Chapter 2 The Human Thermal

System

Chapter 2 The Human Thermal

System

• 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.

• 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 1F or 2F higher in temperature than the surrounding tissues. In general, therefore, the body “core” is usually significantly warmer than the body’s extremities and surfaces.

Isotherms in the body. Left, isotherms in a warm environment; right, in a cold environment.

• 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.

Food Energy(100%)

Free energy potentially available

(95%)

Free energy pool(45%)

Heat(50%)

Heat(5%)

Heat(45-20%)

Structure and chemical integrity of body

Internal work

Skeletal muscle contraction

External work0-25%

Entropy change

Biochemical inefficiency

Summary of the distribution of ingested food energy within the body and its transfer to the environment

• 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.

Example 2-1 body temperature rise with no heat loss

• 某人體重 68kg, 基礎代謝率 (BMR):72kcal/hr, 熱容量 :0.86kcal/kg‧℃ , 在人體無熱量散失的情況下體溫上升的速率為何？

• 公式： BMR= 體重 x 熱容量 x 溫度變化率

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 (Qel+Qsl)• Heat Conduction Through Clothing (Qcl)• Overall Heat Loss From the Surface of the body Q = Qr + Qc + Qd + Qe + Qel + Qsl + Qcl

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vkk TTAkQloss heat Convective B

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• A. Radiative Heat losses from the body

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Example 2-2 Estimation of Radiative Heat Loss from body

• 某人表面積 Ar=1.4m2, 此人體表溫度為33℃, 環境溫度為 29℃ ， es=0.97, Kr=7kcal/hr‧m2‧℃ 。

• 求 radiative heat loss ？

• B. Convective heat losses from the body

環境溫度體表溫度有效對流散熱面積

）對流傳導係數（

對流散熱量

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Example 2-3 Heat loss via forced convection

• 某人體表面積為 1.4m2, 體表溫度為 33℃ 環境溫度為 29 ℃, 其有效對流面積為總面積的 80% ，風速為 0.447m/sec 。

• 求 Heat loss via forced convection ？

• C. Heat loss by diffusion of water through skin

環境水氣分壓體表水蒸氣壓體表面積

量水分透過皮膚擴散散熱

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Example2-4 Heat loss by diffusion of water through skin

• 某人體表面積為 1.8m2, 體表溫度為 33℃ 環境水氣分壓為 0 。

• 求體表水蒸氣壓？• 求 Heat loss by diffusion of waterthro

ugh skin ？

• D. Heat loss by sweet secretion

環境水蒸氣分壓體表水蒸氣分壓濕潤體表面積蒸發散熱係數排汗的散熱量

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Example 2-5 Heat loss by sweet secretion

• 某人濕潤體表面積為 1.5m2, 體表溫度 33℃, 在風速為 0.45m/sec, 室溫 70℉, 1大氣壓下，相對濕度為 30%

• 求 heat loss by sweet secretion?(70℉, 一大氣壓下飽和水蒸氣壓為 18.8mmHg)

• E. Heat loss Associated with respiration

含水量每單位時間呼出空氣之含水量每單位時間吸入空氣之

水轉為水蒸氣之潛熱

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吸入空氣溫度呼出空氣溫度

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感應散熱量

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Example 2-6 heat loss associated with respiration

• 每分鐘吸入 7.2g, 20℃ 乾空氣 , 呼出 37 ℃ 空氣，其中含 0.295g 水蒸氣 , 空氣熱容量為 0.25cal/g‧℃

• 求 heat loss associated with respiration

• F. Heat conductin through clothing

衣物因子衣服溫度體表溫度：體表面積透過皮膚的傳導散熱

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2-3 Heat Transfer within the

Body • The preceding discussion has centered on

heat transfer from the body’s 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.

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•heat transfer from the body

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A. 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 arterie

s and veins often occurs to a significant extent. ：

週微血管循環

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主要動脈

主要靜脈

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To characterize heat transfer in the nude body on an overall basis, Ruch and Patton use the definition

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and determine a “mean” surface temperature Ts according to t

he formular

Ts=0.07Tfeet+0.32Tlegs+0.18Tchest+0.17Tback+0.14Tarms+0.05Thands

+0.07Thead

This formular weights the temperature of each region in proportion to the fraction of surface area

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. Bm

core muscle skin

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ties與血流速成線性關係

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整理係數併成相同型式