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10 Sweat Essay

Why do we sweat?
EVAPORATION of sweat from the surface of the skin

HEAT LOSS from the body to maintain core temperature

Transfer of thermal energy to liquid molecules

Latent heat of vaporization of water
2.43 kj.g-1 (energy required to evaporate water from the skin at 30 °C)
How much can a trained individual sweat during work in the heat?
excess of 3-4 L.h-1 but not for sustained periods
Insensible perspiration, what is it and how much
Water that evaporates before it can form droplets
20-25 g•m^-2•h^-1
How much heat loss at rest through respiratory evaporation?
~10% resting heat loss
Factors affecting sweating
Relative Humidity
Temperature
Air velocity
Clothing
Factors affecting sweating: Relative humidity
Skin vapor pressure and ambient vapor pressure
Together make vapor pressure gradient
Factors affecting sweating: Temperature
Ambient Temperature
Skin Temperature
Core Temperature
Factors affecting sweating: Clothing
Insulation
Exposed skin
Control of Sweating
Control of Sweating
Skin temp vs internal temp vs sweat rate
Skin temp vs internal temp vs sweat rate
Skin temp vs internal temp vs sweat rate with heat acclimation
Skin temp vs internal temp vs sweat rate with heat acclimation
Non-thermal modifiers of sweating
Non-thermal modifiers of sweating
Sweat gland classifications
Eccrine
Apocrine
Apoeccrine
Sweat gland classifications: Eccrine
Thermoregulatory function
Located over most of body surface
Sweat gland classifications: Apocrine
Psychological stimuli
Located in axilla, forehead, palms and soles
Sweat gland classifications: Apoeccrine
Hybrid gland, develops during puberty
Located in the axilla only
Sweat gland parts
Tubular epithelium divided into the duct and secretory coil
Sweat gland parts
Glandular body/Secretory portion of sweat gland: location
Located in the dermis
Glandular body/Secretory portion of sweat gland: location
Glandular body/Secretory portion of sweat gland: main part
Unbranched tubule coiled into a glomerulus (0.2-0.5 mm diameter)
Glandular body/Secretory portion of sweat gland: main part
Glandular body/Secretory portion of sweat gland: what is produced?
Production of precursor fluid (largely protein-free)
Glandular body/Secretory portion of sweat gland: what is the lining and what is it made of?
Epithelial lining – consisting of 3 cell types:
1) Dark (mucoid) cells (cover 90% of luminal surface)
2) Clear cells (produce aqueous fluid)
3) Myoepithelial cells – helical arrangement around secretory coil (role in gland contraction)
Excretory duct of sweat gland: where does it start
Begins in the glandular/secretory body. Initial coiled portion is followed by a continuing straight portion
Excretory duct of sweat gland: where does it start
Excretory duct of sweat gland: cells
Reduced number of myoepithelial cells

Cells lining the duct resemble clear cells

Excretory duct of sweat gland: cells
Excretory duct of sweat gland: what does it do with electrolytes?
Reabsorption of Na+, K+ and Cl- as sweat passes thought the excretory duct
Excretory duct of sweat gland: final pathway and what is secreted?
Excretory duct opens almost onto the epidermal surface, passing through the keratinized mass of the stratum corneum (particularly thick skin)

Hypotonic sweat solution secreted

Excretory duct of sweat gland: final pathway and what is secreted?
Sweat gland density: how many over whole body?
2-5 million sweat glands over the body
Sweat gland density: When is # of sweat glands determined?
Total number of active sweat glands determined by 2 yrs of age
Sweat gland density: Sex differences
– Females have a higher density of heat activated sweat glands (HASG) compared to men
– Total number of HAGS similar between sexes due to a smaller body SA in females
– Females typically have a lower output per gland and are thought to have a lower whole body sweat loss than males for the same relative work load
Regional Sweat Gland Densities
Regional Sweat Gland Densities
Eccrine Sweat Composition
Derived from blood plasma (ultra-filtrate). The concentration is adjusted as the fluid passes through the sweat duct

Eccrine sweat is ~98% water and hypotonic compared to plasma

Na+, Cl-, K+, and lactate show highest concentrations

Variations in electrolyte content with sweat rate, acclimation state and dietary intake

What increases as profuse sweating is prolonged?
Increase in sweat ion concentration with prolonged, profuse sweating
What happens to sweat composition with heat acclimation?
Na+ and Cl- concentrations ? with prolonged, profuse sweating
Na+ and Cl- concentrations ? with heat acclimation
Aldosterone role in sweating
Maintains Na+ levels in the body
Stimulated by ? blood serum Na+ or blood volume
ADH role in sweating
Stimulates water reabsorption at kidneys and ? urine production
Male regional sweat rates at 55% VO2 Max
Male regional sweat rates at 55% VO2 Max
Male regional sweat rates at 75% VO2 Max
Male regional sweat rates at 75% VO2 Max
Female regional sweat rates at 55% VO2 Max
Female regional sweat rates at 55% VO2 Max
Female regional sweat rates at 75% VO2 Max
Female regional sweat rates at 75% VO2 Max
Sweat measurement: Capsule techniques
Absorbent
Sweat collection using filtration paper placed inside a capsule which is glued to the skin.

Ventilated
Capsule glued or pressed to the skin. Analysis
of humidity and temperature change of air.

Sweat measurement: absorbent
Application of hygroscopic material to skin surface

Sweat pads relative in size to each individual.

Calculation of sweat rate
(g•m-2•h-1) based on weight change of pad, application time, and SA.

Sweat Measurement: sweat droplet analysis
Colouring agent and petroleum jelly applied to skin
Magnification to allow observation of sweat drop number and size
When is regulation of Tc compromised?
When Ereq > Emax
Eccrine sweat gland structure and function
Two main sections:
gland/secretory portion – production of precursor fluid
Excretory duct – Na+ and Cl- reabsorption
What sweat type has highest concentration of electrolytes?
Highest concentration of Na+, Cl-, and K+ in eccrine sweat
Control of Sweating: sumary
– Sweating stimulated by an increase Tcore and Tsk – Integration at pre-optic hypothalamus with 9:1 Tcore:Tsk proportional control
– Release of Ach and binding to m3 receptors at sweat gland stimulates sweating
Eccrine sweat glands: summary
– Thermoregulatory role
– Located over most of skin surface with ~ 2-5 million over whole body
– Most densely distributed on the palms, soles and forehead. Lowest densities on the forearm, lower leg, and thigh
Regional sweat rates: summary
– Large inter and intra regional variation
– Highest sweat rates observed on medial and lower posterior torso and forehead
– Lowest sweat rates observed on hands and feet
Sweating: summary
– Evaporation of sweat from the surface of the skin – greatest avenue of heat loss from the body
– Regulation of Tcore is compromised when Ereq > Emax
Eccrine Sweat Gland: Summary
Two main sections:
gland/secretory portion – production of precursor fluid
Excretory duct – Na+ and Cl- reabsorption
Sweat composition: summary
– Highest concentration of Na+, Cl-, and K+ in eccrine sweat
– Sweat electrolyte levels are highly variable, but Na+ and Cl- losses are reduced with heat acclimation and training

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