Physical Principles of Respiratory Care: Key Concepts

Physical principles are essential in respiratory care because they explain how gases move, how equipment works, and how the lungs respond to changes in pressure, volume, flow, temperature, and humidity.

Respiratory therapists use these concepts every day when delivering oxygen, setting up humidifiers, administering aerosol therapy, interpreting ventilator graphics, managing artificial airways, and evaluating patient response to treatment.

A basic understanding of physics helps connect clinical decisions with the behavior of gases and fluids inside the respiratory system and within respiratory care equipment.

What Are the Physical Principles of Respiratory Care?

The physical principles of respiratory care are the basic science concepts that explain how gases, liquids, pressure, flow, temperature, and humidity behave in the respiratory system and in respiratory equipment. These principles help respiratory therapists understand how oxygen moves into the lungs, how carbon dioxide is removed, how mechanical ventilators deliver breaths, and how oxygen and aerosol devices work.

Key principles include gas laws, pressure gradients, airflow, airway resistance, lung compliance, humidity, diffusion, and fluid dynamics. For example, gas moves from an area of higher pressure to an area of lower pressure, which explains both normal breathing and positive-pressure ventilation. Resistance affects how easily air moves through the airways, while compliance describes how easily the lungs expand.

These principles also apply to oxygen therapy, humidification, aerosol delivery, gas storage, pressure monitoring, and ventilator management. In short, they help respiratory therapists understand not only what respiratory equipment does, but why it works.

States of Matter in Respiratory Care

Matter exists in several forms, including solids, liquids, gases, and plasma. In respiratory care, the most clinically important states are liquids and gases because therapists routinely work with oxygen, air, helium, carbon dioxide, water, aerosol particles, and liquid oxygen systems.

Solids have a fixed shape and volume because their molecules are held closely together. Liquids have a fixed volume but take the shape of their container because their molecules can move around one another. Gases have neither a fixed shape nor a fixed volume. They expand to fill any container and can be compressed. This behavior is important when working with compressed gas cylinders, oxygen delivery systems, ventilators, nebulizers, and gas storage devices.

Liquids and gases are both considered fluids because they can flow. This means many of the same basic principles apply to both air moving through the lungs and water moving through tubing.

Note: Respiratory therapists must understand how fluids behave because airflow, airway resistance, humidification, aerosol delivery, and pressure measurement all depend on these principles.

Internal Energy and Molecular Motion

Internal energy refers to the energy contained within matter because of molecular motion and molecular position. It includes potential energy and kinetic energy.

Potential energy is stored energy. It may be related to position, molecular attraction, or chemical structure. Kinetic energy is energy of motion. In gases, molecules move rapidly and collide with one another and with the walls of their container. These collisions help create gas pressure.

This relationship between molecular motion and pressure is important in respiratory care. When gas molecules move faster, temperature rises. As temperature changes, gas pressure and volume may also change. This is why gas laws are so important when studying mechanical ventilation, compressed gases, humidifiers, and liquid oxygen systems.

Heat Transfer

Heat is energy transferred from one object or substance to another because of a temperature difference. Respiratory care involves several types of heat transfer, including conduction, convection, radiation, evaporation, and condensation.

Conduction occurs when heat moves through direct contact. For example, a patient may lose heat when lying on a cold surface. Convection occurs when heat is transferred by the movement of a fluid, such as air or water. Air circulating around an infant in an incubator is an example of convection.

Radiation transfers heat without direct contact. A radiant warmer used for a newborn is a common clinical example. Evaporation occurs when a liquid changes into a gas. This process requires heat energy and produces cooling. Sweating cools the body by evaporation. Condensation occurs when a gas changes back into a liquid, releasing heat in the process. Condensation is commonly seen as water collecting inside respiratory tubing.

Note: These heat transfer processes are important because patients can gain or lose heat during respiratory therapy, especially during aerosol therapy, humidification, mechanical ventilation, and neonatal care.

Temperature and Absolute Temperature

Temperature is related to the average kinetic energy of molecules. When molecules move faster, the temperature increases. When molecules move more slowly, the temperature decreases.

Respiratory therapists commonly encounter Celsius, Fahrenheit, and Kelvin temperature scales. Celsius and Fahrenheit are used in clinical settings, while Kelvin is often required for gas law calculations. Kelvin is an absolute temperature scale, meaning it begins at absolute zero. Absolute zero is the theoretical point at which molecular motion stops.

Gas law calculations require absolute temperature because gas behavior depends on molecular motion. For example, when gas is warmed, its molecules move faster and occupy more space if pressure remains constant. If the gas is in a fixed container, warming it can increase pressure. This matters when working with gas cylinders, ventilators, heated humidifiers, and gas storage systems.

Changes of State

A change of state occurs when matter changes from one physical form to another. Common phase changes include melting, freezing, vaporization, and condensation.

Melting occurs when a solid becomes a liquid. Freezing occurs when a liquid becomes a solid. Vaporization occurs when a liquid becomes a gas. Condensation occurs when a gas becomes a liquid. These processes require or release energy.

Latent heat is the energy required to change the state of a substance without changing its temperature. For example, energy is required to melt ice into water, but even more energy is needed to turn water into vapor. This is clinically relevant because respiratory therapists work with humidifiers, nebulizers, vaporizers, and liquid oxygen systems.

Liquid oxygen is a good example of phase change in respiratory care. Oxygen must be cooled to an extremely low temperature to remain in liquid form. When it warms, it changes back into a gas. Understanding this process helps therapists use oxygen storage systems safely and appropriately.

Properties of Liquids

Liquids have several properties that are important in respiratory care. These include pressure, density, viscosity, buoyancy, cohesion, adhesion, surface tension, and capillary action.

Liquid pressure depends on depth and density. The deeper the liquid column and the denser the liquid, the greater the pressure at the bottom. This principle is used in pressure measurement systems, such as water columns and mercury barometers.

Viscosity is a fluid’s resistance to flow. A thick liquid has greater viscosity and requires more force to move. Blood has greater viscosity than water because it contains cells suspended in plasma. When blood viscosity increases, the heart must work harder to pump it through the circulatory system.

Buoyancy refers to the upward force a fluid exerts on an object. This principle helps explain why objects float or sink. It is also related to the concept of specific gravity, which compares the density of one substance to another.

Cohesion is the attraction between like molecules, while adhesion is the attraction between unlike molecules. These properties contribute to surface tension and capillary action.

Surface Tension and the Lungs

Surface tension occurs because molecules at the surface of a liquid are pulled inward. This creates a force that makes the surface behave as if it has a thin elastic layer. Surface tension causes liquid droplets to assume a round shape because this shape has the smallest possible surface area.

Surface tension is especially important in the lungs. The alveoli are lined with a thin layer of fluid. Without a substance to reduce surface tension, the alveoli would be more likely to collapse, especially during exhalation. Pulmonary surfactant helps reduce alveolar surface tension, making the lungs easier to inflate and helping prevent alveolar collapse.

This concept is especially important in neonatal respiratory care. Premature infants may have insufficient surfactant production, which contributes to respiratory distress syndrome. Without enough surfactant, the lungs become stiff, compliance decreases, and more pressure is required to inflate the alveoli.

Surface tension also relates to aerosol therapy. Aerosol droplets are round because of surface tension. Droplet size affects where particles deposit in the respiratory tract. Larger particles tend to deposit in the upper airway, while smaller particles may reach the lower airways and alveoli.

Evaporation, Vapor Pressure, and Humidity

Water can evaporate at temperatures below its boiling point. Once water becomes vapor, it behaves like a gas and exerts pressure. This is called water vapor pressure.

Humidity is the amount of water vapor in a gas. Warm gas can hold more water vapor than cool gas. This is important because inspired gases must be properly humidified, especially when medical gases are delivered for long periods or when the upper airway is bypassed.

Normally, the nose and upper airway warm and humidify inspired air before it reaches the lower respiratory tract. When a patient has an artificial airway, such as an endotracheal tube or tracheostomy tube, this natural conditioning function is bypassed. Dry gas can irritate the airway mucosa, thicken secretions, impair mucociliary clearance, and increase the risk of mucus plugging.

Note: For this reason, humidification is an important part of respiratory care. Heated humidifiers, heat moisture exchangers, and aerosol devices help maintain airway hydration and patient comfort.

Properties of Gases

Gas molecules move rapidly and randomly. They collide with one another and with the walls of their container. These collisions create pressure. Gas behavior is influenced by pressure, volume, temperature, gas density, and the number of molecules present.

In respiratory care, gases are used constantly. Oxygen, air, helium, carbon dioxide, nitric oxide, and other medical gases may be delivered for therapeutic or diagnostic purposes. Understanding how gases behave helps therapists safely operate oxygen systems, ventilators, flowmeters, blenders, nebulizers, and anesthesia-related equipment.

Gas density refers to mass per unit volume. A dense gas is harder to move through the airways than a less dense gas. This concept helps explain the use of helium-oxygen mixtures in some patients with severe airway obstruction. Helium is less dense than nitrogen, so a helium-oxygen mixture may flow more easily through narrowed airways and reduce the work of breathing in selected situations.

Pressure in Respiratory Care

Pressure is force exerted over an area. In respiratory care, pressure is one of the most important physical measurements. Airway pressures, gas cylinder pressures, blood gas pressures, and atmospheric pressure all influence clinical practice.

Atmospheric pressure is commonly measured with a barometer. At sea level, standard atmospheric pressure is approximately 760 mm Hg, 1 atmosphere, or 14.7 pounds per square inch. Pressure may also be expressed in torr, centimeters of water, or kilopascals.

Respiratory therapists commonly use different pressure units in different clinical situations. Airway pressures on mechanical ventilators are usually measured in centimeters of water pressure. Blood gas values, such as PaO₂ and PaCO₂, are commonly measured in millimeters of mercury or torr. Gas cylinders are usually measured in pounds per square inch.

Pressure gradients cause gas movement. Gas flows from an area of higher pressure to an area of lower pressure. During spontaneous inspiration, expansion of the thoracic cavity lowers alveolar pressure below atmospheric pressure, allowing air to enter the lungs. During exhalation, alveolar pressure becomes higher than atmospheric pressure, causing gas to leave the lungs.

Note: Mechanical ventilation uses the same basic principle but applies it differently. Instead of the patient generating negative pressure inside the thorax, the ventilator applies positive pressure to push gas into the lungs.

Gas Laws in Respiratory Care

Gas laws explain how pressure, volume, temperature, and the amount of gas are related. These laws help respiratory therapists understand ventilation, oxygen delivery, compressed gas systems, humidification, and pulmonary physiology.

Boyle’s Law

Boyle’s law states that pressure and volume are inversely related when temperature remains constant. When volume decreases, pressure increases. When volume increases, pressure decreases. This helps explain normal breathing. When the thoracic cavity expands, alveolar volume increases and pressure decreases, allowing air to flow into the lungs.

Charles’ Law

Charles’ law states that gas volume and temperature are directly related when pressure remains constant. As temperature increases, gas volume increases. As temperature decreases, gas volume decreases. This principle is relevant when gases are warmed during humidification or cooled during storage.

Gay-Lussac’s Law

Gay-Lussac’s law states that gas pressure and temperature are directly related when volume remains constant. If gas in a fixed container is heated, pressure increases. This principle is important for compressed gas cylinders. A cylinder exposed to excessive heat may develop increased internal pressure, creating a safety hazard.

Avogadro’s Law

Avogadro’s law states that equal volumes of gases contain equal numbers of molecules under the same temperature and pressure conditions. This helps explain the behavior of gas mixtures and is part of the broader understanding of gas physics.

Dalton’s Law and Partial Pressures

Dalton’s law states that the total pressure of a gas mixture equals the sum of the partial pressures of each gas in the mixture. Each gas exerts pressure according to its fraction of the total mixture.

This law is essential for understanding oxygenation. Atmospheric air contains approximately 21% oxygen. At sea level, the partial pressure of oxygen is determined by multiplying the oxygen fraction by the barometric pressure. However, inspired gas becomes humidified in the airways, and water vapor must be considered.

Water vapor occupies part of the total pressure in a gas mixture. At body temperature, fully saturated gas contains water vapor pressure of approximately 47 mm Hg. Because water vapor contributes to total pressure, it lowers the partial pressure available to other gases, including oxygen and nitrogen.

Note: This concept is important when calculating inspired oxygen pressure, alveolar oxygen pressure, and gas exchange values. It also explains why humidified gas behaves differently from dry gas.

Henry’s Law and Gas Solubility

Henry’s law explains how gases dissolve in liquids. It states that the amount of gas dissolved in a liquid depends on the pressure of the gas above the liquid and the solubility of the gas.

This principle is important in blood gas transport. Oxygen and carbon dioxide both dissolve in blood, but carbon dioxide is much more soluble than oxygen. The amount of dissolved oxygen in plasma is related to PaO₂. Although most oxygen is carried by hemoglobin, dissolved oxygen still determines the partial pressure measured on an arterial blood gas.

Note: Henry’s law also helps explain why pressure changes can affect gases dissolved in body fluids. It is relevant to oxygen transport, carbon dioxide transport, and certain pressure-related conditions.

Graham’s Law and Gas Diffusion

Diffusion is the movement of gas molecules from an area of higher concentration to an area of lower concentration. In the lungs, oxygen diffuses from the alveoli into the pulmonary capillary blood, while carbon dioxide diffuses from the blood into the alveoli.

Graham’s law states that lighter gases diffuse faster than heavier gases. This helps explain differences in gas movement. Carbon dioxide is heavier than oxygen, but it is much more soluble, so it diffuses efficiently across the alveolar-capillary membrane.

Note: Diffusion is affected by several factors, including surface area, membrane thickness, partial pressure gradients, and gas solubility. Diseases that thicken the alveolar-capillary membrane, reduce alveolar surface area, or impair ventilation can interfere with gas exchange.

Critical Temperature, Critical Pressure, and Medical Gas Storage

Some gases can be liquefied if they are cooled and compressed. Critical temperature is the temperature above which a gas cannot be liquefied by pressure alone. Critical pressure is the pressure required to liquefy a gas at its critical temperature.

Oxygen and helium have very low critical temperatures, so they cannot be liquefied by pressure alone at room temperature. They must be cooled to extremely low temperatures. Liquid oxygen must be stored at very cold temperatures because oxygen boils at about −183°C.

Carbon dioxide and nitrous oxide have critical temperatures above room temperature, so they can be stored as liquids under pressure in cylinders. This is why some gas cylinders contain both liquid and gas phases.

Note: Understanding gas storage helps respiratory therapists safely handle cylinders, liquid oxygen systems, and hospital gas supplies. It also helps explain why different gases require different storage methods.

Flow and Fluid Dynamics

Fluid dynamics is the study of fluids in motion. Since gases and liquids are both fluids, this topic applies directly to airflow through the lungs, flow through artificial airways, oxygen delivery devices, aerosol systems, and ventilator circuits.

Flow is the movement of gas or liquid over time. In respiratory care, gas flow depends mainly on pressure gradients and resistance. A greater pressure difference increases flow, while greater resistance decreases flow.

Resistance is opposition to flow. In the respiratory system, resistance may increase because of bronchospasm, airway edema, secretions, mucus plugging, artificial airways, kinked tubing, or narrowed airways. When resistance increases, the patient must generate more effort to breathe. On a mechanical ventilator, increased airway resistance often causes an increase in peak airway pressure.

Flow may be laminar, turbulent, or transitional. Laminar flow is smooth and orderly, with gas moving in layers. Turbulent flow is chaotic and requires more driving pressure. Transitional flow has features of both patterns.

Turbulent flow is more likely when flow is high, gas density is high, or the airway diameter changes abruptly. In narrowed or obstructed airways, turbulent flow increases the work of breathing. This is clinically important in asthma, chronic bronchitis, upper airway obstruction, and artificial airway management.

Flow, Velocity, and Cross-Sectional Area

Flow and velocity are related but not identical. Flow refers to the volume of gas moving per unit of time. Velocity refers to the speed of gas movement.

When flow remains constant, gas velocity increases as a tube narrows and decreases as a tube widens. This principle applies to both natural airways and oxygen delivery equipment. Narrowed airways increase gas velocity and may contribute to turbulence, resistance, and increased work of breathing.

The Bernoulli principle states that as fluid velocity increases, lateral pressure decreases. This principle helps explain how jets and nozzles work in respiratory equipment. When oxygen passes through a small jet, its velocity increases and pressure around the jet decreases. This pressure change can pull surrounding air into the gas stream.

This process is known as entrainment. Entrainment is used in devices that mix oxygen with room air to deliver a specific oxygen concentration. Larger entrainment ports allow more room air to enter, which increases total flow but lowers FiO₂. Smaller ports or higher oxygen concentrations allow less room air to enter, which decreases total flow but increases FiO₂.

Note: Understanding entrainment is important when using air-entrainment masks, nebulizers, high-flow systems, and oxygen delivery devices that depend on precise gas mixing.

Compliance, Resistance, and Time Constants

Compliance describes how easily the lungs and chest wall expand. A highly compliant lung inflates easily. A poorly compliant lung is stiff and difficult to inflate.

Static compliance reflects the elastic properties of the lungs and thorax when airflow is not occurring. Dynamic compliance includes both elastic resistance and airway resistance because it is measured during airflow. Reduced compliance may occur with acute respiratory distress syndrome, pulmonary edema, atelectasis, pneumonia, and pulmonary fibrosis.

High compliance may occur in emphysema. In this condition, the lungs may inflate easily, but elastic recoil is reduced. This can make exhalation difficult and contribute to air trapping.

Time constants combine resistance and compliance. A time constant represents how quickly a lung unit fills and empties. It is calculated by multiplying resistance by compliance. Lung units with short time constants fill and empty quickly. Lung units with long time constants fill and empty slowly.

This concept is especially important in obstructive lung disease. Patients with asthma, chronic bronchitis, and emphysema often have increased airway resistance, which lengthens the time needed for exhalation. If a ventilator does not allow enough expiratory time, air trapping and auto-PEEP may develop.

Mechanical Ventilation and Physical Principles

Mechanical ventilation depends on pressure, volume, flow, resistance, compliance, and time. Every ventilator breath involves the movement of gas through tubing, artificial airways, and the patient’s respiratory system.

Peak airway pressure reflects the pressure required to move gas through the airways and inflate the lungs. Plateau pressure reflects the pressure needed to keep the lungs inflated when there is no airflow. If peak pressure increases while plateau pressure remains unchanged, airway resistance is likely increased. If both peak and plateau pressures increase, lung compliance may be reduced.

Ventilator graphics help therapists evaluate these relationships. Flow-time graphics can help identify air trapping when expiratory flow does not return to baseline before the next breath begins. Pressure-time graphics help assess pressure delivery, peak pressure, and plateau pressure. Volume-pressure loops can provide information about lung compliance, overdistention, and changes in patient mechanics.

Note: These physical principles help therapists make safer ventilator adjustments and recognize problems such as airway obstruction, secretions, bronchospasm, reduced compliance, auto-PEEP, and patient-ventilator asynchrony.

Aerosol Therapy and Particle Behavior

Aerosol therapy depends on particle size, flow, humidity, and airway anatomy. Aerosols are suspensions of liquid or solid particles in gas. To be effective, aerosol particles must remain suspended long enough to be inhaled and must be small enough to reach the intended area of the respiratory tract.

Larger particles tend to deposit in the mouth, throat, and upper airway. Smaller particles are more likely to reach the lower airways. Very small particles may be exhaled before depositing. Particle behavior is influenced by inertia, gravity, diffusion, airway diameter, breathing pattern, and device performance.

Note: Respiratory therapists use these principles when administering bronchodilators, inhaled corticosteroids, mucolytics, antibiotics, and other aerosolized medications. Proper technique, flow, device selection, and patient breathing pattern can all affect drug delivery.

Oxygen Therapy and Equipment Function

Oxygen therapy depends on the physical behavior of gases. Oxygen must be stored, regulated, transported, blended, and delivered in a controlled manner. Compressed gas cylinders use high pressure to store oxygen. Regulators reduce this pressure to a safe working level. Flowmeters allow the therapist to set the desired gas flow.

Low-flow oxygen devices provide oxygen at flows that may not fully meet the patient’s inspiratory demand. The final inspired oxygen concentration depends partly on the patient’s breathing pattern, inspiratory flow, and room air entrainment. Nasal cannulas and simple masks are examples.

High-flow systems can meet or exceed the patient’s inspiratory demand and provide a more stable oxygen concentration. These devices rely on precise flow control, gas mixing, humidification, and pressure relationships.

Note: Understanding these principles helps therapists choose the correct device, set appropriate flow, recognize equipment problems, and ensure that the patient receives the intended oxygen concentration.

Fluidics and the Coanda Effect

Fluidics refers to the use of fluid-flow principles to control gas movement, pressure, or direction without relying on traditional moving mechanical parts. Some respiratory devices use fluidic principles to regulate flow or trigger specific functions.

The Coanda effect, also called wall attachment, occurs when a fluid stream attaches to a nearby surface. This principle can be used to direct gas flow in certain respiratory devices. Because fluidic systems can control gas flow with few or no moving parts, they may provide reliable function in specific types of equipment.

Note: While these concepts may seem more technical, they help explain how certain respiratory care devices function internally.

Physical Principles of Respiratory Care Practice Questions

1. What is absolute humidity?
The actual amount of water vapor contained in a given volume of gas, usually expressed in mg/L.

2. What is absolute zero?
The theoretical temperature at which molecular motion is minimal and a substance has no measurable heat energy.

3. What is adhesion?
The attractive force between unlike molecules.

4. What does ATPS stand for?
Ambient temperature, pressure, saturated.

5. What is Avogadro’s Law?
Equal volumes of gases at the same temperature and pressure contain the same number of molecules.

6. What is the Bernoulli effect?
As fluid flows through a constricted area, its velocity increases and its lateral pressure decreases.

7. What is boiling?
The process in which a liquid changes to a gas when its vapor pressure equals atmospheric pressure.

8. What is buoyancy?
The upward force exerted by a fluid on an object immersed in it.

9. What is Boyle’s Law?
At a constant temperature, the volume of a gas varies inversely with its pressure.

10. What does BTPS stand for?
Body temperature, pressure, saturated.

11. What is the calculation to convert Celsius to Fahrenheit?
F = (C x 1.8) + 32

12. What is Charles’ Law?
At a constant pressure, the volume of a gas varies directly with its absolute temperature.

13. What is cohesion?
The attractive force between like molecules.

14. What is condensation?
The change of state from a gas to a liquid.

15. What is conduction?
The transfer of heat through direct contact between molecules, most commonly in solids.

16. What is convection?
The transfer of heat by the movement of liquids or gases.

17. What is the calculation to convert Celsius to Kelvin?
K = C + 273

18. What is critical temperature?
The temperature above which a gas cannot be liquefied, regardless of the amount of pressure applied.

19. What is Dalton’s Law?
The total pressure of a gas mixture equals the sum of the partial pressures of each gas in the mixture.

20. What is density?
The ratio of a substance’s mass to its volume.

21. What is the dew point?
The temperature at which a gas becomes fully saturated with water vapor and condensation begins.

22. What is evaporation?
The process in which a liquid changes into a gas at a temperature below its boiling point.

23. How do you convert Fahrenheit to Celsius?
C = (F – 32) x 5/9

24. What is the first law of thermodynamics?
Energy cannot be created or destroyed, only transferred or converted from one form to another.

25. What is fluid dynamics?
The study of fluids in motion and the forces that affect their movement.

26. What is gaseous diffusion?
The movement of gas molecules from an area of higher concentration to an area of lower concentration.

27. What is Graham’s Law?
The rate of diffusion or effusion of a gas is inversely proportional to the square root of its molar mass.

28. What is Gay-Lussac’s Law?
At a constant volume, the pressure exerted by a gas varies directly with its absolute temperature.

29. What is Henry’s Law?
At a constant temperature, the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid.

30. What is the kinetic activity of gases?
Gas molecules move rapidly and randomly, and their velocity increases as temperature increases.

31. What is kinetic energy?
The energy an object has because of its motion.

32. What is laminar flow?
Fluid movement in smooth, parallel layers or streamlines.

33. What is the melting point?
The temperature at which a solid changes into a liquid.

34. What is partial pressure?
The pressure exerted by a single gas within a mixture of gases.

35. What is Pascal’s principle?
Pressure applied to a confined fluid is transmitted equally in all directions.

36. What is percent body humidity?
The ratio of the actual water vapor content of inspired gas compared with the maximum water vapor content at body temperature, which is 37°C.

37. What is Poiseuille’s Law?
A law that describes the pressure needed to produce flow through a tube, with flow strongly affected by the tube’s radius, length, and fluid viscosity.

38. What is potential energy?
Stored energy related to an object’s position or the arrangement of its molecules.

39. What is relative humidity?
The ratio of the actual amount of water vapor in a gas to the maximum amount the gas can hold at the same temperature.

40. What is resistance?
The opposition to fluid or gas flow through a tube or airway.

41. What does STPD stand for?
Standard temperature, pressure, dry.

42. What is surface tension?
The force created by cohesive attraction between liquid molecules at the surface of a liquid.

43. What is thermal conductivity?
The ability of a substance to transfer heat.

44. What are the three main states of matter?
Solid, liquid, and gas.

45. What is turbulent flow?
Irregular, chaotic fluid movement in which molecules do not follow smooth streamlines.

46. What is vaporization?
The change of state from a liquid to a gas.

47. What is viscosity?
A measure of a fluid’s resistance to flow.

48. What is water vapor pressure?
The pressure exerted by water vapor in a gas mixture.

49. What does thermodynamics study?
Thermodynamics studies heat, energy, temperature, and how energy is transferred or transformed.

50. What are two common forms of vaporization?
Boiling and evaporation.

51. What are the three major laws of thermodynamics?
The first law describes conservation of energy, the second law describes increasing entropy, and the third law describes the impossibility of reaching absolute zero.

52. Why is absolute zero important?
Absolute zero represents the theoretical point at which molecular motion is minimal and no measurable heat energy remains.

53. What is Archimedes’ Principle?
An object immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.

54. How does the Bernoulli effect apply to respiratory care?
It explains how gas velocity increases and pressure decreases as flow passes through a narrowed area, which helps describe the function of devices such as jet nebulizers and air entrainment systems.

55. How does Boyle’s Law apply to breathing?
When thoracic volume increases, pressure decreases and air moves into the lungs. When thoracic volume decreases, pressure increases and air moves out of the lungs.

56. What is Celsius?
A metric temperature scale in which water freezes at 0°C and boils at 100°C at standard atmospheric pressure.

57. How does Charles’ Law apply to gases?
It explains that gas volume increases as absolute temperature increases, as long as pressure remains constant.

58. How does convection transfer heat?
Convection transfers heat by the movement and mixing of liquids or gases at different temperatures.

59. What is gas density?
The mass of a gas per unit volume.

60. What is entropy?
A measure of disorder in a system and the amount of energy that is unavailable to perform useful work.

61. What is Fahrenheit?
A temperature scale in which water freezes at 32°F and boils at 212°F at standard atmospheric pressure.

62. What are flow patterns?
Flow patterns describe how a fluid or gas moves through a tube or airway, such as laminar, turbulent, or transitional flow.

63. What causes pressure to decrease as fluid flows through a tube?
Pressure decreases because energy is lost as the fluid overcomes resistance within the tube.

64. Fluid viscosity is directly proportional to what?
The cohesive forces between the fluid’s molecules.

65. What are gases?
Gases are substances that have no fixed shape or volume and expand to fill their container.

66. When must the heart use more energy to pump blood?
The heart must use more energy when blood viscosity increases, such as in polycythemia.

67. What are the four major ways heat can be transferred?
Conduction, convection, radiation, and evaporation or condensation.

68. What are the main patterns of flow?
Laminar flow and turbulent flow.

69. What is plasma?
Plasma is an ionized state of matter made up of free electrons, ions, and neutral particles.

70. What does Poiseuille’s Law explain?
Poiseuille’s Law explains how flow through a tube is affected by pressure, tube radius, tube length, and fluid viscosity.

71. What is polycythemia?
A condition characterized by an abnormal increase in the number of red blood cells, which can increase blood viscosity.

72. What is potential energy in matter?
Potential energy is stored energy related to the position of molecules and the attractive forces between them.

73. What does the second law of thermodynamics describe?
The second law states that energy transfer is never completely efficient and that entropy tends to increase.

74. Is air a compound or a mixture?
Air is a mixture, not a compound.

75. The stronger the cohesive forces in a fluid, the greater the what?
The fluid’s viscosity.

76. Why is thermal conductivity important?
Thermal conductivity determines how efficiently a substance can transfer heat to another substance or environment.

77. What is radiation heat transfer?
The transfer of heat through electromagnetic waves without direct physical contact.

78. What is the absolute humidity of fully saturated gas at 37°C?
Fully saturated gas at 37°C contains approximately 44 mg/L of water vapor.

79. How do you convert Celsius to Kelvin?
Add 273 to the Celsius temperature.

80. What is the lowest theoretical temperature?
Absolute zero.

81. What is the attractive force between unlike molecules called?
Adhesion.

82. Which law states that equal volumes of gases contain the same number of molecules at the same temperature and pressure?
Avogadro’s Law.

83. What effect occurs when fluid velocity increases and lateral pressure decreases through a constriction?
The Bernoulli effect.

84. What is the boiling point?
The temperature at which a liquid’s vapor pressure equals atmospheric pressure.

85. What is the upward force that supports an object in a fluid called?
Buoyancy.

86. What is capillary action?
The movement of liquid through a small tube or narrow space due to adhesion, cohesion, and surface tension.

87. What is the change of state from a gas to a liquid called?
Condensation.

88. What is the transfer of heat by direct contact?
Conduction.

89. Which law states that the total pressure of a gas mixture equals the sum of the partial pressures of its component gases?
Dalton’s Law.

90. What is the ratio of mass to volume called?
Density.

91. What is the term for the amount of energy in a system that is unavailable for useful work?
Entropy.

92. Which law states that the rate of diffusion of a gas is inversely proportional to the square root of its molecular weight?
Graham’s Law.

93. The pressure exerted by a liquid depends on what two factors?
The depth of the liquid and the liquid’s density.

94. Which law states that the amount of gas dissolved in a liquid is proportional to its partial pressure?
Henry’s Law.

95. What is hydrodynamics?
The study of liquids in motion.

96. What is the energy of motion?
Kinetic energy.

97. Which type of flow occurs when fluid moves in smooth, parallel layers?
Laminar flow.

98. Which law states that pressure is directly proportional to surface tension and inversely proportional to radius?
Laplace’s Law.

99. Which principle states that pressure applied to a confined fluid is transmitted equally in all directions?
Pascal’s principle.

100. What is the solubility coefficient?
The volume of gas that dissolves in a given amount of liquid at a specified temperature and pressure.

Final Thoughts

The physical principles of respiratory care explain much of what respiratory therapists do at the bedside. Gas moves because of pressure differences. Resistance affects flow. Compliance affects lung expansion. Temperature and humidity influence airway function.

Gas laws explain changes in pressure, volume, and partial pressure. Fluid dynamics helps explain oxygen delivery, aerosol therapy, artificial airway resistance, and ventilator performance.

By understanding these concepts, respiratory therapists can better interpret patient data, troubleshoot equipment, and make safer clinical decisions. These principles are not just classroom topics. They apply directly to oxygen therapy, mechanical ventilation, airway management, and patient care.