Dermis

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== The Dermis ==

The dermis is located beneath the epidermis and is the thickest of the three layers of the skin (1.5 to 4 mm thick), making up approximately 90 percent of the thickness of the skin. The main functions of the dermis are to regulate temperature and to supply the epidermis with nutrient-saturated blood. Much of the body's water supply is stored within the dermis. This layer contains most of the skins' specialized cells and structures, including:

Blood Vessels
The blood vessels supply nutrients and oxygen to the skin and take away cell waste and cell products. The blood vessels also transport the vitamin D produced in the skin back to the rest of the body.

Lymph Vessels
The lymph vessels bathe the tissues of the skin with lymph, a milky substance that contains the infection-fighting cells of the immune system. These cells work to destroy any infection or invading organisms as the lymph circulates to the lymph nodes.

Hair Follicles
The hair follicle is a tube-shaped sheath that surrounds the part of the hair that is under the skin and nourishes the hair.

Sweat Glands
The average person has about 3 million sweat glands. Sweat glands are classified according to two types:

  1. Apocrine glands are specialized sweat glands that can be found only in the armpits and pubic region. These glands secrete a milky sweat that encourages the growth of the bacteria responsible for body odor.
  2. Eccrine glands are the true sweat glands. Found over the entire body, these glands regulate body temperature by bringing water via the pores to the surface of the skin, where it evaporates and reduces skin temperature. These glands can produce up to two liters of sweat an hour, however, they secrete mostly water, which doesn't encourage the growth of odor-producing bacteria.

Sebaceous glands
Sebaceous, or oil, glands, are attached to hair follicles and can be found everywhere on the body except for the palms of the hands and the soles of the feet. These glands secrete oil that helps keep the skin smooth and supple. The oil also helps keep skin waterproof and protects against an overgrowth of bacteria and fungi on the skin.

Nerve Endings
The dermis layer also contains pain and touch receptors that transmit sensations of pain, itch, pressure and information regarding temperature to the brain for interpretation. If necessary, shivering (involuntary contraction and relaxation of muscles) is triggered, generating body heat.

Collagen and Elastin
The dermis is held together by a protein called collagen, made by fibroblasts. Fibroblasts are skin cells that give the skin its strength and resilience. Collagen is a tough, insoluble protein found throughout the body in the connective tissues that hold muscles and organs in place.

In the skin, collagen supports the epidermis, lending it its durability. Elastin, a similar protein, is the substance that allows the skin to spring back into place when stretched and keeps the skin flexible.

The dermis layer is made up of two sublayers:

The Papillary Layer

The upper, papillary layer, contains a thin arrangement of collagen fibers. The papillary layer supplies nutrients to select layers of the epidermis and regulates temperature. Both of these functions are accomplished with a thin, extensive vascular system that operates similarly to other vascular systems in the body. Constriction and expansion control the amount of blood that flows through the skin and dictate whether body heat is dispelled when the skin is hot or conserved when it is cold.

The Reticular Layer

The lower, reticular layer, is thicker and made of thick collagen fibers that are arranged in parallel to the surface of the skin. The reticular layer is denser than the papillary dermis, and it strengthens the skin, providing structure and elasticity. It also supports other components of the skin, such as hair follicles, sweat glands, and sebaceous glands.

The Subcutis

The subcutis is the innermost layer of the skin, and consists of a network of fat and collagen cells. The subcutis is also known as the hypodermis or subcutaneous layer, and functions as both an insulator, conserving the body's heat, and as a shock-absorber, protecting the inner organs. It also stores fat as an energy reserve for the body. The blood vessels, nerves, lymph vessels, and hair follicles also cross through this layer. The thickness of the subcutis layer varies throughout the body and from person to person.

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