7.8 Work, Energy, and Power in Humans
3 min read•june 18, 2024
The human body is a marvel of conversion, transforming food into usable forms like ATP. This process fuels everything from basic functions to physical activities. Understanding how our bodies use energy is key to grasping the complexities of human physiology.
Energy output can be measured and calculated, helping us quantify our body's and . Factors like body size, age, and genetics influence our , affecting how much energy we use daily. This knowledge is crucial for maintaining health and energy balance.
Energy Conversion and Usage in the Human Body
Energy conversion in human body
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- Human body converts chemical energy from food into usable forms of energy
- Cellular respiration breaks down glucose to release energy in the form of
- Aerobic respiration occurs in the presence of oxygen, yielding 36-38 ATP molecules per glucose molecule
- Anaerobic respiration occurs in the absence of oxygen, yielding only 2 ATP molecules per glucose molecule (lactic acid fermentation)
- Cellular respiration breaks down glucose to release energy in the form of
- Energy is used for various bodily functions and activities
- is the minimum energy required to maintain vital functions at rest (breathing, circulation)
- Physical activity uses energy to fuel muscle contractions and movement
- ATP is consumed by muscle fibers to generate force and motion ( cross-bridge cycling)
- uses energy to maintain a constant body temperature (shivering, sweating)
- Digestion and absorption require energy to process and absorb nutrients from food (, active transport)
Calculation of energy output
- Energy output can be measured in calories (cal) or joules (J)
- 1 = 4.184 joules
- Work (W) is the product of force (F) and displacement (d) in the direction of the force:
- (P) is the rate at which work is done or energy is transferred over time (t):
- Efficiency is the ratio of output to total energy input, expressed as a percentage:
- Example calculations:
- Lifting a 10 kg object 1 meter:
- Running at a power output of 200 watts for 30 minutes:
- If a person consumes 100 kcal and performs 25 kcal of useful work, their efficiency is:
Energy Forms and Conservation
- in the human body is the sum of kinetic and
- is associated with motion (e.g., running, throwing)
- Potential energy is stored energy due to position or configuration (e.g., stretched muscles, elevated limbs)
- The principle of applies to the human body
- Energy can be converted between different forms but cannot be created or destroyed
- This principle is fundamental to understanding energy balance and
Factors Affecting Energy Consumption
Factors affecting metabolic rate
- (BMR) is influenced by several factors
- Body size and composition
- Larger individuals generally have higher BMRs due to more metabolically active tissue (organs, muscle)
- Lean body mass (muscle) has a higher metabolic rate than fat tissue
- Age
- BMR tends to decrease with age due to changes in body composition (muscle loss) and hormonal factors
- Sex
- Men typically have higher BMRs than women due to greater muscle mass and body size
- Genetics
- Some individuals may have inherently higher or lower BMRs due to genetic variations (mitochondrial efficiency)
- Hormonal factors
- Thyroid hormones play a significant role in regulating BMR (T3, T4)
- Conditions such as hyperthyroidism can increase BMR, while hypothyroidism can decrease it
- Body size and composition
- Impact on overall energy consumption
- A higher BMR leads to greater daily energy expenditure, even at rest
- Individuals with higher BMRs may require more calories to maintain their body weight (energy balance)
- Changes in BMR can affect the balance between energy intake and expenditure, influencing weight gain or loss (obesity, malnutrition)
- and
- The laws of thermodynamics govern energy transformations in the body
- Homeostasis involves maintaining a stable internal environment, which requires continuous energy expenditure
Key Terms to Review (28)
ATP (Adenosine Triphosphate): ATP, or adenosine triphosphate, is the primary energy currency in living cells. It is a high-energy molecule that stores and transports chemical energy within the body, powering a wide range of cellular processes essential for life.
Basal metabolic rate: Basal Metabolic Rate (BMR) is the amount of energy expended while at rest in a neutrally temperate environment, in the post-absorptive state. It represents the minimum amount of energy required to keep your body functioning, including breathing and circulation.
Basal Metabolic Rate (BMR): Basal Metabolic Rate (BMR) is the minimum amount of energy the body requires to perform basic life-sustaining functions at rest, such as maintaining organ function, regulating body temperature, and supporting cell growth and repair. It represents the baseline energy expenditure of the body when the individual is in a fasted, rested state.
Calorie: A calorie is a unit of energy that measures the amount of heat required to raise the temperature of one gram of water by one degree Celsius. This term is crucial for understanding how energy is consumed and expended in biological systems, particularly in relation to human metabolism and physical activity.
Carnot efficiency: Carnot efficiency is the maximum possible efficiency that a heat engine can achieve, operating between two thermal reservoirs. It is determined solely by the temperatures of the hot and cold reservoirs.
Conservation of Energy: Conservation of energy is a fundamental principle in physics that states the total energy of an isolated system remains constant, it is said to be conserved over time. Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.
Efficiency: Efficiency is a measure of how well a system or process converts input energy or resources into useful output, with minimal waste or losses. It is a fundamental concept in physics, engineering, and various other fields, as it quantifies the performance and optimization of systems and devices.
Elastic potential energy: Elastic potential energy is the energy stored in an object when it is deformed elastically, such as when a spring is stretched or compressed. It can be calculated using the formula $U = \frac{1}{2} k x^2$, where $k$ is the spring constant and $x$ is the displacement from equilibrium.
Electric power: Electric power is the rate at which electrical energy is transferred by an electric circuit. It is typically measured in watts (W).
Energy: Energy is the capacity to do work or cause change. It is the fundamental currency that powers all physical and chemical processes in the universe, from the motion of subatomic particles to the dynamics of entire galaxies. Energy is a unifying concept that connects diverse areas of physics, including mechanics, thermodynamics, electromagnetism, and quantum mechanics.
Homeostasis: Homeostasis is the process by which living organisms maintain a stable and relatively constant internal environment, despite changes in external conditions. It is a fundamental principle that allows the body to function optimally and adapt to various stresses and challenges.
Internal kinetic energy: Internal kinetic energy is the sum of the kinetic energies of all particles within a system. It plays a crucial role in understanding how energy is distributed and conserved during elastic collisions.
Joule: A joule is the SI unit of work or energy, equivalent to one newton-meter. It measures the amount of work done when a force of one newton displaces an object by one meter in the direction of the force.
Joule: The joule (J) is the standard unit of energy in the International System of Units (SI). It represents the amount of work done or energy expended when a force of one newton acts through a distance of one meter. The joule is a fundamental unit that connects various topics in physics, from work and energy to thermodynamics and electricity.
Kinetic Energy: Kinetic energy is the energy of motion possessed by an object. It is the energy an object has by virtue of being in motion and is directly proportional to the mass of the object and the square of its velocity. Kinetic energy is a crucial concept in physics, as it relates to the work done on an object, the conservation of energy, and various other physical phenomena.
Law of conservation of energy: The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another. The total energy in an isolated system remains constant over time.
Mechanical Energy: Mechanical energy is the sum of kinetic energy and potential energy in a system, representing the total energy available for performing work. This concept encompasses various forms of energy related to motion and position, and is crucial for understanding how objects interact under the influence of forces.
Metabolic rate: Metabolic rate is the amount of energy expended by the body per unit time, often measured in calories or joules. It reflects the energy required to maintain basic bodily functions and support physical activity.
Metabolism: Metabolism refers to the complex set of chemical reactions that occur within living organisms to maintain life. It involves the conversion of food into energy and the building and breaking down of molecules for growth, reproduction, and maintenance of cellular functions. Essentially, metabolism encompasses all the processes that enable organisms to utilize energy from their environment for their survival and activity.
Myosin-actin: Myosin-actin refers to the protein interaction between myosin and actin filaments, crucial for muscle contraction and movement in living organisms. This interaction is central to the mechanism of muscle contraction, where myosin heads bind to actin filaments and pull them, resulting in the shortening of muscle fibers. This process is driven by ATP hydrolysis, which provides the energy necessary for muscle contraction, linking the concepts of work, energy, and power in biological systems.
Peristalsis: Peristalsis is a coordinated series of muscle contractions and relaxations that propel food and other materials through the digestive tract. It is an essential physiological process that facilitates the movement of contents within the gastrointestinal system.
Potential Energy: Potential energy is the stored energy an object possesses due to its position or state, which can be converted into kinetic energy or other forms of energy. This term is central to understanding various physical phenomena and energy transformations in the context of introductory college physics.
Power: Power is the rate at which work is done or energy is transferred. It is the measure of the amount of energy expended per unit of time. Power is a fundamental concept in physics that is essential for understanding various topics, including work, energy, and simple machines.
Thermodynamics: Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy. It explores how energy is transformed and transferred within physical systems, governing the principles that dictate the behavior of matter in various states. The laws of thermodynamics play a crucial role in understanding how energy flows and changes form, influencing everything from engines to biological processes.
Thermoregulation: Thermoregulation is the process by which the body maintains its internal temperature within a narrow range, despite changes in external temperature or metabolic heat production. It is a critical physiological mechanism that allows organisms to function optimally in a variety of environmental conditions.
Useful work: Useful work is the component of work that results in a desired outcome or effective energy transfer. It excludes any energy dissipated as waste, such as heat.
Watt: The watt is the unit of power, which is the rate at which energy is transferred or the rate at which work is done. It is a fundamental unit in the study of physics, particularly in the context of power, energy, and electricity.
Work: Work is a measure of the energy transferred by a force acting on an object as it is displaced. It is the product of the force applied and the distance moved in the direction of the force. Work is a fundamental concept in physics that is central to understanding energy, power, and the laws of motion.