 When it comes to breathing, the lungs are vital organs that allow us to take in oxygen and expel carbon dioxide. The process of respiration starts with the diaphragm, a large muscle located at the base of the lungs.

When we inhale, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity. This causes a decrease in air pressure, which allows air to flow into the lungs.

This process is possible due to the laws and physical principles of respiratory care that take place, which is what we will be discussing in this guide.

## States of Matter

In order for the lungs to work properly, it is important to understand the states of matter and how they apply to respiration. There are three main states of matter: solid, liquid, and gas.

Solids are particles that are close together and have a definite shape. Liquids are particles that are close together but do not have a definite shape. An example of a liquid is water. Gases are particles that are far apart and do not have a definite shape.

The state of matter that is important for respiration is gas. In order for the lungs to work, air must be able to flow in and out during a breathing cycle. This is only possible if the air is in a gaseous state.

## Laws of Thermodynamics

The term thermodynamics refers to the relationship between heat and energy. In order for the lungs to work, they must follow the laws of thermodynamics.

The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. The second law of thermodynamics states that when energy is converted from one form to another, there is always some loss of energy.

The entropy of a system approaches a constant value as the temperature approaches absolute zero. When two systems are in thermal equilibrium with each other, their temperatures and entropy are equal.

The laws of thermodynamics are important for the lungs because they help to explain how energy is converted from one form to another during respiration. ## Change of State

Respiratory therapists can work with both liquids and gases, which means they must be familiar with the concept of change of state.

A change of state is when a substance changes from one state of matter to another. For example, water can exist in all three states of matter: solid (ice), liquid (water), and gas (steam).

## Gas Laws

There are three primary gas laws that respiratory therapists should be familiar with:

1. Boyle’s law
2. Charles’ law
3. Gay-Lussac’s law

Each of these laws explains how gases behave under different conditions.

### Boyle’s Law

Boyle’s law states that the pressure of a gas is inversely proportional to the volume of the gas. This means that as the volume of a gas increases, the pressure of the gas decreases.

### Charles’ Law

Charles’ law states that the volume of a gas is directly proportional to the temperature of the gas. This means that as the temperature of a gas increases, the volume of the gas increases.

### Gay-Lussac’s Law

Gay-Lussac’s law states that the pressure of a gas is directly proportional to the temperature of the gas. This means that as the temperature of a gas increases, the pressure of the gas increases.

## Fluid Dynamics

Fluid dynamics is the study of how fluids (liquids and gases) move. In order for respiratory therapists to understand how the lungs work, they must be familiar with the principles of fluid dynamics.

### Bernoulli’s Principle

One of the most important principles of fluid dynamics is Bernoulli’s principle, which states that as flow increases, the pressure in a fluid decreases.

This principle is important for respiratory therapists to understand because it explains how air flows into and out of the lungs during a breathing cycle.

The Bernoulli equation is used to calculate the pressure of a fluid at any point in a pipe or tube.

## Physical Principles of Respiratory Care Guide Practice Questions:

1. What is absolute humidity?
The mass of water vapor contained in a certain volume of air.

2. What is absolute zero?
No kinetic energy.

The attractive force between unlike molecules.

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

Equal volumes of gases contain the same number of molecules.

6. What is the Bernoulli effect?
Fluid passing through a tube that meets a constriction experiences a significant pressure drop. Fluid that flows through the constriction increases its velocity while the lateral wall pressure decreases.

7. What is boiling?
Heating a liquid to a temperature at which its vapor pressure equals atmospheric pressure.

8. What is buoyancy in liquids?
Occurs because the pressure below a submerged object always exceeds the pressure above it.

9. What is Boyle’s Law?
The volume of 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?
The volume of gas varies directly with its temperature.

13. What is cohesion?
It is the attractive force between like molecules.

14. What is condensation?
Change from gas to liquid.

15. What is conduction?
Transfers heat in solids.

16. What is convection?
Transfers heat in liquids and gases.

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

18. What is critical temperature?
The temperature above which the substance cannot exist in the liquid state.

19. What is Dalton’s Law?
The partial pressure of a gas in a mixture is proportional to its percentage in the mixture.

20. What is the density of a gas?
The ratio of its mass to its volume.

21. What is the dew point?
The temperature at which the water vapor in the air becomes saturated and condensation begins.

22. What is evaporation?
When water enters the atmosphere at a temperature below its boiling point.

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

24. What is the first law of thermodynamics?
Energy cannot be created nor destroyed. Energy gain by a substance = energy lost by surroundings.

25. What is fluid dynamics?
The pressure exerted by a liquid in motion depends on the nature of the flow itself.

26. What is gaseous diffusion?
The movement of molecules from areas of high concentration to areas of lower concentration.

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

28. What is Guy-Lussac’s Law?
The pressure exerted by a gas varies directly with its absolute temperature.

29. What is Henry’s Law?
At a given temperature the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid. The volume of gas dissolved in a liquid is a function of its solubility coefficient and its partial pressure

30. What is the kinetic activity of gases?
Gas molecules travel at high speeds in random fashion with frequent collisions. The velocity of gas molecules is directly proportional to its temperature.

31. What is Kinetic energy?
Energy an object has due to its motion.

32. What is laminar flow?
Fluid moving in discrete cylindrical layers or streamlines.

33. What is the melting point?
The temperature at which melting occurs.

34. What is partial pressure?
The pressure exerted by a single gas in a gas mixture.

35. What is Pascal’s law?
Pressure depends on depth and density.

36. What is percent body humidity?
The ratio of the amount of water vapor in a volume of gas compared to the amount of the water in gas saturated at a normal body temperature of 37* C.

37. What is Poiseuille’s Law?
Predicts the pressure required to produce a given flow.

38. What is potential energy?
Stored energy.

39. What is relative humidity?
The ratio of the amount of water in the air at a given temperature to the maximum amount it could hold at that temperature.

40. What is resistance?
Causes a progressive decrease in fluid pressure as the fluid flows through a tube.

41. What does STPD stand for?
Standard Temperature Pressure Dry.

42. What is surface tension?
A force exerted by like molecules at a liquid’s surface. It is a measure of how difficult it is to stretch or break the surface of a liquid.

43. What is thermal conductivity?
The ability of an object to transfer heat.

44. What are the three states of matter?
Solid, Liquid, and Gas.

45. What is turbulent flow?
Loss of regular streamlines; fluid molecules form irregular currents in a chaotic pattern.

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

47. What is the viscosity of liquids?
Force opposing a fluid’s flow (blood has 5x viscosity of water)

48. What is water vapor pressure?
The atmospheric pressure exerted by the water vapor in the air. It increases as the water vapor in air increases.

49. What are the subjects of thermodynamics?
(1) Science – studying the properties of matter at various temperatures. 2. Kinetics – (speed) of reactions of matter at various temperatures.

50. What are the two types of vapor?
(1) Boiling Point, and (2) Evaporation.

51. What are the 3 physical principles that describe how energy is handled and transferred?
(1) Conservation of Energy. (2) Thermodynamic Equilibrium. (3) Impossibility of Achieving Absolute Zero.

52. What is Absolute Zero?
It is the lowest temperature theoretically attainable (at which the kinetic energy of atoms and molecules is minimal). The temperature at which no kinetic energy exists. Molecules cease to vibrate; object has no measurable heat. It has not actually been achieved.

53. What is Archimedes’ Principle?
The buoyant force on an object is equal to the weight of the fluid displaced by the object. The property of something weightless and insubstantial. Occurs because pressure below submerged objects always exceeds pressure above the object. Gases also exert buoyant forces. Helps keep solid particles suspended in gases (aerosols).

54. What is the Bernoulli Effect?
As the velocity of airflow increases, pressure decreases with total energy remaining constant. Fluid passing through a tube that meets constriction experiences significant pressure drop. Fluid that flows through constriction increases it’s velocity while lateral wall pressure decreases.

55. What is Boyle’s Law?
The relationship between the pressure and volume of a gas at constant temperature; when volume increase, pressure decreases.

56. What is Celsius?
Metric unit for measuring temperature. On this scale water freezes at zero and boils at 100. At -273 degrees C is where kinetic molecular activity stops which is equal to 0 K.

57. What is Charles Law?
A principle that describes the relationship between the temperature and volume of a gas at constant pressure. Volume of a gas varies directly with its temperature. If the temperature goes up, so does volume. If it goes down, so does the volume.

58. What is convection?
Mixing of fluid molecules at different temperatures. Transfers heat in liquids and gases (e.g. forced air heating in homes-fluid movements carry heat).

59. What is density?
The ratio of a gas’ mass to its volume.

60. What is entropy?
A thermodynamic quantity representing the amount of energy in a system that is no longer available for doing mechanical work. Amount of energy in a system not available for work. Lowest amount of organization system can achieve.

61. What is Fahrenheit?
A temperature scale where 32 is freezing and 212 is boiling.

62. What are flow patterns?
The pattern that flow occurs, along with its shape.

63. What is fluid dynamics?
The pressure exerted by a liquid in motion depends on nature of flow itself. Progressive decrease in fluid pressure occurs as fluid flows through tube due to resistance.

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

65. What are gases?
They have no fixed volume or shape; weak attractive forces. Gas molecules exhibit rapid, random motion with frequent collisions. When these molecules collide they tend to bounce off of each other rather than attach to one another.

66. The heart must use more energy when?
When blood viscosity increases, as occurs in polycythemia (increase in red blood cell concentration).

67. What are the four ways that heat transfer can occur?

68. What are the patterns of flow?
Laminar Flow and Turbulent Flow.

69. Plasma is a combo of what?
Neural atoms, free electrons, and atomic nuclei.

70. What is Poiseuille’s Law?
The relationship between the pressure, volume, and temperature of a fixed amount of gas; it predicts the pressure required to produce given flow.

71. What is polycythemia?
A disorder characterized by an abnormal increase in the number of red blood cells in the blood.

72. What is potential energy?
Energy of position (attractive forces between molecules). Weak in gas state. Makes up most of internal energy in solids and liquids.

73. The Second Law of Thermodynamics is called conservation of energy and is described as?
The energy cannot be created, nor destroyed.

74. What are the states of matter?
Solid, Liquid, or Gas.

75. The stronger the cohesive forces the greater the?
Fluid viscosity.

76. What is Thermal Conductivity?
The ability of an object to transfer heat. A measure to quantify heat transfer between objects.

77. What is radiation heat transfer?
Heat transfer that occurs without direct physical contact.

78. Air that is fully saturated with water vapor at 37 degrees Celsius has a water vapor pressure of 47 mmHg and an absolute humidity of what?
43.8

79. The degrees Kelvin = degrees Celsius + what?
273

80. What is the lowest possible temperature that can be achieved and there is no kinetic energy?
Absolute Zero.

81. What is the attractive force between unlike molecules?

82. What is the law that states that the 1-g atomic weight of any substance contains exactly the same number of atoms, molecules, or ions?

83. When a fluid flows through a tube of uniform diameter, pressure decreases progressively over the tube length?
Bernoulli effect.

84. What is the temperature at which vapor pressure of a liquid exceeds the atmospheric pressure?
Boiling point.

85. Archimedes’ principle explains the difference in liquid pressure that creates an upward or supporting force. What is this called?
Buoyancy.

86. A phenomenon in which liquid in a small tube moves upward against gravity?
Capillary action.

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. What is the law stating that the total pressure of a mixture of gases must equal the sum of the partial pressures of all component gases?
Dalton’s law.

90. What is the ratio of the mass of a substance to its volume?
Density

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

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

93. The pressure exerted by a liquid depends on what 2 factors?
Height and weight density (weight per unit volume).

94. What is the law that states the volume of a gas that dissolves in a liquid is equal to its solubility coefficient times its partial pressure?
Henry’s law.

95. What is the study of fluids in motion?
Hydrodynamics.

96. What is the energy of motion?
Kinetic energy

97. In this type of flow, a fluid moves in cylindrical layers, or streamlines?
Laminar flow

98. According to this law, the pressure varies directly with the surface tension of the liquid and inversely with its radius?
Laplace’s law

99. What principle explains that the pressure of a given liquid is the same at any specific depth regardless of the container’s shape because the pressure of a liquid acts equally in all directions?
Pascal’s principle

100. The volume of a gas that will dissolve in 1 ml of a given liquid at standard pressure and specified temperature is called what?
Solubility coefficient.

## Final Thoughts

In order for respiratory therapists to understand how the lungs work, they must be familiar with the principles of fluid dynamics and the laws of thermodynamics.

These principles help to explain how energy is converted from one form to another during respiration, and how gases behave under different conditions.

Hopefully, you can use this information to develop a better understanding of this topic. We have a similar guide on humidity and aerosol therapy that I think you will find helpful. Thanks for reading and, as always, breathe easy, my friend.

### References

The following are the sources that were used while doing research for this article:

• Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020. [Link]
• Pittman, Roland. “The Respiratory System and Oxygen Transport – Regulation of Tissue Oxygenation – NCBI Bookshelf.” National Center for Biotechnology Information, U.S. National Library of Medicine, 2011, www.ncbi.nlm.nih.gov/books/NBK54114.
• “The History and Physical Examination.” PubMed Central (PMC), 2006, www.ncbi.nlm.nih.gov/pmc/articles/PMC7149736.
• “The Lung Microbiome: New Principles for Respiratory Bacteriology in Health and Disease.” PubMed Central (PMC), 1 July 2015, www.ncbi.nlm.nih.gov/pmc/articles/PMC4497592.

Disclosure: The links to the textbooks are affiliate links which means, at no additional cost to you, we will earn a commission if you click through and make a purchase.

## Follow

AN ELITE CAFEMEDIA LIFESTYLE PUBLISHER