Intro to Respiratory Care – Unit III Key Vocabulary

These flashcards summarize essential vocabulary from Unit III covering measurement conditions, thermodynamics, heat transfer, phase changes, humidity, gas laws, and respiratory flow dynamics.

STPD

Standard Temperature and Pressure, Dry: 0 °C, 760 mm Hg, no water vapor.

BTPS

Body Temperature, Pressure, Saturated: 37 °C, barometric pressure (760 mm Hg at sea level), fully saturated with water vapor.

ATPD

Ambient Temperature and Pressure, Dry: room conditions without water vapor.

ATPS

Ambient Temperature and Pressure, Saturated: room conditions fully saturated with water vapor.

First Law of Thermodynamics

Energy cannot be created or destroyed—total energy equals heat added minus work done.

Second Law of Thermodynamics

In a closed system, energy conversions increase entropy to reach the lowest energy state.

Third Law of Thermodynamics

Absolute zero cannot be reached; at that point entropy would be minimal and processes cease.

Conduction (heat)

Transfer of heat by direct contact between hot and cold molecules.

Convection (heat)

Heat transfer via mixing of fluid molecules at different temperatures.

Radiation (heat)

Heat transfer by electromagnetic waves without physical contact.

Evaporation (heat effect)

Heat is taken from surrounding air as liquid becomes vapor, cooling the air.

Condensation (heat effect)

Heat is released to surrounding air as vapor becomes liquid, warming the air.

Melting

Change of state from solid to liquid.

Melting Point

Temperature at which a substance changes from solid to liquid.

Latent Heat of Fusion

Extra calories required to convert 1 g of solid to liquid at its melting point.

Freezing Point

Temperature at which a liquid becomes solid; same as melting point for a substance.

Sublimation

Transition from solid to vapor without passing through liquid (e.g., dry ice).

Vaporization

General change of state from liquid to gas; requires heat energy.

Latent Heat of Vaporization

Energy needed to convert a liquid to vapor without temperature change.

Boiling Point

Temperature at which a liquid’s vapor pressure exceeds atmospheric pressure.

Evaporation (liquid-to-gas)

Conversion of liquid to gas below its boiling point; faster when temperature increases.

Water Vapor Pressure

Partial pressure exerted by water vapor molecules in a gas mixture.

Molecular Water

Invisible water vapor that behaves like a gas; not mist or fog.

Absolute Humidity (AH)

Actual mass of water vapor in a volume of air, expressed in mg H₂O/L.

Relative Humidity (RH)

Ratio of current water vapor content to capacity at a given temperature; RH = content / capacity × 100%.

Body Humidity (BH)

Water vapor content relative to full saturation at 37 °C (capacity = 43.8 mg/L).

Humidity Deficit

Amount of water vapor the body must add to inspired gas to reach saturation at 37 °C; HD = 43.8 – actual content.

Dew Point

Temperature at which gas becomes saturated and condensation begins.

Pascal’s Principle

Pressure exerted by a confined liquid acts equally in all directions.

Buoyancy

Upward force liquids (and gases) exert; pressure below an object is greater than above.

Surface Tension

Attractive force between like molecules at a liquid surface that minimizes surface area.

Laplace’s Law

Pressure inside a sphere ∝ surface tension ÷ radius; smaller radius or higher tension increases pressure.

Pulmonary Surfactant

Lung substance that lowers surface tension, preventing alveolar collapse and over-distension.

Airway Resistance (RAW)

Opposition to gas flow in the airways; ~80 % occurs in nose, mouth, trachea, upper airways.

Lung Compliance

Ease of lung expansion, determined by tissue elasticity and surface tension.

Static Compliance

Measurement using tidal volume, plateau pressure, and PEEP at end-inspiration.

Laminar Flow

Smooth, parallel flow in layers; pressure is linearly related to flow.

Turbulent Flow

Chaotic flow with eddies; resistance rises, favored by high velocity, density, and large tubes.

Transitional Flow

Combination of laminar and turbulent patterns, common in tracheobronchial tree.

Poiseuille’s Law

For laminar flow, pressure difference depends on viscosity, tube length, flow, and inversely on radius⁴.

Reynolds Number

Dimensionless value predicting flow type; < 2000 indicates laminar, > 2000 tends toward turbulence.