Page 37 - 84 human physiolofy part-1

The Integumentary System

Layers

The skin has two major layer which
are made of different tissues and have
very different functions.

Skin is composed of the epidermis
and the dermis. Below these layers lies
the hypodermis or subcutaneous
adipose layer, which is not usually
classified as a layer of skin.

The outermost epidermis consists
of stratified squamous keratinizing
epithelium with an underlying basement
membrane. It contains no blood vessels,
and is nourished by diffusion from the
dermis. The main type of cells which
make up the epidermis are
keratinocytes, with melanocytes and
Langerhans cells also present. The
epidermis can be further subdivided into
the following strata (beginning with the
outermost layer): corneum, lucidum,
granulosum, spinosum, basale. Cells are Diagram of the layers of human skin
formed through mitosis at the innermost
layers. They move up the strata changing shape and composition as they differentiate, inducing
expression of new types of keratin genes. They eventually reach the corneum and become sloughed off
(desquamation). This process is called keratinization and takes place within about 30 days. This layer
of skin is responsible for keeping water in the body and keeping other harmful chemicals and
pathogens out.

Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule.
Arterial shunt vessels may bypass the network in ears, the nose and fingertips.

The dermis lies below the epidermis and contains a number of structures including blood vessels,
nerves, hair follicles, smooth muscle, glands and lymphatic tissue. It consists of loose connective tissue
otherwise called areolar connective tissue - collagen, elastin and reticular fibers are present. Erector
muscles, attached between the hair papilla and epidermis, can contract, resulting in the hair fiber pulled
upright and consequentially goose bumps. The main cell types are fibroblasts, adipocytes (fat storage)
and macrophages. Sebaceous glands are exocrine glands which produce, a mixture of lipids and waxy
substances: lubrication, water-proofing, softening and antibactericidal actions are among the many
functions of sebum. Sweat Glands open up via a duct onto the skin by a pore.

The dermis is made of an irregular type of fibrous connective tissue consisting of collagen and
elastin fibers. It can be split into the papillary and reticular layers. The papillary layer is outermost and
extends into the epidermis to supply it with vessels. It is composed of loosely arranged fibers. Papillary
ridges make up the lines of the hands giving us fingerprints. The reticular layer is more dense and is
continuous with the hypodermis. It contains the bulk of the structures (such as sweat glands). The

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