Comprehensive Respiratory Care Gas Dynamics and Physics Study Guide

Importance of gas physics in respiratory care

Gas physics describes the behavior of gases, and manipulation of gases is fundamental to respiratory care therapeutics and gas delivery.

Variables determining gas behavior

The three major variables that determine the physical behavior of a gas are temperature, pressure, and volume.

Effect of temperature change on a gas

Changing temperature can affect the gas's pressure and volume.

Effect of pressure change on a gas

Changing pressure can alter the gas's temperature and volume.

Effect of volume change on a gas

Changing volume can modify the gas's temperature and pressure.

Primary states of matter

The three primary states of matter discussed in respiratory-care physics are solid, liquid, and gas.

Factors determining state of matter

The two molecular factors that determine the state of matter of a substance are kinetic energy and intermolecular forces.

Molecular arrangement in solids

In a solid, molecules are held tightly by strong intermolecular forces and primarily vibrate in place.

Movement of molecules in liquids vs. solids

Liquid molecules have greater movement and weaker intermolecular attraction compared to solid molecules.

Movement of gas molecules

Gas molecules have extensive freedom of movement and relatively weak intermolecular attraction compared to liquids and solids.

Strongest intermolecular forces

Solids experience the strongest intermolecular attractive forces.

Greatest freedom of movement

Gases have the greatest freedom of molecular movement.

Common temperature scales

The three temperature scales commonly used are Kelvin, Fahrenheit, and Celsius.

Celsius to Fahrenheit conversion formula

The formula to convert Celsius to Fahrenheit is °F = (9/5 × °C) + 32.

Fahrenheit to Celsius conversion formula

The formula to convert Fahrenheit to Celsius is °C = 5/9 × (°F − 32).

Celsius to Kelvin conversion formula

The formula to convert Celsius to Kelvin is K = °C + 273.

Key temperature points on the Kelvin scale

Absolute zero is 0 K, the freezing point of water is 273 K, and the boiling point of water is 373 K on the Kelvin scale.

Key temperature points on the Celsius scale

Absolute zero is −273°C, the freezing point of water is 0°C, and the boiling point of water is 100°C on the Celsius scale.

Key temperature points on the Fahrenheit scale

Absolute zero is approximately −460°F, the freezing point of water is 32°F, and the boiling point of water is 212°F on the Fahrenheit scale.

Molecular motion and temperature

Temperature measures the average energy of molecular motion in a substance.

Difference between heat and temperature

Heat represents the total energy of molecular motion, whereas temperature represents the average energy of molecular motion.

Major mechanisms of heat transfer

The major mechanisms of heat transfer are radiation, conduction, convection, and evaporation/condensation.

Conduction

Conduction is the transfer of heat through direct physical contact.

Convection

Convection is the transfer of heat through the movement of a fluid, such as air or liquid.

Radiation

Radiation is the transfer of heat through electromagnetic energy without requiring direct contact.

Effect of evaporation on body heat

Evaporation removes heat as liquid water changes to vapor.

Cooling effect of diaphoresis

Sweating (diaphoresis) cools a patient by evaporative heat loss from the skin.

Factors determining liquid pressure

The pressure exerted by a liquid is determined by the depth or height of the liquid and its density.

Density in relation to fluids

Density refers to the weight or mass per unit volume of a liquid.

Pascal's principle

Pascal's principle states that pressure applied to a confined fluid is transmitted throughout the fluid.

Importance of cmH₂O in respiratory care

Centimeters of water (cmH₂O) is a commonly used unit for measuring respiratory pressures.

Buoyancy

Buoyancy is an upward force exerted by a fluid on an object immersed in it.

Direction of fluid pressure on immersed objects

Fluid pressure acts in all directions on an immersed object.

Buoyancy force and fluid depth

As fluid depth increases, the buoyancy force also increases.

Cohesion

Cohesion is the attraction between molecules of the same substance.

Adhesion

Adhesion is the attraction between molecules of different substances.

Surface tension in liquids

Surface tension is caused by cohesive forces between liquid molecules at the surface.

Vulnerability of small alveoli to collapse

Small alveoli are particularly vulnerable to collapse due to higher surface tension producing greater collapsing pressure.

Alveolar collapse and radius

As alveolar radius decreases, the tendency toward collapse increases.

Kinetic theory of gas molecules

According to kinetic theory, gas molecules are in rapid, continuous motion.

Behavior of gas molecules in a container

Gas molecules collide with one another and with the walls of the container.

Energy during molecular collisions

During ideal molecular collisions, there is no net loss of energy when temperature remains constant.

Kinetic energy and absolute temperature relationship

Kinetic energy is directly proportional to absolute temperature.

Molecular motion with increasing temperature

As gas temperature increases, molecular velocity and kinetic energy also increase.

Production of gas pressure

Gas pressure inside a container is produced by collisions of gas molecules with container walls.

Effect of collision frequency on pressure

As the frequency of molecular collisions increases, pressure also increases.

Factors influencing molecular collision frequency

The three factors influencing the number of molecular collisions in a gas are container size, molecular velocity, and number of molecules.

Effect of decreasing container size on collisions

Decreasing container size increases the frequency of molecular collisions.

Effect of increasing molecular velocity on collisions

Increasing molecular velocity raises the frequency and/or force of collisions.

Avogadro's number

Avogadro's number is 6.02 × 10²³ particles per mole.

Molar volume of an ideal gas at STPD

One mole of an ideal gas occupies 22.4 L at standard temperature and pressure (STPD).

Molecular mass of O₂

The molecular mass of O₂ is approximately 32 g/mol.

Density calculation of a gas at STPD

Density at STPD can be calculated by dividing its molecular weight in grams per mole by 22.4 L/mol.

Approximate density of oxygen at STPD

The approximate density of oxygen at STPD is 1.43 g/L.

Approximate density of helium at STPD

The approximate density of helium at STPD is 0.18 g/L.

Density difference between helium and oxygen

Helium is less dense than oxygen due to its lower molecular weight.

Determining density of a gas mixture at STPD

The density of a gas mixture at STPD is determined by calculating the weighted molecular mass from the fraction of each gas, then dividing by 22.4 L/mol.

Approximate density of a mixture containing 20% O₂ and 80% He at STPD

The approximate density of a mixture containing 20% O₂ and 80% He at STPD is approximately 0.43 g/L.

Barometric pressure

Barometric pressure is atmospheric pressure, commonly referenced to the height of a column of a substance such as mercury.

1 atmosphere in mmHg

1 atmosphere is equal to 760 mmHg.

Atmospheric pressure and altitude

Atmospheric pressure decreases as altitude increases.

Effect of altitude on sealed containers

A sealed container from high altitude can become crushed when brought to sea level due to greater external atmospheric pressure.

Humidity in respiratory gas physics

Humidity refers to the amount of water vapor present in a gas.

Temperature and water vapor capacity

The temperature most strongly affects the amount of water vapor that air can hold.

Effect of temperature increase on water vapor capacity

As temperature increases, the water-vapor-holding capacity of air also increases.

Evaporation definition

Evaporation is the conversion of liquid water into water vapor.

Condensation definition

Condensation is the conversion of water vapor into liquid water.

Absolute humidity definition

Absolute humidity is the actual amount of water vapor contained in a volume of gas.

Another term for absolute humidity

Specific humidity is another term used for absolute humidity.

Maximum water vapor content at body temperature

The maximum water vapor content of fully saturated gas at body temperature (37°C) is 43.8 mg/L.

Water vapor pressure at body temperature

The water vapor pressure of fully saturated gas at 37°C is 47 mmHg.

Relative humidity definition

Relative humidity is the ratio of the amount of water vapor actually present to the maximum amount the gas could hold at that temperature and pressure.

100% relative humidity meaning

100% relative humidity indicates that the gas contains the maximum amount of water vapor possible at its current temperature and pressure.

Equation for calculating partial pressure in a dry mixture

Partial pressure = fractional concentration × total pressure.

Example of partial pressure calculation for gas mixture

At 760 mmHg, the partial pressure of each gas in a mixture containing 50% oxygen and 50% argon is 380 mmHg each.

Fraction of atmospheric air that is oxygen

Approximately 21% of atmospheric air is oxygen.

Approximate partial pressure of oxygen in dry atmospheric air at sea level

The approximate partial pressure of oxygen in dry atmospheric air at sea level is 0.21 × 760 ≈ 160 mmHg.

Effect of altitude on inspired oxygen partial pressure

Inspired oxygen partial pressure decreases as altitude increases, even though atmospheric oxygen remains approximately 21%.

Partial pressure of oxygen at 517 mmHg

At a barometric pressure of 517 mmHg, the approximate partial pressure of oxygen in dry atmospheric air is 0.21 × 517 ≈ 109 mmHg.

Partial pressure of oxygen at 253 mmHg

At a barometric pressure of 253 mmHg, the approximate partial pressure of oxygen in dry atmospheric air is 0.21 × 253 ≈ 53 mmHg.

Water vapor and available pressure in humidified mixtures

Water vapor reduces the pressure available to other gases in a humidified mixture because it exerts its own partial pressure.

Pressure to subtract for partial pressure calculations

Water vapor pressure (PH₂O) must be subtracted from barometric pressure before calculating the partial pressure of a gas in humidified air.

Equation for available pressure in humidified gas

Available pressure = PB − PH₂O.

Pressure available to He and O₂ in a humidified mixture example

In a mixture that is 70% He and 30% O₂ with PB of 750 mmHg and PH₂O of 30 mmHg, the available pressure is 750 − 30 = 720 mmHg.

Partial pressure of helium in that humidified mixture

In that humidified mixture, the partial pressure of helium is 0.70 × 720 = 504 mmHg.

Partial pressure of oxygen in that humidified mixture

In that humidified mixture, the partial pressure of oxygen is 0.30 × 720 = 216 mmHg.

Combined gas-law relationship emphasized in lecture

The combined gas-law relationship emphasized in the lecture is P₁V₁/T₁ = P₂V₂/T₂.

Absolute temperature requirement in gas-law calculations

Absolute temperature must be used in gas-law calculations, measured in Kelvin.

Effect of decreasing gas volume on pressure

If gas volume decreases while temperature remains constant, pressure increases.

Effect of decreasing gas pressure on volume

If gas pressure decreases while temperature remains constant, volume increases.

Gas compression example calculation

A gas occupies 450 mL at 30 psi and is compressed to 55 psi; the new volume is V₂ = (30 × 450) ÷ 55 ≈ 245 mL.

Biological process demonstrating Boyle's law

Breathing/ventilation is a major biological process that demonstrates Boyle's law.

Effect of increasing absolute temperature on gas volume

If absolute temperature increases while pressure remains constant, gas volume increases.

Effect of decreasing absolute temperature on gas volume

If absolute temperature decreases while pressure remains constant, gas volume decreases.

Second temperature calculation example

A gas occupies 500 mL at 37 K and occupies 450 mL at the same pressure; the second temperature is T₂ = (450 × 37) ÷ 500 = 33.3 K.

Gay-Lussac's law equation

The equation for Gay-Lussac's law is P₁/T₁ = P₂/T₂.

Pressure increase in a rigid cylinder with temperature rise

When temperature increases in a rigid cylinder, pressure increases.

Tank pressure change due to temperature

Tank pressure can change even when the number of gas molecules has not changed because a temperature change alters molecular kinetic energy.

Temperature conversion for Gay-Lussac calculations

Convert Celsius to Kelvin by adding 273 before using Celsius values in Gay-Lussac calculations.

Impact of heating a gas in a cylinder

If a cylinder at 400 psig and 25°C is warmed to 35°C, the approximate resulting pressure is about 413 psig using 298 K and 308 K.