Heart Energy Per Day Versus Truck Energy Consumed Over 20 Miles

Quote of the Day

The pursuit of wealth is now largely the pursuit of information and its application to the means of production ... One can postulate that in the next few decades the attraction and management of intellectual capital will determine which institutions and nations survive and prosper, and which will not.

— Walter Wriston, Citibank Chairman, in an essay carried in Foreign Affairs magazine.


Introduction

Figure 1: Factoid That I Do Not Believe.

Figure 1: Factoid That I Do Not Believe.

During my web browsing, I sometimes encounter a statement that just seems too incredible to believe. Figure 1 contains such a statement. It states that the human heart "creates" daily an energy equivalent to driving a truck for 20 miles. I thought about this statement for a minute and it makes no sense. I drive a van (similar in mileage to a small truck) that uses roughly a gallon of gasoline for every 20 miles driven – on a very good day. Now a gallon of gasoline is roughly 4 kilograms of a highly energetic substance. A human living for a day will use roughly 2000 kilocalories (kcal – food calories are measured in kilocalories) a day. If you look at the relative energy levels of one gallon of gasoline versus a 2000 kcal, you can see that a gallon of gasoline has far more energy than the entire daily energy use of a human (Figure 2). Of course, the heart would only use a fraction of the total body energy budget.

Figure 2: Total Daily Human Energy Use About 20% That of Gallon of Gasoline.

Figure 2: Total Daily Human Energy Use About 20% That of Gallon of Gasoline.

Let's see if we can get more insight into the power levels involved with the heart. This is a good illustration of how to do some basic pump analysis from an energy standpoint.

Background

First, we need to gather a few facts.

  • The heart has an average mechanical power output of 1.3 W.

    We can roughly estimate the hearts mechanical power and daily work output as shown in Figure 3.

    Figure 3: Heart Mechanical Power Computations.

    Figure 3: Heart Mechanical Power and Work Computations.

  • The heart uses an average of ~6 W of chemical power input to generate 1.3 W of mechanical power output.

    I have seen quotes for the chemical energy consumed by the heart to be ~5 W. Using a my crude model, I compute about 6 W in Figure 4 assuming a chemical-t0-mechanical conversion efficiency of 20% (reference). If you want to know more about this efficiency figure, see this post for more details.

    Figure 4: Chemical Power Consumed By the Heart.

    Figure 4: Chemical Power Consumed By the Heart.

  • A truck travels 20 miles for every gallon of gas.

    That is the mileage of my Ford E150 van, which is similar to a Ford F150 pickup truck.

  • Burning a kilogram of gasoline releases 42.4 MJ.

    This is the value reported on the Wikipedia.

Analysis

Figure 5 shows my calculation of the chemical energy used by the heart in a day compared with the chemical energy used to move a truck 20 mile. The heart uses far less energy per day (~1%) than is contained in one gallon of gasoline.

Figure 4: Comparison of Heart Daily Heart Energy to Energy a Truck Uses Over 20 Miles.

Figure 5: Comparison of Heart Daily Heart Energy to Energy a Truck Uses Over 20 Miles.

Conclusion

I had to take a closer look at the statement that a heart beating for a day consumes as much energy as a truck moving 20 miles. I have shown that the magnitudes of these values are far different.

Appendix A: References on Cardiac Efficiencies

Figure 6 contains a reference table of reported cardiac chemical-to-mechanical work efficiencies. I decided to use 20% as a rough value that is close enough for my purposes.

Figure 5: Reported Cardiac Chemical-to-Mechanical Work Efficiencies.

Figure 6: Reported Cardiac Chemical-to-Mechanical Work Efficiencies.

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16 Responses to Heart Energy Per Day Versus Truck Energy Consumed Over 20 Miles

  1. Pingback: An Estimate of the Heart’s Chemical to Mechanical Efficiency | Math Encounters Blog

  2. Yep says:

    you realize the the statement was in relation to the % of energy used in total for the truck is the same as the heart right? Just the saying is short......you have to see whats missing more clearly.

     
    • mathscinotes says:

      No, I do not realize that. Could you explain your statement a bit more? I would love to see how others interpret this statement differently.

      There is actually more to this quote than I show in this blog post. The rest of the quote compares the heart energy to driving to the Moon.

      Every day, the heart creates enough energy to drive a truck 20 miles. In a lifetime, that is equivalent to driving to the moon and back.

      Again, how does a percentage related to this?

      mathscintoes

       
    • Kurt says:

      So apperently a gallon of gas equals 42.4 mj
      A heart creates. 8.4 mj
      The heart use.11 in its day creates 8.4 mj now that Efficent, so the heart used all that energy it would more than like pass the truck chew up42.4 mj and only harnessing a minuscule amount.
      There for if the heart was big gross pig on energy than yes it makes no sense.
      The fact that it produces eight times the energy to than it needs... in think to the moon and back is like a stroll in the park.

       
  3. CarlK says:

    I think that your math is as sound as needed to show that the quote doesn't hold. The heart doesn't create enough energy to drive a truck 20 miles a day. The part about going to the moon and back is just some simple math as well on the part of the original quote, as 20 miles per day for an average of 76 years would be 554800 miles total. They are saying that the moon would be a 477800 mile round trip on average, as we're not taking into account variance in the elliptical orbit. This is a simplistic notion really, but the entire quote was simplistic by nature.

     
  4. Rob says:

    Thanks for the analysis. I saw that quote and thought it surely must be *very* wrong. It would have been nice to see just how far the heart would have moved your truck in one day. At 1% of the available energy, it would be 320metres but the chemical to mechanical energy conversion is inefficient (6:1.3) so it's reduced to 69m. Does that sound right?

     
    • mathscinotes says:

      The conversion efficiencies of the heart and of an automobile are remarkably similar. The chemical-mechanical conversion efficiency of the heart is 1.3 W/6 W = 21.7%. The Wikipedia reports that the chemical-mechanical conversion efficiency of an iron engine is ~20%. Let's assume they are equal for the sake of argument – the error bars on these numbers are pretty big. Since the conversion efficiencies are equal, we can just setup the following comparisons of total energy consumed to distance.
      Heart Mileage
      This is one way to look at it.

      mark

       
  5. rich says:

    It actually really depends on how you look at it. "Drive" is kinda vague.

    Gasoline has a lot of energy. But cars running at high speeds require more energy. And the thermo efficiency of combustion engines would require more gas than needed to drive the car.

    The heart certainly would not provide the same energy as the gasoline needed.

    But, in terms of moving the car, the heart (or rather the entire human's metabolic process) would create enough energy to move a truck on neutral 20 miles. If you pushed a truck at 100N of force over 20 miles, you'd use less than 2000 calories of energy.

    Still, I believe the quote was made by someone with no real knowledge of the physics and just wanted a popular meme.

     
    • Kender Ostlund says:

      Thats actually a really interesting point. If you consider a really healthy human is pushing the truck, averaging 20 miles a day is tough but possible assuming totally flat ground. Using less than 2000 calories seems unlikely though but thats just me saying when I work out a lot in a day I tend to eat a lot and pushing a truck 20 miles seems like quite the workout so definitely not scientific. That being said its definitely not the heart alone and so it doesn't really fit the semantics of the original quote about the heart but is kinda interesting that the heart plays an integral part in an organism that is efficient enough to "love you to the moon and back" in a lifetime feasibly. Thats assuming that 20 miles a day is actually enough to get to to the moon and back in a day. (Just checked it comes out to about 65 years of pushing at 20 miles per day to get to moon and back.)

       
  6. Amey says:

    How much Beats does humans have in a day? Have you considered it?

     
    • mathscinotes says:

      The analysis does use the heart's beat rate (assumed RBeat=70 per minute), so I did consider it. There are 1440 minutes in a day, so the total number of beats per day is 1440·70=100,800 per day.

      mark

       
  7. Coach Mike says:

    I think it is important to compare power created with power needed vs. potential energy of the fuel involved.

    So if the heart produces 1.3W of power per day...or 0.00175 Horsepower (HP)...

    HP Required to move an object of Weight "W" 0.25 miles in Time "T" hours is given by: HP=W/[T/5.825]^3.

    Using a 26,000 lb truck and 0.00175 HP, T is approximately 0.398 hours per 0.25 miles.
    This translates into 0.62 miles per hour.
    0.62 x 24 hours per day ~ 15.075 miles per day

    Distance to the moon =238,900 miles. Round trip = 477,800

    477,800 miles / 15.075 miles per day ~31,695 days to make the round trip at 1.3W.

    31,695 days / 365 days per year ~ 87 years to make the round trip at 1.3W.

    So, taking the 26,000 lb truck "to the moon and back" consumes approximately 87 years of heart power...and that's the power of love....

     
    • Kender Ostlund says:

      Good point that just because the heart uses less chemical energy in a day than the truck does to move 20 miles does not mean the heart does not produce enough energy in a day to move a truck 20 miles.

       
  8. Pingback: How Many Red Blood Cells Do We Lose A Day? | Math Encounters Blog

  9. Tom says:

    Working pressure is 120mmhg-70mm dystolic=50mmhg
    That is the actual differential.
    Tom.

     

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