Laws of Thermodynamics and Related Concepts

Laws of Thermodynamics

• The total amount of energy in a system is equal to the amount of heat put into the system minus the work done.
• Energy changes from one form to another, and entropy increases in a closed system to achieve the lowest possible energy state.
• At absolute zero, all processes cease, and entropy is at a minimum value.
• Triple point of water: solid, liquid, gas.

Heat Transfer

Conduction

• Transfer of energy via direct contact between hot and cold molecules.
• Mainly occurs in solids.
• Metals feel cold because of heat transfer.
• Ventilator heated humidifiers.

Convection

• Involves mixing of fluid molecules at different temperatures.
• Air is warmed and circulated to carry heat elsewhere.
• Used in infant incubators.

Radiation

• Heat transfer without direct physical contact, even in a vacuum.
• Examples: sun warming the earth, heat from a light bulb, radiant heat from an electric stone burner.
• Radiant energy is used to keep newborn infants warm under a radiant warmer.
• Evaporation and condensation are discussed later.

Phase Changes

Liquid-Solid Phase Changes

• When a solid is heated, its molecular kinetic energy increases, leading to increased molecular vibrations.
• If enough energy is applied, intermolecular attractive forces weaken, and the solid changes into a liquid (melting).

Melting

• Changeover from solid to liquid state.
• Melting point: the temperature at which this change occurs.
• Latent heat of fusion: extra heat energy required for this change (calories per 1 g of solid to a liquid).

Freezing

• Heat energy is transferred from a liquid into a solid.
• Kinetic energy decreases, and molecules regain a solid structure.
• Freezing returns energy to its surroundings.
• Freezing and melting points are the same; the same energy is required for both (0°C & 32°F).

Sublimation

• Transition from a solid to a vapor without becoming a liquid.
• Example: dry ice (frozen carbon dioxide).

Liquid Pressure

• Liquids exert pressure that depends on height (depth) and weight density (weight per unit volume).
• PL = h \times dW where PL is liquid pressure, h is height, and dW is weight density.
• Pascal’s Principle: the pressure of a given liquid is the same at any specific depth, regardless of the container's shape; the pressure acts equally in all directions.

Buoyancy (Archimedes’ Principle)

• Objects submerged in water appear to weigh less than in air.
• Liquids exert a buoyant force because the pressure below a submerged object always exceeds the pressure above it; this creates an upward supporting force.
• Buoyant force = weight density x volume.
• Buoyancy helps keep solid articles suspended in gases; these suspensions are called aerosols.

Liquid-Vapor Phase Changes

• As water temperature reaches 100°C, a new change of state begins: from liquid to vapor (gas).
• Two types of vaporization: boiling and evaporation.
• Boiling point of a liquid is the temperature at which its vapor pressure exceeds atmospheric pressure.
• Changes in molecules and kinetic energy play a role.
• The heat energy required to vaporize a liquid is the latent heat of vaporization.

Evaporation

• Change of a liquid into a gas at temperatures lower than its boiling point.
• Evaporates exert their own partial pressure, and molecular water turns into vapor pressure.
• Heated to saturated air/water vapor to humidity.
• The heat energy required for evaporation comes from the air next to the water surface.

Water Vapor Pressure

• Water converted to vapor is called molecular water, which obeys the same physical principles as other gases and exerts a pressure called water vapor pressure.
• Remember, water vapor isn't the same as mist or fog.

Influence of Temperature on Evaporation

• The warmer the air, the more vapor it can hold.
• If water is heated, its kinetic energy increases, and more molecules escape from its surface.
• The temperature of a gas affects both its capacity to hold molecular water and the water vapor pressure.

Humidity

• The amount of water vapor or moisture in the atmosphere involves the kinetic activity of water molecules in the air.
• To determine the actual amount or weight of water vapor in a gas, the water vapor content or absolute humidity must be measured.

Absolute Humidity (AH)

• Measured by weighing the actual amount of water vapor extracted from air.
• Represents the actual water vapor content.
• If a gas is only half saturated with water vapor, its water vapor pressure and absolute humidity are only half of its fully saturated state.
• Air fully saturated with water vapor at 37°C and 760 mmHg.

Relative Humidity (RH)

• The ratio of its actual water vapor content to its saturated capacity (the amount of water vapor that could be held) at a given temperature, expressed as a percentage.
• When the water vapor content of a volume of gas equals its capacity, the RH is 100%; the gas is fully saturated with water vapor.
• RH never exceeds 100%, and a further decrease in temperature causes condensation.

Condensation

• The opposite of evaporation; a gas turns back into a liquid.
• Condensed moisture deposits on any available surface.
• Condensation returns heat to and warms the surrounding environment, whereas vaporization of water cools the adjacent air.
• The temperature at which condensation begins is called the dew point.
• When RH is at 100%, even slight cooling (below its dew point) causes more water vapor and condensation.

Humidity and Humidifiers

• A humidifier maintains molecular water in a gas.
• Evaporation keeps a liquid below its boiling point, maintaining more water vapor pressure in the air.
• Keeping relative humidity below 100% (less than 100% saturated) maintains more molecular water and vapor pressure to maintain humidity.
• Certain humidifiers maintain a larger surface area to increase evaporation and water vapor.
• If too much condensation occurs during mechanical ventilation, temperature can be increased to increase evaporation and keep RH below 100%.

Percent Body Humidity (%BH) and Humidity Deficit

• The percent body humidity (%BH) of a gas is the ratio of its actual water vapor content to the water vapor capacity in saturated gas at body temperature (37°C).

• The %BH is the same as the RH except the capacity is fixed at 43.8 mg/L.

• The humidity deficit associated with a %BH less than 100% represents the amount of water vapor the body must add to the inspired gas to achieve saturation at body temperature (37° C).

• HD = capacity (fixed 43.8) - actual \ water \ vapor \ content

• Actual water vapor content at its capacity 37° C is 43.8.

Influence of Pressure on Vaporization

• High temperatures increase vaporization, whereas high pressures impede this process.
• The surrounding air pressure if high, there are more opposing air molecules and vaporization decreases.
• Low atmospheric pressures increase vaporization; boiling occurs at lower temperatures at higher altitudes.
• Temperature and surface tension are inversely related.

Influence of Surface Area on Evaporation

• The greater the available surface area of the gas in contact with air, the greater is the rate of liquid evaporation.

Surface Tension

• Results from an imbalance of intermolecular forces existing at a gas-liquid surface.
• Cohesive forces affect molecules inside a drop equally from all directions; on the surface, there is an imbalance.
• Surface molecules contract into the smallest possible area.
• Reducing droplet diameter by half increases surface area by a factor of four.

Laplace's Law

• Refers to spherical structures that demonstrate the interaction between distending pressure and surface tension forces as a sphere’s radius varies inversely.
• The lungs' 300 million alveoli are considered tiny spheres independently expanding and contracting equally in all directions of their curvature.
• P = \frac{2 \times T}{r}

Pulmonary Surfactant

• A phospholipid produced by Alveolar type II epithelial cells that reduces surface tension in the lung.
• The ability of pulmonary surfactant to reduce surface tension decreases as surface area increases.
• When this surface area decreases, the ability of pulmonary surfactant to reduce surface tension increases, this changing surface tension to match lung volume which helps stabilize the alveoli.

Lung Recoil

• Lung recoil involves elasticity and surface tension in the alveoli.
• Surfactant molecules mix with high intermolecular molecules and push liquid to the surface.

Effects of Inadequate Surfactant

• Increased lung recoil (tendency to deflate on inflation).
• Reduced lung compliance.
• Increased collapsing and distending pressures.
• Greater effort to reduce recoil required.
• Causes increased work of breathing.
• Increase accessory muscle use (diaphragm and other intercostal muscles that drive respiration).
• Eventually will fatigue from overwork.
• Eventually ventilatory failure.

Infant Respiratory Distress Syndrome

• Caused by pulmonary pre-maturity (surfactant deficiency).
• When pulmonary surfactant is lacking and surface tension remains constant; as the alveolar radius decreases, the inflation pressure rises.

Action of Pulmonary Surfactant

• Presence with the liquid lining of the alveoli alters the surface tension forces as alveolar size fluctuates during ventilation.
• During lung expansion (Inhalation), alveolar size (radius) increases.
• The surface tension forces also increase because the concentration of pulmonary surfactant is distributed over a larger alveolar area and becomes less effective in reducing the surface tension of the liquid lining.
• As exhalation proceeds to end-expiration, alveolar size (radius) diminishes, and surface tension forces briefly decrease.
• The presence of pulmonary surfactant in the liquid lining layer is confined to a smaller area; therefore, its concentration within each alveolus increases and its effect on surface tension forces is heighted.

Benefits of Pulmonary Surfactant

• Prevents over-distention on inhalation.
• Helps prevent collapse of alveoli during exhalation.
• Helps stabilize alveoli across the lungs.