Human Physiology Body s Heat Balance

Size: px

Start display at page:

Download "Human Physiology Body s Heat Balance"

Samson Montgomery
7 years ago
Views:

1 Human Physiology Body s Heat Balance Arsen K. Melikov October , CTU in Prague International Centre for Indoor Environment and Energy Department of Civil Engineering, Technical University of Denmark 1 Human Physiology: Body Temperature Core (internal) temperature average core temperature: 37 C individual differences: 36 to 38 C daily variations: ± 1C (minimum in the morning; maximum in the afternoon) kept approx. constant (in the limits of body s capacity to keep heat balance) maximum core temperature: 42 to 43 C fatal core temperature: > 43 C shivering starts with the decrease of the core temperature shivering stops at core temperature < 33 C; follows unconsciousness core temperature < 25 C is fatal 2 1

2 Human Physiology: Body Temperature Measures of the Core temperature oral temperature (most common and least accurate) rectal temperature (most consistent but least responsive to sudden changes) esophageal temperature (temperature of blood flowing through the heart) ear s tympanic membrane temperature (under warm conditions indicates the temperature of the hypothalamus and brain stem, which are sensed by the primary neurophysiological mechanism regulating body temperature) 3 Human Physiology: Body Temperature Skin temperature average skin temperature (AST): 33.2 C ± 1 C local skin temperature: C (17-40 C) uniform at warm environment non-uniform at cold environment (hands, feet, legs & arms colder than head & torso) pain limit: AST 45 C AST 31 C uncomfortably cold AST 30 C shivering cold AST 29 C extremely cold 4 2

3 Human Physiology: Body Temperature Skin temperature temperature of extremities is critical factor for comfort in the cold hand-skin temperature: 20 C uncomfortably cold 15 C extremely cold 5 C painful t air - 35 C lower limit for useful outdoor activity 30 min exposure at t air = -75 C reported skin surface temperature measured by: - radiometer - thermocouples -etc. 5 Human Physiology: Body Temperature Local skin temperature Skin temperaturte measurement with a thermistor harness 6 3

4 Thermal Comfort: Body s Heat Balance Heat production: continuous, in rest 1 W/kg (body weight) Transport of heat: Conduction through tissues Blood flow to the skin Temperature control: hypothalamus (human thermostat) thermal receptors (human temperature sensors) cold & warm receptors static & dynamic discharge of thermal receptors response to mean temperature and rate of temperature change 7 Thermal Comfort: Heat Balance Regulation Cold environment temperature centre starts tensions in the muscles start of metabolic process heat production (up to 3 times basal metabolism) muscle work heat production (up to 10 times basal metabolism) cold receptors impulses to the brain vaso-constriction decreased blood flow decreased heat flow heat loss by conduction through the skin remains 8 4

5 Thermal Comfort: Heat Balance Regulation Warm environment high skin temperature small temperature gradient between body core and skin surface small heat exchange by conduction vasodilation increased blood flow to skin surface (10 times increase; hands & feet 30 times) evaporation of sweat controlled (by hypothalamus) evaporation secretion of water from sweat glands uncontrolled continuous evaporation water diffusion through the skin (persipation insensibili) water in exhalation ( 40 g/h) 9 Heat Balance Equation: S = M ± W ± R ± C ± K E RES [W/m 2 ] S rate of heat storage M rate of metabolic heat production W rate of mechanical work accomplished R rate of heat exchange by radiation C rate of heat exchange by convection K rate of heat exchange by conduction E rate of heat exchange by evaporation RES rate of heat exchange by respiration 10 5

6 Body surface area Metabolism, M Α D = m l A D DuBois surface area [m 2 ]; M mass [kg]; L height [m] A D = 1.8 m 2 for a 1.73 m tall, 70 kg man Metabolic rate increases in proportion to exercise intensity Unit used is met : 1 met = 58.1 W/m 2 = 50 kcal/(h m 2 ) Varies over wide range: 0.7 met - sleeping, ~8 met - wrestling Maximum capacity: 20 met - healthy man at age 20, 7 met - at age 70 Maximum rate for women is 30% lower than for man 11 Metabolism 12 6

7 Metabolism 13 Mechanical work - expressed in body s mechanical efficiency: μ =W/M - μ > is unusual, typically μ is close to 0 - μ max = under optimal conditions (e.g. bicycle ergometer) - for office work W<<M - estimates for M are often inaccurate - W = 0 used for design of HVAC give more conservative estimate Example of estimation of mechanical work: 90 kg person walking up a 5% grade at 1.0 m/s would be lifting an 882 N (90 kg 9.8 N/kg) weight over a height of 0.05 m every second, for a work rate of 44 (Nm)/s = 44 W. This rate substracted from M gives the net heat generated. 14 7

8 Respiration, RES: small, 2 5 W/m 2 RES = C res + E res C res = M (34 t a ) E res = M (5.87 p a ) C res - is sensible heat loss by convection E res - is latent heat loss by evaporation of heat and water vapour from the respiratory tract p a - is ambient water vapour pressure (in kpa) t a - is ambient air temperature 15 Evaporation, E: E = E rsw + E dif E dif diffusion of water through the skin (approx. 10 W/m 2 ) E rsw evaporation of sweat from the skin surface by regulatory sweating: W/m 2, sweating limitations, large individual differences, acclimatisation occurs - skin wettedness, w, is the fraction of the skin covered with water to account for the total evaporation rate - E max occurs when the skin is completely wet (w=1) - skin wettedness caused by diffusion is skin wettedness caused by regulatory sweating is w rsw w rsw = E rsw /E max E dif = (1-w rsw )0.06E max w = E rsw /E max E = w E max 16 8

9 Convection, C: C = h c (t sk t a ) [W/m 2 ] h c = f (v, Tu, geometry, Δt, etc.) C increases with the increase of v and decrease of t a free convection: Gr>Re 2 (v < 0.1 m/s) forced convection: Re 2 >Gr (v > 0.2 m/s) mixed convection: Gr Re 2 (0.1 m/s < v < 0.2 m/s) h c convection heat transfer coefficient [W/m 2 C], t a air temperature [ C], t sk average skin surface temperature [ C], v air velocity [m/s], Tu Turbulence intensity [%], Δt=(t sk t a ) [ C], Re Reynolds number, Gr Grashof number 17 Table of convection coefficients 18 9

10 Radiation, R: R = h r (t sk t r ) [W/m 2 ] h r = 4 εσ(a r /A D )[ (t cl +t r )/2] 3 h r radiativ heat transfer coefficient [W/(m 2 K)] t r mean radiant temperature [ C] t cl average temperature of clothing surface [ C], ε average emissivity of clothing or body suface [-] σ Stefan-Boltzmann constant, 5.67x10-8 [W/(m 2 K 4 )] A r effctive radiation area of body, [m 2 ] A r /A D is 0.70 for a sitting person and 0.73 for a standing person. ε is close to unity (typcally 0.95). For typical indoor temperature h r is nearly constant, 4.7 [W/(m 2 K)] and if emissivity is significantly less than unity adjust h r by h r =4.7 ε, where ε is the area weighted average emissivity for the clothing/body surface. 19 Clothing thermal insulation F cl = (t cl t o )/ (t sk t o ) F cle thermal efficiency of clothing [-] Clothing thermal insulation I cl in clo units: 1 clo = [m 2 K/ W] 20 10

11 Clothing insulation 21 11

Chapter 12. Temperature Regulation. Temperature Regulation. Heat Balance. An Overview of Heat Balance. Temperature Regulation. Temperature Regulation

Chapter 12. Temperature Regulation. Temperature Regulation. Heat Balance. An Overview of Heat Balance. Temperature Regulation. Temperature Regulation Chapter 12 Body core temperature regulation Critical for: Cellular structures Metabolic pathways Too high Protein structure of cells destroyed Too low Slowed metabolism Cardiac arrhythmias Homeothermic