83 Efficiency of the Human Body

The Energetic Functions of the Body

We have learned so far that your body takes in , and then does to convert that into for , for storage, and .  The following diagram summarizes the basic energetic functioning in the human body. (Electric potential energy is important to nerve conduction and other processes in the body, and we have mentioned that chemical potential energy is actually a form of electric potential energy, but we will not specifically discuss electric potential energy in this textbook.)

A box labeled "the body does work" has an arrow labeled "input" pointing inward from the left. The arrow starts from a box labeled "Chemical Potential Energy." An arrow labeled "output" points outward from the body box to the right and toward a pair of boxes labeled "Potential Energy" and "Kinetic Energy." The potential energy box contains the terms "chemical, electrical, spring, gravitational." The terms chemical and electrical are connected to the terms "growth/repair, nerve function, cellular metabolism, storage." The terms spring, gravitational, and kinetic energy are connected to the the term "motion." An arrow labeled "heat" points outward and upward from the top of the central purple box and toward a box labeled "thermal energy." Small, thin arrows connect the output and input arrows to the word "efficiency," and separately the input and heat arrows to the word "entropy."
The most basic functions of the human body mapped to the main concepts covered in this textbook.

The body is capable of storing and thermal energy internally. Remembering that thermal energy is just the kinetic energy of atoms and molecules, we recognize that these two types of energy are stored microscopically and internal to the body. Therefore, we often lump these two types of microscopic energy into the (U). When on object is warmer then its surroundings then will be transferred from object to surroundings, but if the object is cooler than its surroundings then thermal energy will transferred into the object from its surroundings. The amount of thermal energy exchanged due to temperature differences is often called (Q). When heat is transferred out of the body to the environment, we say call this , as indicated in the previous figure.  We will learn more about how temperature and heat transfer are related in the next unit.

The First Law of Thermodynamics

The states that energy cannot be created or destroyed. Therefore, if the body does useful work to transfer to its surroundings (W_{by}), or transfer to the environment as , then that energy must have come out of the body’s . We observe this in nature as the :

(1)   \begin{equation*} \Delta U =  Q - W_{by} \end{equation*}

Your body uses stored internally to do , and that process also generates thermal energy, which you release as . The internal combustion engines that power most cars operate in similar fashion by converting chemical potential energy in fuel to thermal energy via , then converting some of the thermal energy into and some into . Your body is capable of releasing the chemical potential energy in your food without combustion, which is good, because you are not capable of  using your from your to do . Machines that can use to do work, such as a combustion engine, are known as .  Heat engines are still governed by the , so any must have been thermal energy that was not used to do work. The thermal energy input that can be used to do work rather than wasted as determines the of the heat engine.

Reinforcement Exercises

You apply a 50 N force in order to slide an object 3 m. How much did you do?

If you exhausted 500 worth of while doing the useful work above, how much did your change?

Body Efficiency

The of the human body in converting into is known as the of the body.  We often calculate the body’s mechanical efficiency, as a percentage:

(2)   \begin{equation*} e_{body} = \frac{W_{by}}{Chemical\, Potential\, Energy\, Used}\times 100\,\bold{\%} \end{equation*}

generated during the chemical reactions that power muscle contractions along with in joints and other tissues reduce the efficiency of humans to about 25 %.[1]

“Alas, our bodies are not 100 % efficient at converting food energy into mechanical output. But at about 25 % efficiency, we’re surprisingly good considering that most cars are around 20 %, and that an Iowa cornfield is only about 1.5 % efficient at converting incoming sunlight into chemical [potential energy] storage.”  [2]For an excellent discussion of human and comparisons with other machines and fuel sources, see MPG of a Human by Tom Murphy, the source of the previous quote.

Everyday Example: Energy to Climb Stairs

Assuming a 20% in climbing stairs, how much does your decrease when a 65 kg person climbs a 15 high flight of stairs? How much does the person transfer to the environment as ?

First, lets calculate the change in :

    \begin{equation*} \Delta PE_g =mg\Delta h = (65\,\bold{kg}) (9.8\,\bold{m/s/s})(15\,\bold{m}) = 9,555\, \bold{J} \end{equation*}

The person did in converting in their body to mechanical energy, specifically . However, they are only 20% efficient, which means that only 1/5 of the chemical potential energy they use goes into doing useful work. Therefore the change in chemical potential energy must have been 5x greater than the mechanical work output

    \begin{equation*} Chemical\, Potential\, Energy\, Used = (5 \times 9,555\, \bold{J})= 47,775\,\bold{J} \end{equation*}

The used came out of the person’s so:

    \begin{equation*} \Delta U = - 47,775 \,\bold{J} \end{equation*}

We can use the to find the thermal energy exhausted by the person:

(3)   \begin{equation*} \Delta U =  Q - W_{by} \end{equation*}

Rearranging for Q:

    \begin{equation*} Q = \Delta U + W_{by} = -  47,775 \,\bold{J}+ 9,555\, \bold{J} = -38,220\, \bold{J} \end{equation*}

We find that the heat is negative, which makes sense because the person exhausts thermal energy out of the body and into to the environment while climbing the stairs.

Alternatively, we could have known right off that the must be 4/5 of the total loss of , because only 1/5 went into doing useful . So the exhaust heat should be:

    \begin{equation*} Q = \Delta U \times (4/5)= -  47,775 \,\bold{J} \times 4/5 = -38,220\, \bold{J} \end{equation*}

Food Calories

For historical reasons we often measure and in units of (cal) instead of .  There are 4.184 Joules per calorie. We measure stored in food with units of 1000 calories, or kilocalories (kcal) and we sometimes write kilocalories as Calories (Cal) with with capital C instead of a lowercase c. For example, a bagel with 350 Cal has 350 kcal, or 350,000 cal. Converting to Joules, that would be 350,000 \,\bold{cal} times (4.184 \,\bold{J}/bold{cal}) =1,464,400\, \bold{J} in the bagel.

Everyday Examples

What fraction of a bagel would you need to eat in order to make up for the 47,775 J  loss of internal energy (as ) that we calculated in the previous everyday example about climbing stairs?

There are 1,464,400  J/bagel

Therefore we need to eat:

    \begin{equation*} \frac{47,775 \,\bold{J}}{1,464,400\, \bold{J}/bagel} = 0.03\, Bagels\, or\, 3\, \bold{\%}\, of \, a\, bagel \end{equation*}

Reinforcement Exercises

How many of energy are stored in a 260 Calorie candy bar?

How many candy bars would you need to eat to make up for the loss of we found in the previous everyday example?

Conservation of Mass and of Energy

We often talk about “burning” calories in order to lose weight,  but what does that really mean scientifically?. First, we really we mean lose that is the measure of how much stuff is in our bodies and weight depends on where you are (it’s different on the moon). Second, our bodies can’t just interchange and — they aren’t the same physical quantity and don’t even have the same units. So how do we actually lose mass by exercising? We don’t actually shed the atoms and molecules that make up body tissues like fat by “burning” them. Instead, we break down the fat molecules into smaller molecules and then break bonds within those molecules to release , which we eventually convert to and .  The atoms and smaller molecules that resulting from breaking the bonds combine to form carbon dioxide and water vapor (CO2 and H2O) and we breath them out. We also excrete a bit as H2O in sweat and urine. The process is similar to burning wood in campfire — in the end you have much less of ash than you did original wood. Where did the rest of the mass go? Into the air as CO2 and H2O. The same is true for the fuel burned by your car. For more on this concept see the video below.

The really amazing fact is that your body completes this chemical process without the excessive temperatures associated with burning wood or fuel, which would damage your tissues. The body’s trick is to use enzymes, which are highly specialized molecules that act as catalysts to improve the speed and of chemical reactions, as described and animated in the beginning of the following video:

General Efficiency

Similar to the body , the efficiency of any energetic process can be described as the amount of energy converted from the input form to the desired form divided by the original input amount. The following chart outlines the efficiencies of various systems at converting energy various forms. The chart does not account for the cost, hazard risk, or environmental impact associated with the required fuel, construction, maintenance, and by-products of each system.

The Efficiency of the Human Body Compared to Other Systems
System Input Energy Form Desired Output Form Max Efficiency
Human Body Chemical Potential Mechanical 25 %
Automobile Engine Chemical Potential Mechanical 25 %
Coal/Oil/Gas Fired Stream Turbine Power Plants Chemical Potential Electrical 47%
Combined Cycle Gas  Power Plants Chemical Potential Electrical 58 %
Biomass/Biogas Kinetic Electrical 40%
Nuclear Kinetic Electrical 36%
Solar-Photovoltaic Power Plant Sunlight (Electromagnetic) Electrical 15%
Solar-Thermal Power Plant Sunlight (Electromagnetic) Electrical 23%
Hydroelectric and Tidal Power Plants Gravitational Potential Electrical 90%+


Check out the energy systems tab in this simulation to visualize different energy conversion systems

Energy Forms and Changes


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