The SkinThe Skin

The skin is the largest organ in the body.The skin is the largest organ in the body.It is composed of It is composed of twotwo layers: layers:

The The epidermisepidermis on the outside. on the outside.The The dermisdermis is underneath. is underneath.

The thickness of the skin varies according to its The thickness of the skin varies according to its position e.g. the skin on the soles of the feet is the position e.g. the skin on the soles of the feet is the thickest.thickest.

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Hair Shaft

Sebaceous Gland

Sweat Gland Duct

Erector Muscle

Nerve

Blood Capillary

Sweat Gland

Adipose TissueHair PapillaBlood Vessel

Epidermis

Dermis

Hair Follicle

Malphigian Layer

Structure of the EpidermisStructure of the EpidermisCorneous LayerCorneous Layer The surface is known as the corneous layer. The surface is known as the corneous layer. The corneous layer is made up of flat dead cells.The corneous layer is made up of flat dead cells. These cells form a protective, waterproof layer on These cells form a protective, waterproof layer on

the skin.the skin. These cells are constantly been worn away.These cells are constantly been worn away. These cells are replaced by an upward growth These cells are replaced by an upward growth

from beneath.from beneath. Under this layer are layers of living cells.Under this layer are layers of living cells.

Malpighian LAYERMalpighian LAYER This is at the bottom of the dermis.This is at the bottom of the dermis.

It is a layer of actively growing cells.It is a layer of actively growing cells.

It contains the It contains the pigment melaninpigment melanin which which determines the colour of the skin.determines the colour of the skin.

The Structure of the DermisThe Structure of the Dermis

The dermis is a thick layer consisting of The dermis is a thick layer consisting of connective tissue.connective tissue.

It has sweat glands, hair follicles, sebaceous It has sweat glands, hair follicles, sebaceous glands, nerves, capillaries and lymphatics.glands, nerves, capillaries and lymphatics.

At the base is a layer of fat cells.At the base is a layer of fat cells. This layer of fat cells is called adipose tissue.This layer of fat cells is called adipose tissue. The adipose tissue acts as an energy reserve and The adipose tissue acts as an energy reserve and

helps insulate the body.helps insulate the body.

Sweat GlandsSweat Glands

They are coiled up tubes.They are coiled up tubes. They are lined with secretory cells which takes They are lined with secretory cells which takes

fluid from the capillaries surrounding them.fluid from the capillaries surrounding them. This liquid is released through the sweat ducts This liquid is released through the sweat ducts

which open on to the skin surface as pores.which open on to the skin surface as pores. The fluid is known as sweat.The fluid is known as sweat. Sweat consists of water, salt and urea.Sweat consists of water, salt and urea. The capillaries bring oxygen and nutrients to the The capillaries bring oxygen and nutrients to the

skin and remove carbon dioxide.skin and remove carbon dioxide.

Sebaceous GlandsSebaceous Glands These produce sebum, a greasy substance.These produce sebum, a greasy substance. This lubricates the hair and helps to keep the skin This lubricates the hair and helps to keep the skin

supple and waterproof.supple and waterproof. A tiny muscle called the erector muscle attached A tiny muscle called the erector muscle attached

to each hair follicle contracts when we are cold or to each hair follicle contracts when we are cold or frightened creating “goose pimples”.frightened creating “goose pimples”.

This provides insulation.This provides insulation.

NervesNerves

Nerves are plentiful in areas such as the Nerves are plentiful in areas such as the fingertips.fingertips.

Different types of nerve receptors enable us to Different types of nerve receptors enable us to feel sensations such as temperature, pressure feel sensations such as temperature, pressure and pain.and pain.

Functions of the SkinFunctions of the Skin

Protection:Protection: The malphigian layer screens the body from the The malphigian layer screens the body from the

harmful ultraviolet rays of the sun.harmful ultraviolet rays of the sun. The corneous layer keeps the skin waterproof and The corneous layer keeps the skin waterproof and

prevents infection by micro-organisms and loss of prevents infection by micro-organisms and loss of moisture.moisture.

ExcretionExcretionWater and traces of waste such as salt and urea Water and traces of waste such as salt and urea

pass onto the surface of the through the sweat pass onto the surface of the through the sweat ducts and pores.ducts and pores.

Temperature RegulationThe 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.

VasodilationVasodilation: : When we get too warm, capillaries near the When we get too warm, capillaries near the

surface of the skin dilate.surface of the skin dilate. This allows blood to lose heat. (this is what This allows blood to lose heat. (this is what

happens when we blush)happens when we blush)

Vasoconstriction:Vasoconstriction: When we are cold the capillaries contract.When we are cold the capillaries contract. This reduces the volume of blood to the skin.This reduces the volume of blood to the skin. We get pale, thereby lessening heat loss.We get pale, thereby lessening heat loss. Sweat production is also reduced to retain heat.Sweat production is also reduced to retain heat.

Sensory OrganSensory Organ

The skin responds to sensations such as heat, The skin responds to sensations such as heat, cold and pain.cold and pain.

The ends in the skin transmit this information to The ends in the skin transmit this information to the brain.the brain.

The brain may act on this information, as when The brain may act on this information, as when we remove our fingers from a hot object.we remove our fingers from a hot object.