Function and Structure of the Skin

Function of the Skin

• The largest organ in the body
• Protects the body from the external environment
• Excretes waste products like sweat
• Absorbs substances like medications
• Regulates body temperature
• Produces vitamin D
• Provides sensation

Structure of the Skin

• Composed of two layers: the epidermis and the dermis

The Epidermis

• Superficial layer of the skin
• Composed of flat cells filled with keratin
• The deep layer has cells that regenerate
• Avascular, but contains sensory nerve endings

### The Dermis

• Deep layer of the skin
• Composed of connective tissue
• Contains:
  • Blood vessels
  • Lymph vessels
  • Sensory nerve endings
  • Smooth muscle
  • Hair follicles
  • Sweat glands
  • Sebaceous glands

Lines of Cleavage (Langer's Lines)

• Collagen fibers in the deep part of the dermis are arranged in parallel rows
• Longitudinally in limbs, circumferentially in the neck and trunk
• Important to consider during surgery to minimize scarring

Skin Appendages

• Hairs:
  • Root embedded in the skin
  • Shaft projects on the surface
  • Hair follicle formed by the root and surrounding epidermal cells
  • Arrector pili muscle:
    • Band of smooth muscle innervated by sympathetic nerve fibers (involuntary)
    • Contraction moves hair to a more vertical position and squeezes sebaceous gland
• Sebaceous Glands:
  • Duct opens into hair follicle
  • Secretes sebum (an oily material that softens the skin and lubricates the hair)
• Sweat Glands:
  • Duct opens on the surface of the skin
  • Secretes sweat
  • Apocrine sweat glands:
    • Located in axilla and external genitalia
    • Function at puberty and secrete sweat with a characteristic odor
• Nails:
  • Keratinized plate
  • Located on the dorsal surface of the distal phalanx of the fingers and toes

The Fascia

• Connective tissue that surrounds and supports muscles and organs
• Two types: superficial and deep fascia

Superficial Fascia

• Loose, except in some areas (sole, palm, scalp)
• Functions:
  • Bad heat conductor (keeps the body temperature)
  • Gives the body smooth rounded appearance (particularly in females)
  • Facilitates movement of the skin over the underlying structures
• Contains:
  • Nerves
  • Blood vessels
  • Fat
  • Muscles, like platysma

Deep Fascia

• Dense and usually forms membranes
• Forms special structures:
  • Investing fascia
  • Intermuscular septa
  • Sheathes for neurovascular bundles
  • Aponeurosis
• Functions:
  • Gives attachment to some muscles
  • Intermuscular septa prevent the spread of blood or pus
  • Muscle pump helps venous return
  • Retinacula hold tendons in place to facilitate the action of muscles
  • Palmar and plantar aponeuroses protect the deep structures

The Muscles

• Red, fleshy, contractile tissue responsible for movement
• Three types: smooth, cardiac, skeletal

Smooth Muscle

• Location: wall of internal viscera and blood vessels
• Nerve supply: autonomic
• Contraction: involuntary
• Striations: absent

Cardiac Muscle

• Location: heart (myocardium)
• Nerve supply: autonomic
• Contraction: involuntary
• Striations: present

Skeletal Muscle

• Location: attached to the skeleton
• Nerve supply: somatic
• Contraction: voluntary
• Striations: present

Skeletal Muscles

• Definition: muscles attached to the skeleton to generate movement
• Parts of a skeletal muscle:
  • Muscle belly: red fleshy contractile part between the two tendons
  • Tendon: white fibrous non contractile part (rounded cord-like) or aponeurosis (flat sheet)
• Attachment:
  • Origin: more fixed attachment
  • Insertion: more mobile attachment
• Can be attached to: bone, cartilage, ligament, fascia, skin, other muscle
• Classification:
  • Prime movers: initiate a particular movement