Skin

Question 1

Skin is what % of body weight?

Answer 1

15%
Largest organ in the body

Question 2

Functions of the skin 4

Answer 2

• protection against external physical, chemical and biological threats
  ◦ Discuss immune
• Prevents excess water loss
• Thermoregulation
• Sensory

Question 3

What are the 3 mechanisms of heat loss by the skin

Answer 3

• Control of blood low regulated
  ◦ Convection
  ◦ Radiation
  ◦ Conduction

Question 4

Skin receives what % of cardiac output

Answer 4

8%

Question 5

Normal skin blood flow

Answer 5

300mls/min

Question 6

Blood flow variance to the skin varies from

Answer 6

‣ Blood flow varies from 1-150mL/min per 100g of skin –> can increase to 3000mls/min

Question 7

Variation in skin blod flow determined by

Answer 7

‣ Variation due to metabolism and temperature regulation
* Metabolic requirements are low
* Heat loss primarily by sweating but also from warm skin by radiation, conduction and convection

Question 8

Capacitance of skin capillaries, venules

Answer 8

1.5L

Question 9

Innervation of the skin by

Answer 9

Adrenergic fibres

Question 10

What is a skin shunt vessel? How large is it? Where is it found?

Answer 10

◦ Shunt vessels - mainly in hands, feet, ears,nose, lips
‣ 50-100 micrometers diametre

Question 11

Describe the function and role of skin shunt vessels

Answer 11

◦ Shunt vessels - mainly in hands, feet, ears,nose, lips
‣ 50-100 micrometers diametre
‣ Smooth vascular muscle innervated by alpha adrenergic receptors regulated by SNS
‣ When body temp 30degrees or greater sympathetic activity decreases (anterior hypothalamus) causing vasodilation, enhanced by bradykinin from sweat glands (activated by cholinergic SNS), fall in total vascular resistance leads to increased cardiac output, massive increase in skin blood flow accommodated by AV shunts and deep venous plexus
‣ Increased area for heat transfer
‣ Cutaneous blood flow increasing 30 fold in heat stress, decreases 10 fold in cold stres

Question 12

Describe the countercurrent mechanisms in the skin

Answer 12

◦ Cutaneous veins provide counter current system for heat conservation
‣ Calibre under control by noradrenaline
‣ In cold conditions blood returning from deep veins acquire heat from arteries reducing body heat loss

Countercurrent mechanisms of the skin
* Countercurrent blood flow in the skin is a mechanism of heat preservation and differs to kidneys both in function but also structure as countercurrent structures are vascular exchange between artery and vein, whereas in kidneys this occurs between plasma filtrate lumens as well as between plasma filtrate and capillary flow.
* Arterial blood flow in the skin is closely related to veinous drainage ensuring arterial heat is conducted to blood in returning veins (cooler) to reduce heat lost in peripheries
* With increasing shunt through precapillary vasoconstriction and shunt vessels the fraction of heat lost in peripheral circulation is reduced, and where these shunts are closed due to precapillary vasoconstriction heat loss is increased

Question 13

Sweat is what?

Answer 13

Secretion from sweat glands containing, water, electrolytes and heat

Question 14

Sweat glands on what % of skin

Answer 14

99%

Question 15

What controls sweat glands?

Answer 15

SNS 0 cholinergic/muscurinic innervation

Question 16

Roles of sweat 4

Answer 16

◦ Thermoregulation - every g of evaporating water represents 0.58kcal of heat loss
‣ Only works if evaporation, if wiped away no benefit
◦ Lubrication/gripping - the reason SNS makes you sweat. No longer functional in humans
◦ Excretory - water and electrolytes - max volume 1.5-2L/hr and 12L per day
◦ Antimicrobial

Question 17

Describe the contents of sweat

Answer 17

◦ Average osmolality 120msom/kg but variable –> 60-200
◦ Na 50mmol/L - 30 - 65 (decreased by aldosterone causing 5-350mmol loss per day)
◦ K 4mmol/L,
◦ Calcium 1mmol/L,
◦ Mg 1mmol/L,
◦ Cl 40mmolL and
◦ 16mmol/L from other anions

Question 18

How does sweat change in hot environemnts

Answer 18

Reduced Na content
Increased sweating

Question 19

Insensible losses per day - both in fluid and sweat

Answer 19

◦ Water passing through the skin as diffusion and evaporative losses from respiratory tract
◦ Key characteristics
‣ Pure water loss (no solute) –> different to sweat, and diffusion rather than sweat gland
‣ Cannot be prevented
‣ Minimal loss 800ml per day - 400ml from skin, 400ml respiratory tract
‣ Associated heat loss from this 464kcal or 25% of basal heat production (800ml)

Question 20

A patient with a high body temperature and sweating indicates what?

Answer 20

Sweaty febrile patient = body temperature rising when skin dry and shivering present, sweating only when set point decreases again. Once sweating the temperature is starting to fall