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.
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.
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.
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.
- 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.
- A truck travels 20 miles for every gallon of gas.
- Burning a kilogram of gasoline releases 42.4 MJ.
This is the value reported on the Wikipedia.
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.
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.