of ACTH can also result from a tumor outside the
pituitary gland. For example, an oat-cell tumor in the lung secretes ACTH, which
in turn triggers an increase in the secretion of glucocorticoids.
Skin System
 |
Structure of the Skin
The skin consists of an outer,
protective layer (epidermis) and an inner, living layer (dermis). The top layer
of the epidermis is composed of dead cells containing keratin, the horny protein
that also makes up hair and nails.
The skin is made up of two layers, the epidermis and the dermis. The epidermis, the upper or outer layer of the skin, is a tough, waterproof, protective layer. The dermis, or inner layer, is thicker than the epidermis and gives the skin its strength and elasticity. The two layers of the skin are anchored to one another by a thin but complex layer of tissue, known as the basement membrane. This tissue is composed of a series of elaborately interconnecting molecules that act as ropes and grappling hooks to hold the skin together. Below the dermis is the subcutaneous layer, a layer of tissue composed of protein fibers and adipose tissue (fat). Although not part of the skin itself, the subcutaneous layer contains glands and other skin structures, as well as sensory receptors involved in the sense of touch. |
Epidermis
About 90 percent of the cells in the
epidermis are keratinocytes, named because they produce a tough, fibrous protein
called keratin. This protein is the main structural protein of the epidermis,
and it provides many of the skin’s protective properties. Keratinocytes in the
epidermis are arranged in layers, with the youngest cells in the lower layers
and the oldest cells in the upper layers. The old keratinocytes at the surface
of the skin constantly slough off. Meanwhile, cells in the lower layers of the
epidermis divide continually, producing new keratinocytes to replace those that
have sloughed off. As keratinocytes push up through the layers of the epidermis,
they age and, in the process, produce keratin. By the time the cells reach the
uppermost layer of the epidermis, they are dead and completely filled with the
tough protein. Healthy epidermis replaces itself in a neatly orchestrated way
every month.
Scattered among the keratinocytes in the
epidermis are melanocytes, cells that produce a dark pigment called melanin.
This pigment gives color to the skin and protects it from the sun’s ultraviolet
rays. After being produced in the melanocytes, packets of melanin called
melanosomes transfer to the keratinocytes. There they are arranged to protect
the deoxyribonucleic acid (DNA), or genetic material, of the keratinocytes.
All people have roughly the same number of
melanocytes. Differences in skin color, such as that between light-skinned
people of European descent and dark-skinned people of African descent, result
from differences in the amount of melanin produced and how melanosomes are
arranged in the keratinocytes. Particularly in people with light skin, melanin
sometimes accumulates in patches, forming freckles, age spots, or liver spots.
In people of almost any skin tone, exposure
to the sun causes melanocytes to increase their production of melanin to help
protect the skin. This process results in a darkening of the skin tone to form a
suntan. The suntan fades when keratinocytes containing the extra melanin are
sloughed off. A type of melanin called pheomelanin makes redheaded people more
sensitive to the sun. A total lack of melanin, a genetic condition called
albinism, makes people extremely sensitive to the sun. People with albinism can
be members of any race and have very light skin, hair, and eyes.
The epidermis also contains a type of
immune cell known as a Langerhans cell. Produced in the bone marrow, Langerhans
cells take up sentrylike positions in the epidermis, where they help cells of
the immune system recognize potentially dangerous microorganisms and
chemicals.
Another cell in the epidermis is the Merkel
cell, found in sensitive, hairless areas such as the fingertips and lips.
Located in the deepest layer of the epidermis, Merkel cells contact nerve
endings in the dermis below and function as a type of touch receptor.
Dermis
Unlike the epidermis, the dermis or lower
layer of the skin is richly supplied with blood vessels and sensory nerve
endings. The dermis also contains relatively few cells compared to the
epidermis—instead, it is made up mainly of fibrous proteins and other large
molecules.
The main structural component of the dermis
is a protein called collagen. Bundles of collagen molecules pack together
throughout the dermis, accounting for three-fourths of the dry weight of skin.
Collagen is also responsible for the skin’s strength. Another protein in the
dermis, elastin, is the main component of elastic fibers. These protein bundles
give skin its elasticity—the ability to return to its original shape after
stretching. Collagen and elastin are produced by cells called fibroblasts, which
are found scattered throughout the dermis.
The upper part of the dermis is known as
the papillary layer. It is characterized by dermal papillae, tiny, fingerlike
projections of tissue that indent into the epidermis above. In the thick skin on
the palms and soles, the epidermis conforms to the shape of the underlying
dermal papillae, forming ridges and valleys that we know as fingerprints. These
ridges provide traction that helps people grasp objects and surfaces.
Some dermal papillae contain touch
receptors called Meissner’s corpuscles, and many contain loops of tiny blood
vessels. The extensive network of blood vessels in the dermal papillae plays an
important role in the regulation of body temperature. The blood vessels dilate
in hot environments to help dissipate heat, and they constrict to conserve heat
in cold environments. Approximately one-fourth of the body’s blood flows through
the skin at any given time.
 |
| Shunt Vessels on the under the skins during hot , loses much heat and during cold it loses less heat so that body temperature cant fall below 37 C. |
The lower layer of the dermis is called the
reticular layer. It is made primarily of coarse collagen and elastic fibers.
Skin appendages such as glands and hair follicles are often anchored in the
reticular layer of the dermis. The reticular layer also contains several
different types of sensory receptors, nerve cells specialized to detect various
stimuli, including pain, heat, cold, itch, and pressure. For example, Pacinian
corpuscles are receptors found in the deep dermis of weight-bearing surfaces,
such as the soles of the feet. They are composed of concentric layers of cells,
much like an onion, and transmit vibrational stimuli. Sensory receptors are more
dense in hairless areas, such as the fingertips and lips, making these areas
especially sensitive.
