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Estimate the rate at which heat can be conducted from the interior of the body to the surface. Assume that the thickness of human tissue (\SI{0.2}{\watt\per\kelvin\per\meter}) is $\SI{4.0}{cm}$, that the skin is at \SI{34}{\degreeCelsius} and the interior at \SI{37}{\degreeCelsius}, and that the surface area is $\SI{1.5}{\meter\squared}$. Compare this to the measured value of about $\SI{230}{W}$ that must be dissipated by a person working lightly.
The cooling rate by conductivity is: \begin{align} \Phi &= \frac{\Delta Q}{\Delta t}\\ &= \lambda \frac{\Delta T}{\Delta x} A\\ &= \SI{22.5}{W} \end{align} Compared to \SI{230}{W}, this clearly shows the necessity of convective cooling by the blood.
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