Transformation of Energy in the Human Living Body

Heat and Transfer of Heat

Heat is a fundamental form of energy that plays a crucial role in biological systems. In the human body, heat is transferred and transformed through various mechanisms, affecting physiological processes and homeostasis.

• Heat Transfer: Heat always flows from a hotter body to a colder body.

• Heat as Energy: Heat can be transformed into work, making it a form of energy.

• Definition: Heat is energy transferred due to a temperature difference.

Kinetic Theory

The kinetic theory explains the behavior of matter based on the motion of its constituent particles. It is essential for understanding thermal properties and energy transfer in biological systems.

• Matter Composition: Matter consists of atoms and molecules in continuous chaotic motion.

• States of Matter:

  • Gas: Atoms/molecules move randomly and collide with each other and the container walls.

  • Solid: Atoms are bound together and vibrate around average positions.

  • Liquid: Intermediate behavior between solids and gases.

• Kinetic Energy: Moving particles possess kinetic energy.

• Internal Energy: The total energy of motion within a substance.

• Thermal Motion: The random movement of particles due to thermal energy.

• Temperature: A quantitative measure of hotness; proportional to the internal energy of matter.

Equations:

• Average kinetic energy of a molecule in an ideal gas: where

• Relationship between pressure, volume, and temperature: where is the total number of gas molecules.

Specific Heat and Latent Heat

Specific heat and latent heat are important thermal properties that determine how substances absorb and release energy.

• Specific Heat: The quantity of heat required to raise the temperature of 1 g of a substance by 1 degree.

• Latent Heat: The energy required to convert a solid to a liquid or a liquid to a gas.

• Units: , (1 cal = 4.184 J)

Transfer of Heat

Heat can be transferred by several mechanisms, each with distinct physical principles and biological relevance.

• Conduction: Direct transfer of heat through a material.

• Convection: Transfer of heat by the movement of fluids.

• Radiation: Transfer of energy by electromagnetic waves.

• Diffusion: Movement of particles from high to low concentration, contributing to energy distribution.

Conduction

Conduction is the process by which heat is transferred through direct contact of particles within a solid or between solids in contact.

• Equation: where is the area perpendicular to heat flux, is the length, is the temperature difference, and is the thermal conductivity coefficient.

Convection

Convection occurs in fluids, where heated molecules move away from the heat source, carrying energy with them.

• Equation: where is the area exposed to convective currents, is the temperature difference, and is the convection coefficient.

Radiation

Radiation is the emission of energy as electromagnetic waves, which can transfer heat without direct contact.

• Internal energy causes nuclei and electrons to vibrate, emitting electromagnetic radiation.

• This process is called thermal radiation.

Diffusion

Diffusion is the process by which molecules move from regions of high concentration to regions of low concentration, facilitating energy and material transfer in biological systems.

• Equation for average distance:

• Total distance traveled:

• Time required:

• Example (in water): Mean free path cm, cm/s, time to diffuse 1 cm:

Diffusion through Membranes

Diffusion through biological membranes is essential for the transport of oxygen, nutrients, and waste products.

• Depends on pore size and the properties of the diffusing molecule.

• Example: Oxygen diffuses to corneal cells from tear fluid; contact lenses can impede this process.

Thermodynamics

First Law of Thermodynamics

The first law of thermodynamics states that energy is conserved in all processes. In biological systems, this principle governs energy intake, expenditure, and transformation.

• Statement: Energy, including heat, is conserved; it can be converted from one form to another, but the total amount remains constant.

• Application: For constant internal temperature and weight, energy intake must equal work done plus heat lost.

• Imbalance: A difference between intake and output energy changes the sum of internal and chemical energy.

Second Law of Thermodynamics

The second law of thermodynamics describes the direction of spontaneous processes and the concept of entropy.

• Statement: Spontaneous change in a system proceeds from an arrangement of lesser probability to greater probability (increase in entropy).

Difference Between Heat and Other Forms of Energy

Heat and work are distinct forms of energy, with different properties and conversion efficiencies.

• Work: Energy in ordered motion.

• Heat: Energy in random motion.

• Complete conversion of heat to work is impossible due to the random nature of heat (Second Law).

• Maximum ratio of work: where is the higher temperature and is the lower temperature.

Energy Requirements and Regulation in the Human Body

Energy Required by People

Living systems require energy to maintain vital functions. The metabolic rate quantifies energy consumption per unit surface area.

• Basal Metabolic Rate (BMR): The rate of energy expenditure at rest.

• Equation for surface area: where is mass (kg), is height (m).

• Example: A 70-kg man of height 1.55 m has a surface area of about 1.70 and a BMR of 70 Cal/hr.

• Metabolic rate is proportional to

Energy from Foods

Food provides chemical energy, which is oxidized to release energy for biological functions.

• Glucose oxidation: A gram of glucose releases 3.81 Cal of energy.

• Example: Total daily energy expenditure for a person with 1.7 surface area is about 3940 Cal.

Regulation of Body Temperature

Homeostasis of Body Temperature

Maintaining a constant body temperature is vital for proper physiological function. The human body employs various mechanisms to regulate temperature.

• Normal body temperature: 37°C

• Protein damage occurs above 44–45°C

• Heart stoppage occurs below 28°C

Heat Production and Loss

• During moderate activity, most energy is converted to heat inside the body.

• Heat must be conducted to the skin and released.

• Example: 70-kg man consumes 260 Cal/hr, with 208 Cal converted to heat.

Heat Transfer Mechanisms in the Body

• Conduction through tissues is limited; most heat is transported by blood in the circulatory system.

• Heat is transferred to the skin and then to the environment by conduction, convection, radiation, and evaporation.

Control of Skin Temperature

• Skin temperature must be lower than internal body temperature for heat to flow out.

• Heat removal from skin occurs via convection, radiation, and evaporation.

• Contact with good thermal conductors (e.g., metal) increases heat removal by conduction.

Convection from Skin

• Equation: where is skin area exposed to air, is skin temperature, is air temperature, is convection coefficient.

• Example:

Radiative Heat Loss

• Equation: where is radiative surface area, is skin temperature, is radiative surface temperature, is radiation coefficient, is emissivity.

• Radiative heat loss: 63 Cal/hr

Radiative Heating by the Sun

• Solar energy at the top of the atmosphere: 1150 Cal//hr

• Cloud cover can reflect up to 75% of solar radiation.

• Dark skin absorbs about 80% of radiation; light skin about 60%.

• Light-colored clothing decreases radiative heating by about 40%.

Evaporation

• In warm climates, evaporation of sweat is a major cooling mechanism.

• Convection and radiation alone may not be sufficient during physical activity.

Resistance to Cold

• Cold increases the rate of heat outflow from the skin.

• Heat loss rate depends on temperature, wind velocity, and humidity.

• A mild wind (30 cm/sec) can cause a temperature drop of more than 5°C.

• The body responds by decreasing heat outflow and increasing heat production.