84 Efficiency of the Human Body
The Energetic Functions of the Body
All bodily functions, from thinking to lifting weights, require energy. The many small muscle actions accompanying all quiet activity, from sleeping to head scratching, ultimately become thermal energy, as do less visible muscle actions by the heart, lungs, and digestive tract. The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate. The total energy conversion rate of a person at rest is called the basal metabolic rate (BMR) and is divided among various systems in the body, as shown the following table:
|Organ||Power consumed at rest (W)||Oxygen consumption (mL/min)||Percent of BMR|
|Liver & spleen||23||67||27|
|Totals||85 W||250 mL/min||100%|
The First Law of Thermodynamics
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 (). 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 (). 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 states that energy cannot be created or destroyed. Therefore, if the body does useful work to transfer to its surroundings (), or transfer to the environment as , then that energy must have come out of the body’s . We observe this throughout nature as the :
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 dumping 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.
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:
The mechanical efficiency of the body is limited because energy used for metabolic processes cannot be used to do useful work. Additional generated during the chemical reactions that power muscle contractions along with in joints and other tissues reduces the efficiency of humans even further..
“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.” 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 m high flight of stairs? How much does the person transfer to the environment as ?
First, lets calculate the change in :
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
The used came out of the person’s so:
We can use the to find the thermal energy exhausted by the person:
Rearranging for :
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:
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:
Conservation of Mass and of Energy
The digestive process is basically one of oxidizing food, so energy consumption is directly proportional to oxygen consumption. Therefore, we can determine the the actual energy consumed during different activities by measuring oxygen use. The following table shows the oxygen and corresponding energy consumption rates for various activities.
|Activity||Energy consumption in watts||Oxygen consumption in liters O2/min|
|Sitting at rest||120||0.34|
|Sitting in class||210||0.60|
|Walking (5 km/h)||280||0.80|
|Cycling (13–18 km/h)||400||1.14|
|Ice skating (14.5 km/h)||545||1.56|
|Climbing stairs (116/min)||685||1.96|
|Cycling (21 km/h)||700||2.00|
|Cycling, professional racer||1855||5.30|
We often talk about “burning” calories in order to lose weight, but what does that really mean scientifically?. First, we really we mean lose because 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 first 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 second video below.
- Significant content in this chapter was adapted from: OpenStax, College Physics. OpenStax CNX. Aug 30, 2019 http://firstname.lastname@example.org ↵
- "The maximum work and mechanical efficiency of human muscles, and their most economical speed" by A. V. Hill,National Library of Medicine, U.S. National institutes of Health ↵
- "MPG of a Human" by Tom Murphy, Do The Math, UCSD Physics Department ↵
- "1917.120 - Fixed stairways." by Occupational Safety and Health Administration, United States Department of Labor ↵
- "Managing Water in the West: Hydroelectric Power" by Bureau of Reclimation, U.S. Department of the Interior ↵
- "Efficiency in Electricity Generation" by EURELECTRIC “Preservation of Resources” Working Group’s “Upstream” Sub-Group in collaboration with VGB, Union of the Electricity Industry ↵
- "Efficiency of Energy Conversion Devices" by John E. Dutton e-Education Institute, Penn State College of Earth and Mineral Sciences ↵
energy stored in the chemical bonds of a substance
the total of a systems thermal energy and chemical potential energy, the total energy stored microscopically within the system
energy stored in the microscopic motion of atoms and molecules (microscopic kinetic energy)
An amount of thermal energy transferred due to a difference in temperature.
heat transferred to the environment rather than being used to do useful work
Energy cannot be created or destroyed, only transferred from one type to another and/or from one object to another
the sum of potential and kinetic energy
the change in internal energy of a system is equal to the heat added to the system minus the work done by the system
A quantity representing the effect of applying a force to an object or system while it moves some distance.
the process of burning, the rapid chemical combination of a substance with oxygen, involving the production of heat and light
work done on the external environment, such as moving objects, as apposed to work done internally, such as pumping blood
devices for converting thermal energy to useful work and exhaust heat
ratio of useful work performed to total energy expended
the effectiveness of a machine in transforming the energy input to the device to mechanical energy output
a force that acts on surfaces in opposition to sliding motion between the surfaces
potential energy stored in objects based on their relative position within a gravitational field
unit of energy equivalent to 4.184 Joules
International standard (SI) unit of Energy
a measurement of the amount of matter in an object made by determining its resistance to changes in motion (inertial mass) or the force of gravity applied to it by another known mass from a known distance (gravitational mass). The gravitational mass and an inertial mass appear equal.
A quantity representing the capacity of an object or system to do work.