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Chapter 2 The Human Thermal System

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Title: Chapter 2 The Human Thermal System


1
Chapter 2 The Human Thermal System
2
  • In this chapter we discuss the magnitude and
    source of heat production in the body, the ways
    in which heat is transported internally from one
    region of the body to another, and the various
    means by which heat is lost to the external
    environment. We shall quantify the current state
    of knowledge where possible, and solve a few
    representative problems.

3
  • 2-1 Heart Production
  • The organs that are most active mechanically and
    chemically, and which therefore produce the most
    heat (liver, heart, brain), generally run 1?F or
    2?F higher in temperature than the surrounding
    tissues. In general, therefore, the body core
    is usually significantly warmer than the bodys
    extremities and surfaces.

4
Isotherms in the body. Left, isotherms in a warm
environment right, in a cold environment.
5
  • The heat produced in the body is derived from the
    breakdown, synthesis, and utilization of food.
    The body utilizes almost all food by a scheme
    that involves the formation of a compound called
    adenosine triphosphate (ATP), and the subsequent
    use of this ATP to supply energy for muscle
    contraction, chemical synthesis, etc.

6
Summary of the distribution of ingested food
energy within the body and its transfer to the
environment
7
  • The end goal of foodstuff breakdown is to provide
    simple raw materials which the cells of the body
    can to synthesize high energy compounds from
    lower energy compounds. The most important such
    high energy molecule is adenosine triphosphate
    (ATP). This compound, synthesized from adenosine
    diphosphate(ADP), has an energy 8 kcal/mole
    greater than ADP, and it is this energy which is
    released in a controlled manner when needed by
    the body.

8
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9
Example 2-1 body temperature rise with no heat
loss
  • ????68kg, ?????(BMR)72kcal/hr, ???0.86kcal/kg??
    ,??????????????????????
  • ??BMR??x???x?????

10
2-2 Loss of Heat to the Environment
  • Radiative Heat Losses from the body (Qr)
  • Convective Heat Losses from the Body(Qc)
  • Heat Loss by Diffusion of Water Through Skin (Qd)
  • Heat Loss by Sweat Secretion (Qe)
  • Heat Loss Associated with Respiration (QelQsl)
  • Heat Conduction Through Clothing (Qcl)
  • Overall Heat Loss From the Surface of the body
  • Q Qr Qc Qd Qe Qel Qsl Qcl

11
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12
  • A. Radiative Heat losses from the body

13
Example 2-2 Estimation of Radiative Heat Loss
from body
  • ?????Ar1.4m2, ???????33?,?????29?,es0.97,
    Kr7kcal/hr?m2???
  • ?radiative heat loss?

14
  • B. Convective heat losses from the body

15
Example 2-3 Heat loss via forced convection
  • ???????1.4m2, ?????33??????29 ?,????????????80,??
    ?0.447m/sec?
  • ?Heat loss via forced convection?

16
  • C. Heat loss by diffusion of water through skin

17
Example2-4 Heat loss by diffusion of water
through skin
  • ???????1.8m2, ?????33????????0?
  • ????????
  • ?Heat loss by diffusion of waterthrough skin?

18
  • D. Heat loss by sweet secretion

19
Example 2-5 Heat loss by sweet secretion
  • ?????????1.5m2, ????33?,????0.45m/sec, ??70?,
    1????,?????30
  • ?heat loss by sweet secretion?
  • (70?,????????????18.8mmHg)

20
  • E. Heat loss Associated with respiration

21
Example 2-6 heat loss associated with respiration
  • ?????7.2g, 20????,??37 ???,???0.295g???,??????0.25
    cal/g??
  • ?heat loss associated with respiration

22
  • F. Heat conductin through clothing

23
2-3 Heat Transfer within the Body
  • The preceding discussion has centered on heat
    transfer from the bodys surface to the
    surrounding environment, and involved various
    equation and correlations relating Ts, Ps, Ta,
    and Pa to several heat loss modes. From the body
    core to the environment there occurs a series of
    resistances, such as muscle, fat, skin, trapped
    air under clothing exterior. Each region has its
    own temperature, lying between Tc and Ta.

24
  • heat transfer from the body

25
  • role of blood circulation in internal heat
    Transfer
  • The circuleation affects internal heat
    distribution in three major ways
  • It minimizes temperature differences within the
    body.
  • It controls effective body insulation in the skin
    region.
  • Countercurrent heat exchange between major
    arteries and veins often occurs to a significant
    extent.

26
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27
To characterize heat transfer in the nude body on
an overall basis, Ruch and Patton use the
definition
and determine a mean surface temperature Ts
according to the formular Ts0.07Tfeet0.32Tlegs
0.18Tchest0.17Tback0.14Tarms0.05Thands0.07Thea
d This formular weights the temperature of each
region in proportion to the fraction of surface
area
28
  • B. simple model for heat transfer between core
    and skin
  • For simplicity, a rectangular geometry is
    assumed. Each region is considered to be
    characterized by some sort of average temperature
    Tc, Tm, or Ts. Metabolic heat production is
    ignored, steady state is assumed, and the rate of
    blood flow from each region to the next is taken
    as (g/sec), a constant.

29
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