Anatomy & Physiology

Anatomy & Physiology:

The Skin:

The skin is the largest organ of the body. The skin functions in a number of different ways to protect us from external elements:

• Prevents the absorption of harmful substances
• Helps regulate body temperature
• Acts as a barrier to keep out infection
• Melanin in the skin protects us from the harmful effects of UV light
• Provides a waterproof coating that prevents us from becoming dehydrated
• Provides an energy reserve in the form of stored fat

The skin covers the entire surface of the body and weighs approximately one ninth of our total body weight. It is thinnest on the eyelids and thickest on the soles of the feet. The skin is continually shedding and renewing itself. We are able to feel sensations such as pain or heat because of sensors in the skin which transmit messages to the brain and outer skin plays a major role in maintaining body temperature and in protecting the body from harm

The skin varies in colour due to age, race inherited factors and external factors such as climate. The skin can vary in thickness depending upon where it is on the body, i.e., eye and lip skin is very thin whereas hand and foot skin is thicker. The thickness of the skin can affect its colour, for example thin skin will look more pink as the blood in subcutaneous tissue will show through, whereas thicker skin, such as on the soles of the feet, tends to look yellow.

There are also medical reasons for skin colour to change such as with rashes where the skin will be more red, heart or lung conditions which will turn the skin blue, or jaundice which will yellow the skin.

With age and sun damage the skin will lose some of its elasticity due to lack of collagen, at which time wrinkles will appear. The skin secretes an oily substance known as ‘sebum’ which will help to maintain the skin’s suppleness, although there are no sebaceous glands on the palms of the hand or soles of the feet.

The skin also secretes sweat which is usually the result of temperature changes – this is known as insensible perspiration, but can be due to fear or nervousness which is known as sensible perspiration. This last form is produced by apocrine sweat gland

Skin Structure

The skin is made up of three layers called:

• Epidermis
• Dermis
• Subcutaneous

Epidermis

The epidermis is the upper portion of the skin and consists of five layers:

• Horny layer (stratum corneum): outer most layer

Outermost layer of the epidermis, made up of several layers of flattened, mostly dead overlapping cells. These cells help to reflect UV light.

Black skin, which evolved to withstand strong UV light, has a thicker stratum corneum than Caucasian skin. It takes about 3 weeks for the epidermal cells to reach the stratum corneum from the stratum germinativum. The cells are then shed through a process called desquamation.

• Clear layer (stratum lucidum)

This layer is only found in thicker areas of the skin such as the palms of the hands or the soles of the feet.

Found below the horny layer and consists of dead keratinized cells without a nucleus. The cells are transparent, which allows the passage of sunlight into the deeper layers.

• Granular layer (stratum granulosum)

In this layer the cells being to die. These cells have what looks like granules within them caused by the nuclei breaking up. These granules are known as kerathohyalin granules and later form keratin.

Keratinisation is the process that cells undergo when they change from living cells with a nucleus to dead cells without a nucleus. This layer links the living cells of the epidermis to the dead cells above.

• Prickle cell layer (stratum spinosum)

Made up of cells which have a spikey surface to connect with surrounding cells. This is the layer that begins to synthesise keratin.

In this layer the cells are living. Pigment granules called melanin may be found here.

• Basal layer (stratum germinativum): inner most layer

Column shaped cells responsible for producing new epidermal cells. Cells divide and move up to higher layers. The remaining cells divide to fill the gaps. This process of cell division is caused mitosis.

As the new cells are produced they push older cells above them towards the surface of the skin, until they finally reach the horny layer. It takes 3 – 6 weeks for the skin cells to be pushed up from the basal layer to the horny layer.

• Germinative Zone

This zone is the epidermis contains two other important cells – Langerhan and Melanocyte cells.

Langerhan cells absorb and remove foreign bodies that enter the skin. They move out of the epidermis and into the dermis below, then finally enter the lymph system – the body’s waste disposal system.

Melanocyte cells are responsible for the production of melanin in the skin. These protect the other epidermal cells from the harmful effects of UV. Melanin helps determine our skin colour; the more melanin present, the darker our skin tone.

Cell regeneration occurs in the epidermis by the process of mitosis (cell division). It takes approximately a month for a new cell to complete its journey from the basal cell layer where it is reproduced to the granular layer where it becomes keratinised, to the horny layer where it is desquamated.

The dermis lies below the epidermis, and connects with the basal layer and is often described as the “true skin”. It is responsible for the strength and elasticity of the skin. It contains lots of specialised cells and structures, including nerves, blood vessels, glands and hair follicles.

It consists of two layers:

• Papillary layer

This is the upper section and contains small tubes called capillaries, which carry blood and lymph. It also has nerve endings. This layer provides nutrients for living layers of epidermis. It contains a thin arrangement of collagen fibre.

• Reticular layer
• Consists of two types of protein:
• Elastin fibres which give the skin its elasticity
• Collagen fibres which give the skin its strength

These fibres are held in a gel-like substance called ‘ground substance’. The collagen and elastin fibres form a strong network which gives us our youthful appearance.

As we age, these fibres in the skin begin to harden and fragment; the network starts to break down and our skin starts to lose its elasticity and show visible signs of ageing. Blood circulation to the skin declines; nutrients do not reach the surface, resulting in sallow skin. The fatty layer beneath the skin grows thinner so we look more drawn as our bone structure is more prominent. The reticular layer is vital to our skin’s health and appearance and so it is essential that it is looked after in order to prevent signs of ageing.

Subcutaneous layer

The subcutaneous layer is situated below the dermis. It consists of adipose tissue (fat) and areolar tissue. The adipose tissue helps to protect the body against injury and acts as an insulating layer against heat loss, helping to keep the body warm. The areolar tissue contains elastic fibres, making this layer elastic and flexible. Muscle is situated below the subcutaneous layer and is attached to bone.

Adipose Tissue

This is a loose connective tissue whose specific purpose is to store fat. Adipose tissue is found under the skin and around organs, it acts as a food reserve. As it is also a poor conductor of heat it assists in maintaining body temperature by preventing heat loss.

It is thought that massage affects the adipose tissue as it softens the hard fat under the skin and helps to disperse it.

The distribution of the fat layer under the skin varies according to sex, age and lifestyle. Women tend to have a thicker layer of adipose tissue than men, giving the female form a softer outline. Following the menopause, women tend to put on weight in the more masculine areas such as the waist and abdomen rather than the hips and thighs.