physiology of the skin

Question 1

functions of the epidermis

Answer 1

• Waterproofing
• Physical barrier
• Immune function
• Vitamin D synthesis
• UV protection
• Thermoregulation

Question 2

functions of the dermis

Answer 2

• Thermoregulation
• Vitamin D synthesis
• Sensory organ

Question 3

functions of the subcutis

Answer 3

• Thermoregulation
• Energy reserve
• Vitamin D storage
• Endocrine organ
• Shock absorber

Question 4

what causes the skin to be a waterproof barrier

Answer 4

tight junctions in stratum granulosum

epidermal lipids and keratin in stratum corneum

Question 5

why does skin wrinkle when wet?

Answer 5

• Mediated by sympathetic nervous system
• Due to vasoconstriction in dermis
• Improves grip

Question 6

how does the skin act as a physical barrier

Answer 6

• Structure of skin helps resist trauma
• Stratified epithelium helps resist abrasive forces
• Fat in subcutis acts as shock absorber

Question 7

how does the skin synthesise vit D

Answer 7

• 7-dyhydrocholesterol in plasma membranes of epidermal keratinocytes and dermal fibroblasts converted to previtamin D3 by UVB

Question 8

how does the skin store vit D

Answer 8

• Lipid soluble – can be stored in subcutis adipocytes

Question 9

name 2 hormones which site of action is in the skin

Answer 9

• Androgens act of follicles and sebaceous glands

* Thyroid hormones act on keratinocytes, follicles, dermal fibroblasts, sebaceous glands, eccrine glands

Question 10

name 3 hormones which have their site of synthesis in the skin

Answer 10

vit D3
5 alpha-reductase -> 5alpha-dihydrotestosterone
insulin-like growth factor

Question 11

how does UV-a and UV-b damage skin

Answer 11

• Burns
• Suppress action of Langerhans cells
• Photo-aging
• DNA damage (skin cancers)

Question 12

what 3 things does skin colour depend upon

Answer 12

• Melanin
• Carotenoids
• Oxy/deoxyhaemoglobin

Question 13

how is melanin synthesised and transported

Answer 13

• Synthesised in melanosomes within melanocytes from tyrosine
• Transported via dendrites to adjacent keratinocytes

Question 14

what are the 2 types of melanin

Answer 14

• Pheomelanin (red/yellow)

* Eumelanin (brown/black)

Question 15

what type of melanin do all skin types contain more of?

Answer 15

eumelanin

Question 16

what is the role of melanin

Answer 16

• Photoprotective – scatters/filters UV light
• Prone to photodegradation
• Lots of melanin = less able to utilize UV light to make vitamin D

Question 17

how does our skin respond to sunlight

Answer 17

• Immediate pigment darkening - photooxidation of existing melanin, redistribution of melanosomes, occurs within minutes and lasts hours-days.
• Persistent pigment darkening (tanning): UVA&raquo_space; UVB, oxidation of melanin, occurs within hours, lasts 3-5 days
• Delayed tanning: increased melanin synthesis. Occurs 2-3 days after UV exposure, maximal at 10-28 days

Question 18

how is the skin a barrier to infection?

Answer 18

• A range of peptides synthesised by granular layer keratinocytes have antimicrobial properties

Question 19

what immune cells are present in the epidermis

Answer 19

Langerhans cells

Question 20

what immune cells are present in the dermis

Answer 20

o	Regulatory T cells
o	Natural killer cells
o	Dendritic cells
o	Macrophages
o	Mast cells

Question 21

what happens when the immune organ of the skin is challenged

Answer 21

• LC migrate to dermis and lymph nodes and activate a T-cell response
• Keratinocytes proliferate & secrete cytokines
• Leucocytes enter skin from blood

Question 22

how is the skin a sensory organ

Answer 22

• Merkle cells - basal epidermis (Light touch)
• Encapsulated mechanoreceptors in dermis
o Pacinian corpuscles (Pressure/Vibration)
o Meissner corpuscles (Touch)
• Myelinated and unmyelinated sensory nerve endings in dermis (Pain, Itch, Temperature)

Question 23

what insulates to maintain body temp

Answer 23

subcutaneous fat

Question 24

how is heat lost from the skin?

Answer 24

• Cutaneous blood flow
o Deep vascular plexus (lower reticular dermis)
o Superficial vascular plexus (upper reticular dermis)
o Loops of blood vessels from superficial plexus extend to reticular dermis
• Eccrine sweating

Question 25

why are humans endothermic homeotherms

Answer 25

heat is generated through metabolism

Question 26

what does evaporation depend upon

Answer 26

• Surface area exposed to environment
• Temp and relative humidity of ambient air
• Convective air currents

Question 27

heat storage equation

Answer 27

Heat storage = metabolism – work – evaporation +/- evaporation =/- conduction +/- convection

Question 28

describe the autonomic regulation of blood flow in dermal vascular plexuses

Answer 28

• Sympathetic alpha-noradrenergic: vasoconstriction
• Sympathetic cholinergic: vasodilation
• (Both in hairy skin. Hairless skin only has cholinergic innervation)
Sympathetic cholinergic nerves that govern sweating may be the same as those controlling active vasodilation
Nitric oxide may play a role in active vasodilation

Question 29

how does piloerection arise

Answer 29

• Arrector pili muscles innervated by sympathetic α1-adrenergic fibres
• Contraction raises cutaneous hairs
• Likely little significant impact on heat conservation

Question 30

what acts as an energy store in the skin

Answer 30

subcutaneous fat

white adipose connective tissue