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Chapter 2 The Human Thermal System

Chapter 2 The Human Thermal System.

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Chapter 2 The Human Thermal System

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  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 body’s 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. Food Energy (100%) Entropy change Heat (5%) Free energy potentially available (95%) Biochemical inefficiency Heat (50%) Free energy pool (45%) Structure and chemical integrity of body Heat (45-20%) Internal work Skeletal muscle contraction External work 0-25% 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 materialswhich 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. 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

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

  10. A. Radiative Heat losses from the body

  11. 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？

  12. B. Convective heat losses from the body

  13. 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？

  14. C. Heat loss by diffusion of water through skin

  15. 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 waterthrough skin？

  16. D. Heat loss by sweet secretion

  17. 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)

  18. E. Heat loss Associated with respiration

  19. 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

  20. F. Heat conductin through clothing

  21. 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.

  22. Tair Tskin (同電阻串併聯) Rc Rtotal RSC Rr • heat transfer from the body

  23. 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. ：

  24. 皮膚 主要動脈 close 熱交換 open 主要靜脈 週微血管循環

  25. To characterize heat transfer in thenude body on an overall basis, Ruch and Patton use the definition and determine a “mean” surface temperature Ts according to the 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

  26. 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.

  27. △Zms core muscle skin Tc Tm Ts Ta △Zcm 整理係數併成相同型式 where k’cm and k’ms are effective thermal conductivities 與血流速成線性關係

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