The Skin  The skin is the largest organ in the body.  It is composed of two layers:  The epidermis on the outside.  The dermis is underneath.  The thickness of the skin varies according to its position e.g. the skin on the soles of the feet is the thickest. © PDST Home Economics

Hair Shaft Sebaceous Gland Sweat Gland Duct Erector Muscle Nerve Blood Capillary Sweat Gland Adipose Tissue Hair PapillaBlood Vessel Epidermis Dermis Hair Follicle Malphigian Layer

Structure of the Epidermis Corneous Layer  The surface is known as the corneous layer.  The corneous layer is made up of flat dead cells.  These cells form a protective, waterproof layer on the skin.  These cells are constantly been worn away.  These cells are replaced by an upward growth from beneath.  Under this layer are layers of living cells.

Malpighian LAYER  This is at the bottom of the dermis. It is a layer of actively growing cells. It contains the pigment melanin which determines the colour of the skin.

The Structure of the Dermis  The dermis is a thick layer consisting of connective tissue.  It has sweat glands, hair follicles, sebaceous glands, nerves, capillaries and lymphatics.  At the base is a layer of fat cells.  This layer of fat cells is called adipose tissue.  The adipose tissue acts as an energy reserve and helps insulate the body.

Sweat Glands  They are coiled up tubes.  They are lined with secretory cells which takes fluid from the capillaries surrounding them.  This liquid is released through the sweat ducts which open on to the skin surface as pores.  The fluid is known as sweat.  Sweat consists of water, salt and urea.  The capillaries bring oxygen and nutrients to the skin and remove carbon dioxide.

Sebaceous Glands  These produce sebum, a greasy substance.  This lubricates the hair and helps to keep the skin supple and waterproof.  A tiny muscle called the erector muscle attached to each hair follicle contracts when we are cold or frightened creating “goose pimples”.  This provides insulation.

Nerves  Nerves are plentiful in areas such as the fingertips.  Different types of nerve receptors enable us to feel sensations such as temperature, pressure and pain.

Functions of the Skin Protection:  The malphigian layer screens the body from the harmful ultraviolet rays of the sun.  The corneous layer keeps the skin waterproof and prevents infection by micro-organisms and loss of moisture.

Excretion Water and traces of waste such as salt and urea pass onto the surface of the through the sweat ducts and pores. Temperature Regulation  The skin regulates body temperature in two ways.  By evaporation of sweat: Heat is taken from the surface of the body in order to evaporate the sweat.  This cools the body.

Vasodilation:  When we get too warm, capillaries near the surface of the skin dilate.  This allows blood to lose heat. (this is what happens when we blush) Vasoconstriction:  When we are cold the capillaries contract.  This reduces the volume of blood to the skin.  We get pale, thereby lessening heat loss.  Sweat production is also reduced to retain heat.

Sensory Organ  The skin responds to sensations such as heat, cold and pain.  The ends in the skin transmit this information to the brain.  The brain may act on this information, as when we remove our fingers from a hot object.