Pulmonary Diffusion Vector Illustration

Pulmonary Diffusion: Overview and Practice Questions (2023)

by | Updated: Nov 30, 2023

Pulmonary diffusion is a vital physiological process that plays a fundamental role in the exchange of gases within the respiratory system.

It is the mechanism by which oxygen from inhaled air is transported into the bloodstream while carbon dioxide, a waste product of metabolism, is eliminated from the body.

Understanding the principles of pulmonary diffusion is crucial for grasping the mechanics of respiration and the maintenance of oxygenation in the human body.

What is Pulmonary Diffusion?

Pulmonary diffusion is the process by which oxygen moves from the alveoli in the lungs into the blood, and carbon dioxide moves from the blood into the alveoli for exhalation. This gas exchange occurs across the alveolar-capillary membrane, driven by concentration gradients, ensuring the body receives the oxygen it needs while expelling carbon dioxide.

Pulmonary Diffusion Labeled Illustration

What are the Functions of Pulmonary Diffusion?

Pulmonary diffusion serves several essential functions in the respiratory system, including the following:

  • Oxygen Uptake: It allows the blood to receive oxygen from the inhaled air in the alveoli. This oxygenated blood is then transported to various tissues and organs throughout the body.
  • Carbon Dioxide Elimination: Pulmonary diffusion facilitates the removal of carbon dioxide, a waste product of cellular metabolism, from the blood. This CO2 is then exhaled from the body.
  • Maintenance of Blood pH: By regulating the levels of oxygen and carbon dioxide in the blood, pulmonary diffusion helps maintain the body’s blood pH, ensuring it remains within the optimal range.
  • Regulation of Gas Concentrations: It ensures that gas concentrations in the blood remain balanced, supporting cellular functions and metabolic processes.

Pulmonary diffusion ensures efficient gas exchange, which is vital for maintaining cellular respiration and overall metabolic function.

What Causes Diffusion Problems in the Lungs?

Diffusion problems in the lungs can be caused by various factors that interfere with the effective exchange of gases. Some examples include:

  • Interstitial Lung Diseases: Conditions such as pulmonary fibrosis, sarcoidosis, and silicosis can cause thickening of the alveolar walls, impairing gas exchange.
  • Emphysema: This condition damages the alveoli, reducing the surface area available for gas exchange.
  • Pulmonary Edema: The buildup of fluid in the alveoli can hinder the passage of gases.
  • Chronic Bronchitis: Chronic inflammation and mucus production can obstruct airflow and affect gas exchange.
  • Acute Respiratory Distress Syndrome (ARDS): This is a severe lung condition causing fluid buildup in the alveoli, leading to impaired oxygenation.
  • Pneumonia: Infection in the lungs can fill alveoli with pus and fluid, impeding gas diffusion.
  • Pulmonary Embolism: A blood clot in the pulmonary artery can reduce blood flow to parts of the lung, affecting gas exchange in the affected areas.
  • Alveolar Hypoventilation: Reduced ventilation of the alveoli due to issues like neuromuscular diseases can lead to reduced oxygen and increased carbon dioxide in the blood.
  • Anemia: Reduced hemoglobin levels can affect the capacity of the blood to carry oxygen, even if lung diffusion is normal.
  • Altitude: High altitudes have lower oxygen levels, which can challenge the lungs’ ability to oxygenate the blood effectively.
  • Smoking: Chronic smoking can damage the alveoli and narrow the airways, leading to reduced surface area and efficiency for gas exchange.

Note: Regular exposure to harmful substances, genetic predisposition, and other systemic diseases can also contribute to diffusion problems in the lungs. Identifying and managing these issues early is crucial to prevent severe complications.

Pulmonary Diffusion Practice Questions

1. What causes gas to move in and out of the lungs?
Pressure gradients

2. What causes an individual gas to cross the alveolar-capillary membrane?
Diffusion gradients

3. What is Dalton’s law?
The law of partial pressure which states that the total pressure is equal to the sum of the partial pressure of each separate gas.

4. What gases are in the atmosphere?
Nitrogen, oxygen, carbon dioxide, water vapor, and trace gases.

5. How do you calculate the partial pressure for each gas?
Barometric pressure multiplies by the percentage of the gas in the atmosphere.

6. What law states that the rate of gas transfer across a sheet of tissue is directly proportional to the surface area of the tissue, to the diffusion constants, the difference in partial pressure of the gas between the two sides of the tissue, and is inversely proportional to the thickness of the tissue?
Fick’s law

7. What law states that the rate of gas diffusion through a liquid is directly proportional to the solubility coefficient of the gas and indirectly proportional to the square root of the gram-molecular weight of the gas?
Graham’s law

8. What law states that the amount of a gas that dissolves in a liquid at a given temp is proportional to the partial pressure of the gas?
Henry’s law

9. What term means that the transfer of a gas across the alveolar wall is a function of the amount of blood that flows past the alveoli?
Perfusion limited

10. What term means that the movement of gas across the alveolar wall is a function of the integrity of the alveolar-capillary membrane itself?
Diffusion limited

11. Why is the PaO2 different than the atmospheric O2?
Because when air is inhaled, by the time the molecules reach the alveoli, they are diluted by the addition of CO2 and H2O molecules.

12. What law uses this equation: Ptotal = P1+P2+P3?
Dalton’s law

13. What law uses this equation: PAO2 = [Pb – PH2O] X FiO2 – PaCO2 X (1.25)?
Alveolar gas equation

14. What law uses this equation: V gas = [AD (P1 – P2)]/T?
Fick’s law

15. What referes to the movement of gas from an area of high pressure (high concentration) to an area of low pressure (low concentration)?
Pressure gradient

16. What is the movement of individual gas molecules from an area of high pressure to an area of low pressure?
Gas diffusion

17. At what body temperature is the gas in the alveoli fully saturated?

18. Why is the partial pressure of oxygen in the atmosphere higher than the partial pressure of oxygen in the alveoli?
Because alveolar oxygen must mix or compete with alveolar CO2 pressure and alveolar water vapor pressure, which are not as high in the atmosphere.

19. How much water vapor pressure is exerted in the lung?
47 mmHg

20. What is the ideal alveolar gas equation?
PAO2 = [PB – PH2O] FIO2 – PaCO2(1.25)

21. What is the normal respiratory exchange ratio?
200/250 = 0.8

22. What are the layers that CO2 and O2 must go through?
Alveolar fluid, alveolar epithelium, basal alveolar membrane, interstitial space, basal capillary membrane, capillary epithelium, and RBC.

23. How can you calculate the difference between alveolar and arterial oxygen gradients?
PAO2 – PaO2

24. Why calculate the difference between the alveoli and the blood?
Because it helps identify problems associated with the A-C capillary membrane.

25. What are the normal gradients?
5-15 mmHg

26. What law is the amount of gas that diffuses from one point to another by exchange?
Fick’s law

27. What cells are the most important and need to get enough oxygen and complex sugars?
Brain cells and myocardial cells

28. What is the significance of Henry’s and Graham’s laws in regard to O2 and CO2 exchange?
CO2 diffuses about 20x faster than O2

29. Which law says that CO2 is 24x more soluble than oxygen?
Henry’s law

30. Which law says that oxygen is lighter, thereby it diffuses 1.7x faster than CO2?
Graham’s law

31. What variables change in the alveolar air equation when at a higher altitude?
Barometric pressure

32. What atmospheric gas typically has the greatest partial pressure in the alveoli while breathing room air?

33. What is the FiO2 at 30,000 ft?

34. What does the ‘a’ in PaO2 stand for?
a = arterial

35. What does the ‘A’ in PAO2 stand for?
A= alveolar

36. Which gas has a greater pressure gradient between the alveolus and pulmonary capillary blood flow?
Oxygen has a greater gradient than CO2.

37. What is the equation for Fick’s law?
A x D x (P1 – P2) / T

38. What is the normal gas equilibrium met in the capillary transit time?

39. What does perfusion-limited mean?
This term refers to how much blood flow is going through the alveolar-capillary membrane.

40. What is normal alveolar water vapor pressure?
47 mmHg

41. What is the normal alveolar CO2 pressure?
40 mmHg

42. What is the normal atmospheric pressure?
760 mmHg

43. What is the solubility coefficient of oxygen at 37 C and 760 mmHg?
0.0244 mL/mm Hg/mL H2O

44. What is the solubility coefficient of carbon dioxide at 37 C and 760 mmHg?
0.592 mL/mm Hg/mL H2O

45. What is simple definition for pulmonary diffusion?
The movement of gases across the alveolar-capillary membrane

46. What is kinetic energy?
The driving force responsible for diffusion.

47. What does diffusion do if alveolar oxygen pressure decreases?
It decreases

48. What does Henry’s law state?
That carbon dioxide is more soluble than oxygen.

49. What test is done to measure diffusion capacity?

50. What pulmonary disorders can increase the alveolar-capillary thickness?
Pulmonary edema, pneumonia, interstitial lung diseases, acute respiratory distress syndrome (ARDS), and RDS in newborns.

51. How does oxygen and carbon dioxide diffuse across the alveolar-capillary membrane?
Oxygen molecules diffuse across the alveolar-capillary membrane into the blood, while the carbon dioxide molecules diffuse out of the capillary blood and into the alveoli.

52. What are diffusion gradients?
Individual gas partial pressure differences that help move a gas across the alveolar-capillary (AC) membrane.

53. What are all the gases in the earth’s atmosphere?
Nitrogen (NO2), oxygen (O2), carbon dioxide, and other trace gases (e.g., argon).

54. Depending on the surrounding temperature and pressure, water can exist as what?
Liquid, gas, or solid.

55. What do you call the water in gaseous form?
Water vapor

56. What is the absolute humidity of alveolar gas?
44 mg/L

57. What is the process of diffusion?
Passive movement of gas molecules from an area of high partial pressure to an area of low partial pressure until both areas are of equal pressure.

58. In the presence of certain pulmonary diseases, the time available to achieve oxygen equilibrium in the alveolar-capillary system may not be adequate. Such diseases include what?
Pulmonary edema, pneumonia, and interstitial lung diseases.

59. When can oxygen toxicity develop?
Within 24 hours in response to high partial pressures of inspired oxygen (PO2) and with longer exposure times to inspired oxygen concentrations (FIO2) above 0.50.

60. What is Henry’s law?
A law that states that the amount of a gas that dissolves in a liquid at a given temperature is proportional to the partial pressure of the gas.

61. Which type of COPD affects pulmonary diffusion?

62. What is the meaning of perfusion-limited?
The transfer of gas across the alveolar wall is a function of the amount of blood that flows past the alveoli.

63. What is the meaning of diffusion-limited?
The movement of a gas across the alveolar wall is a function of the integrity of the alveolar-capillary membrane itself.

64. What is the partial pressure of oxygen, assuming a normal barometric pressure of 760 mmHg, if the percentage of oxygen is 0.4?
304 mmHg

65. How many feet below sea level must an individual descend to exert a total pressure on the body of 3 atmospheres (2,280 mmHg)?
66 feet

66. Which of the following gas laws states that in a mixture of gases, the total pressure is equal to the sum of the partial pressure of each gas?
Dalton’s law

67. What is the normal percentage of carbon dioxide in the atmosphere at sea level?

68. What is the alveolar water vapor pressure at sea level?
47 torr

69. What is the patient’s alveolar oxygen tension if he is receiving an FiO2 of 0.60 on a day when the barometric pressure is 725 mmHg, and if the Paco2 is 50 torr?
344 torr

70. What is the normal transit time for blood within the alveolar-capillary system?
0.75 seconds

71. What law states that the rate of gas diffusion is inversely proportional to the weight of the gas?
Graham’s law

72. What is gas diffusion according to Fick’s law?
Directly proportional to the difference in partial pressure of the gas between the two sides.

73. What is kinetic energy?
The driving force responsible for diffusion.

74. Where does pulmonary diffusion occur?
A-C membrane

75. What is pulmonary diffusion also known as?
Gas exchange

FAQs About Pulmonary Diffusion

What is Pulmonary Diffusion Capacity?

Pulmonary diffusion capacity is the amount of gas transferred from the alveoli in the lungs to capillary blood during gas exchange.

Once air from the lungs crosses the alveolar-capillary membrane, it is picked up by red blood cells and transported to the tissues of the body.

Which of These Factors is Most Critical in Determining the Rate of Pulmonary Diffusion?

The gas partial pressure difference is the most important factor in determining the rate of pulmonary diffusion. This is also known as the pressure gradient.

Partial pressure is defined as the pressure that each gas would exert if it were present alone. In other words, it measures how much a given gas contributes to the total pressure.

The partial pressure gradient is what drives the diffusion process and is determined by subtracting the partial pressure of a gas on one side from the partial pressure of the gas on the other side.

Why Does Oxygen Diffusion from the Alveolus to the Pulmonary Capillary Occur?

Oxygen diffusion from the alveoli to the pulmonary capillary occurs because the alveolar PO2 is greater than the capillary PO2.

The alveolar PO2 is the partial pressure of oxygen in the alveoli, and the capillary PO2 is the partial pressure of oxygen in the pulmonary capillaries.

When these two values are subtracted, it results in a positive number, and this gradient is what drives the diffusion of oxygen from the alveoli to the blood.

What is the Difference Between Diffusion and Perfusion?

Diffusion is the process of how gases are transported across a membrane, while perfusion is the process of how blood is transported through the body.

Both diffusion and perfusion are necessary for gas exchange to take place.

Pulmonary diffusion refers to the diffusion of gases between the alveoli and blood, while pulmonary perfusion refers to the movement of blood through the pulmonary arteries and veins.

What is Diffusion Capacity in a Pulmonary Function Test?

Diffusion capacity (DLCO) is a measure of how well the lungs are able to exchange oxygen and carbon dioxide.

This test is used to diagnose lung conditions such as chronic obstructive pulmonary disease (COPD).

The test involves breathing in a mixture of gases and measuring how much oxygen and carbon dioxide are in your blood before and after. The difference between the two values is the diffusing capacity.

Final Thoughts

Pulmonary diffusion remains an indispensable mechanism for sustaining life by facilitating the exchange of gases essential for cellular respiration.

A thorough grasp of pulmonary diffusion is essential not only for the comprehension of normal respiratory function but also for diagnosing and managing various respiratory disorders.

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.


  • Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
  • Cardiopulmonary Anatomy & Physiology: Essentials of Respiratory Care. Cengage Learning, 2019.
  • Rrt, Des Terry Jardins MEd, and Burton George Md Facp Fccp Faarc. Clinical Manifestations and Assessment of Respiratory Disease. 8th ed., Mosby, 2019.
  • Zhi H, Ji X, Zhao Z, Liang H, Zhong S, Luo Y, Zhong M, Zhan C, Gao Y, Deng X, Li S, Li J, Zhong N, Jiang M, Chen R. Risk factors for impaired pulmonary diffusion function in convalescent COVID-19 patients: A systematic review and meta-analysis. EClinicalMedicine. 2022.

Recommended Reading