ANATOMY & PHYSIOLOGY

The skin is the body's largest organ. It covers and protects everything inside the human body. Skin holds everything together. There are three main layers to the skin:

1. The Epidermis

2. The Dermis

3. The Subcutaneous Layer

Skin-Anatomy.jpg

The anatomy of the skin

THE EPIDERMIS

The epidermis is the part of the skin that you can see. Even though it looks quite inert, the epidermis is continually hard at work. At the bottom of the epidermis, new skin cells are forming. The skin cells germinate in the bottom layer and work their way up to the top of the epidermis. This area is known as the corneum (cornea being the name for the top surface, like the cornea of the eye).

This process takes from two to four weeks and is known as keratinisation. As newer cells continue to move up, older cells near the top die and rise to the surface of your skin. Once a skin cell has reached the surface of the epidermis it is called the stratum corneum. So what you actually see on the surface are dead skin cells. There are actually five sub-layers to the epidermis, the stratum corneum being the outer 'crust'.

There are several sub-layers in the epidermis. These have Latin names that may seen familiar to you. 

STRATUM CORNEUM

This is the outer layer of the skin that's also known as the horny layer. It keratinised cells are continually being shed and replaced by underlying cells rising up to the surface.

STRATUM LUCIDUM

This consists of small transparent cells through which light can be passes. The stratum lucidum is usually only found on the soles of the feet and the palms of the hands.

STRATUM GRANULOSUM

This layer consists of cells that are undergoing a change into the horny substance called keratin. These cells will be pushed up to become the stratum corneum.

STRATUM SPINOSUM

Consists of cells that are in the early stages of keratinisation. This layer contains melanin granules that give the skin its colour.

STRATUM BASALE/GERMINATIVUM

Also known as the basal layer, this is composed of several layers of differently shaped cells. The deepest layer is responsible for the growth of the epidermis through cell division. It also contains melanocyte cells that produce the pigment melanin. This protects the sensitive cells below from the destructive effects of excessive exposure too ultraviolet rays of the sun.

THE DERMIS

Directly beneath the epidermis is the dermis. The dermis is a highly sensitive and vascular layer as it contains nerve endings, blood vessels, oil glands and sweat glands. It also contains collagen and elastin that give the skin its structure and elasticity. This layer is connected to the blood and lymph systems. Unlike the epidermis.

FUNCTIONS OF THE DERMIS

The nerve endings in your dermis provide sense perception. They work with the brain and nervous system, so that the brain gets a message if the skin has been touched. If something hot has been touched, the nerve endings in the dermis send a message to the brain that then immediately commands the muscles to take the body part away from the hot object.

The dermis is also full of tiny blood vessels. These keep skin cells healthy by bringing then the oxygen and nutrients they need and by taking away waste. The dermis is also home to the oil glands, also. These are called sebaceous glands and they are always producing sebum which is the skin's own natural oil. It rises to the surface of your epidermis to keep your skin lubricated and protected. It also makes your skin waterproof.

LAYERS OF THE DERMIS

The dermis consists of two layers: the upper papillary and the deep reticular layer, in which specialised cells are found. The papillary layer lies directly beneath the epidermis. This layer mainly consists of loosely woven connective tissue. It has small cone-shaped projections of elastic tissue that point upward into the epidermis. This layer also contains some of the melanin skin pigment

In the deep reticular layer, below the papillary layer, there are:

  • Fibroblasts that produce collagen

  • Mast cells that create histamine and heparin

  • Histiocytes that also produce histamine

  • Leucocytes that are white blood cells that help fight infection and disease

The tough elastic collagen fibres are interwoven with elastin fibres in this layer and are responsible for the elasticity and tone of the skin. Sweat and sebaceous glands are also present. Small bundles of involuntary muscle fibres, known as arrector pili muscles, are attached to the hair follicles. The deep reticular layer also contains veins and arteries which link up with papillary capillaries. The lymph vessels form a network through the dermis allowing the removal of waste from the skin.

THE SUBCUTANEOUS LAYER

The third and bottom layer is called the subcutaneous layer. It is made mostly of fat and helps your body stay warm and absorb shocks. The subcutaneous layer also helps hold your skin to all the tissues underneath it.

This layer is where the roots of your hair start. Each hair grows out of a tiny tube in the skin called a follicle. Every follicle has its roots way down in the subcutaneous layer and continues up through the dermis. The lips, the palms of your hands, and the soles of your feet are the only areas of the body that don't have hair follicles. There are more than 100,00 follicles on your head. Connected to each follicle in the dermis layer is a tiny sebaceous gland that releases sebum onto the hair. This lightly coats the hair with oil, giving it shine and protection.

DUCT GLANDS

Within the skin there are two types of duct glands that extract materials from the blood to form new substances: 

1. Sudoriferous glands (or sweat glands) that excrete sweat

2. Sebaceous glands (oil glands) that secrete sebum

1. SUDORIFEROUS GLANDS

The sudoriferous or sweat glands, regulate body temperature and help to eliminate waste products from the body. Their activity is greatly increased by heat, exercise, emotions and certain drugs. There are two types of sudoriferous glands - the eccrine and apocrine sweat glands. 

The eccrine sweat glands consist of a coiled base and a tube-like duct that terminates at the skin surface to form the sweat pore. The eccrine sweat glands are located all over the body but are more numerous on the palms and the soles. Apocrine sweat glands don't develop fully until puberty. These glands are found in the axilla and in the genital area. They are associated with hair follicles and excitement appears to be the major inducer of apocrine secretion, a sticky milky sweat.

Th excretion of sweat is controlled by the nervous system. Normally, one to two of liquid containing salts, are eliminated daily through the sweat pores in the skin.

2. SEBACEOUS GLANDS

The sebaceous or oil glands consist of little sacs with ducts opening into the hair follicles. The glands secrete sebum, a fatty oil-like substance that's secreted onto the hair shaft below the opening of the hair follicle and then onto the surface of the skin. Sebum lubricates the skin and preserves the softness of the hair, with the exception of the palms and soles, these glands are found in all parts of the body, particularly in the face.

The blood and tissue fluid supply nourishment to the skin. As they circulate through the skin, the blood and tissue fluid provide essential materials for growth, nourishment and repair of the skin, hair and nails. In the subcutaneous layer, below the reticular dermis, is a network of arteries and lymphatic vessels that send smaller branches to the hair papillae, hair follicles and skin glands.

FUNCTIONS OF THE SKIN

The skin is responsible for numerous important functions including:

  • Sensation

  • Heat Regulation

  • Absorption

  • Protection

  • Excretion and Elimination

  • Secretion

  • Vitamin D formation

  • Melanin formation

  • Wound healing 

  • Vascular reactions

1. SENSATION

The nerve endings in the skin act as a warning system to indicate heat, cold, pain, pressure and other external factors. The nerve receptors are located at different levels in the skin. Touch and pain receptors are close to the surface, which are sensitive to extra stimuli detection such as temperature, pain, pressure, etc. Pacinian corpuscles lie deeper in the skin as they respond to pressure.

2. HEAT REGULATION

Skin can help if the body is too hot or too cold. Blood vessels, hair, and sweat glands cooperate to keep the body at just the right temperature (37 degrees Celsius). Temperature is controlled by the hypothalamus, the brain's in-built thermostat.

If the body becomes hot, blood vessels get the signal from the hypothalamus to release body heat. They do this by bringing warm blood closer to the surface of the skin, which is why you sometimes get a red face when hot. To cool down, sweat glands generate sweat to release body heat into the air. Once the sweat reaches the surface, it evaporates from the skin, the cooling effect of which lowers body temperature.

When the body is cold, blood vessels keep the body from losing heat by contracting as much as possible and keep the warm blood away from the skin's surface. Goose-pimples or the pilomotor reflex makes special tiny muscles called the arrector pili pull on your hairs so they stand up very straight. This allows the body to hold a small cushion of air next to the skin which helps to reduce heat loss.

3. ABSORPTION 

The pores on the skin's surface enable moisture to penetrate. The hair follicle, sebaceous gland opening and the skin itself are capable of absorption, as is the sweat gland, to a lesser degree. Also, penetration is affected by the health and condition of the skin.

4. PROTECTION

The stratum corneum is the part of the skin which protects the body against the environment. The skin is waterproof and acts to contain body fluids while preventing entry of large quantities of fluid through the epidermis.

5. EXCRETION AND ELIMITATION

Some toxins and perspiration are excreted from the body through the skin by the sweat glands. These toxins normally take the form of waste salts and water.

6. SECRETION

Secretion is the production of sebum from the sebaceous gland. This is the skin's moisturiser, keeping the skin soft and supple and intact. The surface of the skin is resistant to bacteria. Most bacteria are harmless, non-pathogenic types, however some pathogenic bacteria may be present without any reaction, although they can sometimes cause inflammation. When skin control fails, skin disorders can occur, e.g. boils and folliculitis.

The acid mantle of the skin is slightly acidic (the pH value of healthy skin is 5.5). It is made up from sebum, which can be affected by decreased activity of glands, sunlight, skin preparations, hygiene, nutrition and excessive perspiration. Sebum destroys or inhibits the growth of bacteria. Excessive sweating reduces acidity and encourages growth of bacteria, while softening the skin's surface.

7. VITAMIN D FORMATION

There is a fatty substance called ergosterol in the skin which ultraviolet light converts into vitamin D. Vitamin D circulates in the blood and is used with calcium and phosphorus in the formation and maintenance of bones. Excess vitamin D is stored in the liver.

8. MELANIN FORMATION

In the sun, the body makes melanocyte-stimulating hormones (MSH), which causes the melanocytes in the basal layer of the epidermis to produce the pigment melanin. Melanin darkens the skin to protect the underlying structures. This protects the body from the harmful effects of the sun's rays, as dark colours absorb radiation.

9. WOUND HEALING 

This is one of the many remarkable functions of the skin that we often take for granted. When the skin is cut, blood clots and cell debris fill the gap. Phagocytes begin to remove the clot and cell debris. Fibroblasts then secrete collagen fibres that begin to bind the surfaces together. The clot becomes a scab at around 3-5 days. The clot between the cut surfaces is completely removed and the scab separates. As the wound site gradually becomes stronger, the scar becomes less vascular, eventually appearing as a pale line.

10. VASCULAR REATIONS

If an organism invades the epidermis, there could be an inflammatory response. Histamine is released, vascular dilation, oedema and leucocytosis occur. The leucocytes engulf and destroy organisms as soon as they pierce the cuticle.