THERMODYNAMICS

1. Introduction

3.0 Introduction: Before we get into the first law of thermodynamics, we need to understand the relation between heat and work and the concept of internal energy. Just like mass, energy is always conserved, i.e., it can neither be created nor destroyed, but it can be transformed from one form to another. Internal energy is a thermodynamic property of the system that refers to the energy associated with the molecules of the system, which includes kinetic energy and potential energy.

Whenever a system goes through any change due to the interaction of heat, work and internal energy, it is followed by numerous energy transfers and conversions. However, during these transfers, there is no net change in the total energy.

Similarly, if we look at the first law of thermodynamics, it affirms that heat is a form of energy. What it means is that the thermodynamic processes are governed by the principle of conservation of energy. The first law of thermodynamics is also sometimes referred to as the law of conservation of energy.

3.1 Principle of Energy Conservation

Conservation of energy is a fundamental law of physics, stating that energy can neither be created nor destroyed, only transformed from one form to another. A general law of physics that applies to all energy forms in all systems (mechanical, electrical, chemical, etc.)

In biological systems, energy is constantly being converted:

·         Chemical energy (food) converts to mechanical energy (movement).

·          From chemical energy converts to thermal energy (body heat).

Therefore, in an isolated system such as the universe, if there is a loss of energy in some part of it, there must be a gain of an equal amount of energy in some other part of the universe. Although this principle cannot be proved, there is no known example of a violation of the principle of conservation of energy.

2. First Law of Thermodynamics

3.2 First Law of Thermodynamics

The law states that Energy cannot be created or destroyed, only transformed from one form to another. A specific application of the conservation of energy to thermodynamic systems, involving heat (Q), work (W), and internal energy (U).

Additionally, the heat energy added to a system is used either to increase its internal energy or to do work (like expanding or moving something). For instance:

When you eat food (energy input), your body uses that energy to:

                    i.            Maintain internal temperature (increase ΔU)

                  ii.            Perform work like muscle movement or heartbeat (W)

No energy is lost—just converted from chemical energy in food to heat, motion, etc.

It is mathematically expressed as 

Where:

ΔU = change in internal energy of the system (in joules, J)

Q = heat added to the system (in joules, J)

W = work done by the system (in joules, J)

1.      A gas absorbs 500J of heat and does 200 J of work by expanding.
What is the change in internal energy of the gas?

2.      A gas is compressed with 150 J of work done on it, and it releases 100 J of heat to the environment.
What is the change in internal energy?

3.      A rigid container (no volume change) is filled with gas. It absorbs 300 J of heat.
How much is the change in internal energy?
How much work is done?

4.      A gas has constant pressure in a system. There is a loss of 45 J of heat in the surroundings around the system. 450 J of work is done on the system. Find the system’s internal energy.

3. Forms of Energy in the Human Body

Forms of Energy in the Human Body

1. Chemical Energy: Chemical energy stored in Adenosine Triphosphate (ATP), glucose, and fats is the main source of energy for the body. During exercise, this energy is converted into other forms to power muscle contractions and maintain body functions.

2. Mechanical Energy: Mechanical energy is produced when muscles contract, creating movement. It allows the body to perform physical tasks like walking, running, lifting, or jumping.

3. Kinetic Energy: Kinetic energy is the energy of motion. It is observed in the movement of limbs during exercise and also in internal processes, such as the beating of the heart and the circulation of blood through vessels.

4. Potential Energy: Potential energy is related to the position or posture of the body. For example, when a person stands up, lifts a leg, or holds a weight above the ground, potential energy increases due to elevation.

5. Electrical Energy: Electrical energy is used by the nervous system to send signals from the brain to the muscles. These signals coordinate movement and responses during physical activity.

6. Thermal Energy: Thermal energy is generated as a byproduct of metabolism, especially during exercise when energy demands are high. This causes body temperature to rise, and the body responds by sweating and increasing blood flow to the skin to cool down—this process is called thermoregulation.

4. Application to Metabolism

3.4 Application to Metabolism

The First Law of Thermodynamics says that energy cannot be created or destroyed, only transformed from one form to another. In the human body, this law applies to metabolism—the process by which the body converts food into energy. When we eat, the body takes the chemical energy stored in food and transforms it into other forms: mechanical energy for moving muscles, electrical energy for sending nerve signals, and thermal energy (heat) to maintain body temperature.

Any extra energy not used immediately is either stored (as fat or glycogen) or released as heat. So, the energy from food is not lost—it is just reused, transformed, or stored, depending on what the body needs. This is how the body follows the First Law of Thermodynamics in everyday activities like walking, thinking, or even sleeping.

Metabolism is the sum of all chemical processes in the body.

Catabolism: refers to the breakdown of molecules (e.g., glucose) to release energy. Example include Cellular respiration.

-Anabolism: refers to the synthesis of complex molecules, requiring energy.

About 60–70% of the energy from food is converted into heat, maintaining body temperature.
The rest powers cellular work, muscle contraction, active transport, etc.

5. Work Done by Organs of the Body

3.5  Work Done by Organs of the Body

1.  Heart: The heart pumps about 5 to 6 liters of blood every minute throughout the body. It converts chemical energy from ATP into mechanical energy, which helps maintain blood pressure and keep blood circulating. The amount of work done by the heart in a single day is roughly equivalent to lifting one ton to a height of 3 meters.

2. Lungs: The lungs expand and contract to allow air to move in and out during breathing. This process requires the contraction of muscles, mainly the diaphragm and intercostal muscles between the ribs.

3. Muscles: Muscles perform mechanical work that enables movement, maintains posture, and generates heat, especially during shivering. This mechanical work is powered by the conversion of ATP into ADP, inorganic phosphate, and energy.

 4. Kidneys: The kidneys use energy to actively transport ions and glucose during the reabsorption process. Despite being only about 0.5% of the body’s mass, the kidneys consume around 10% of the body’s oxygen due to their high energy demand.

5. Brain: The brain has a very high metabolic demand and uses about 20% of the body’s energy while at rest. Most of this energy is used for electrochemical work, including nerve signalling and synaptic transmission.

6. Energy Balance in the Body/Clinical Relevance

3.6 Energy Balance in the Body

        i.            When energy intake exceeds energy output, the body is in positive energy balance, leading to weight gain through fat storage.

      ii.            When energy output exceeds intake, the body is in negative energy balance, causing weight loss through the breakdown of fat and muscle.

3.7 Clinical Relevance

1. Fever increases the body’s metabolic rate, resulting in more heat production.
2. Hyperthyroidism causes an excessive metabolism, leading to a higher energy usage.
3. Hypothermia causes the body to conserve energy by lowering metabolic rate.
4. Exercise physiology studies how the body converts energy during physical activity.
5. Calorimetry measures the heat output from the body to estimate metabolic rate.