The Skin

 

The integumentary system consists of the skin and its derivatives such as hair, nails, glands and several specialised receptors.

The skin is an organ because it consists of various tissues joined together to perform specific functions including:

 

• Protection

• Sensory organ (receives stimuli)

• Temperature regulation

• Synthesis of vitamin D

 

Protection

• The skin acts as a mechanical barrier against microorganisms e.g. bacteria, viruses, fungi

• The skin is waterproof and therefore prevents against water loss (its benefit is not about water getting in, but about preventing water getting out)

• Protects the inside of the body from harmful UV rays. However, the skin cells themselves can be damaged when skin burns. This causes the DNA in the skin cells to mutate causing skin cancer

 

Sensory organ

The skin (especially in the fingers) contains millions of specialised receptors (sensory neurons) that detect stimuli such as temperature, touch, pressure and pain.

 

Messages about these stimuli are relayed to control centres in the brain, via the sensory (afferent) pathways of the nervous system, where they are compared with a ‘set point’ (an established normal range)

 

 

 

Temperature regulation

 

 

In response to the environmental temperature or strenuous exercise, the skin helps to maintain the core body temperature (37⁰)

 

The skin contains sweat glands that produce sweat when the body gets too hot. Body heat is transferred to the water in the sweat and as it heats up it turns to vapour. The sweat evaporates which cools the body down

 

However, if the air is very humid the sweat can’t evaporate, and instead just rolls down the skin. The body will only cool down if the sweat evaporates

 

The skin contains blood vessels that can control the amount of blood that flows through. The blood vessels can dilate and constrict to control heat.

 

Vasodilation (widening of blood vessels) causes an increase in blood flow in the skin, increasing heat loss

 

Vasoconstriction (narrowing of blood vessels) causes a decrease in blood flow in the skin, decreasing heat loss

 

Synthesis of Vitamin D

Vitamin D is actually a steroid hormone which begins its synthesis in the skin. This requires exposure to ultraviolet radiation.  

 

This hormone is important for the absorption of calcium in the small intestine. Calcium is vital for bone growth.

Lack of calcium can result in rickets which causes deformed bones.

 

Structure of the skin

 

Technically, the skin is the largest organ of the body. It has several natural portals (openings) including:

• Mouth

• Nostrils

• Anus

• Urethral canal

• Vaginal canal (females)

 

All these openings potentially are areas where pathogens can enter the body.

 

The skin is made up of two layers:

 

• Epidermis 

• Dermis

 

The Epidermis

The epidermis consists of stratified squamous epithelium and includes several types of cells, and has five different layers including the stratum germinativum and the stratum corneum

 

• Stratum germinativum (basale) - basal layer

This is the bottom layer of cells which are capable of continued cell division. This is the area which actively grows. This layer contains melanin protein which is responsible for the skin colour and absorption of UV light. It is here the cells can become cancerous forming a melanoma (a most aggressive form of skin cancer)

 

• Stratum Corneum

This is the top layer of dead cells and contains keratinocytes. These produce keratin that makes the cell waterproof.

 

This outer layer of skin contains ridges and grooves which are unique to each person. Their function is to facilitate gripping of the hand and foot (when running) by increasing friction.

 

The Dermis

This layer is much thicker and consists of connective tissue containing areolar tissue and some collagen and elastic fibres. This provides the skin with strength, extensibility (ability to stretch) and elasticity (ability to return to original shape after stretching or contraction)

 

The dermis has many blood vessels and nerve endings that form specialised receptors for temperature, pain, touch and pressure. It also contains glands and hair follicles.

 

The sebaceous glands coat the surface of the hair follicle and produce oil that assists with waterproofing. Sweat glands also penetrate both the dermis and epidermis.

 

Hair erector muscles originate low down on the hair follicle and when they contract the hair follicle becomes more erect, forming 'goose bumps'.

 

Subcutaneous layer

Below these layers is a subcutaneous layer which consists of adipose tissue (fat cells)