Gas Exchange Overview Illustration

Gas Exchange: Overview and Practice Questions (2024)

by | Updated: Mar 14, 2024

Gas exchange is a fundamental physiological process that occurs in the lungs, enabling the human body to obtain the oxygen it needs for cellular functions and to expel carbon dioxide, a waste product of metabolism.

This process is not only vital for sustaining life, but its efficiency can also be a critical indicator of respiratory health.

Understanding the mechanics, regulation, and pathophysiology of gas exchange is essential for clinicians, researchers, and even laypeople, as it has direct implications for conditions like asthma, chronic obstructive pulmonary disease (COPD), and various forms of respiratory failure.

What is Gas Exchange?

Gas exchange is the biological process by which gases are transferred between an organism and its environment. In humans, the primary site for gas exchange is the alveoli, tiny air sacs within the lungs. Here, oxygen from the air we breathe in diffuses through the alveolar walls into the bloodstream, while carbon dioxide—a waste product of cellular respiration—moves in the opposite direction, from the blood to the alveoli, to be exhaled.

Gas exchange process vector illustration

The actual exchange occurs across a thin respiratory membrane that separates the air in the alveoli from the blood in the surrounding capillaries. This membrane is highly permeable to gases, allowing for efficient exchange.

Hemoglobin molecules in the red blood cells bind to oxygen and carry it from the lungs to various tissues throughout the body, where it is used for metabolic processes.

Concurrently, carbon dioxide produced as a waste product in tissues is transported back to the lungs via the bloodstream to be eliminated from the body through exhalation.


Summary: The process of gas exchange is essential for sustaining life, as it enables the cells in the body to perform aerobic respiration, which is crucial for generating energy. A failure or inefficiency in the gas exchange system can lead to respiratory conditions such as hypoxia (low levels of oxygen in the body) or hypercapnia (elevated levels of carbon dioxide), which can be life-threatening if not properly managed.

What is Pulmonary Diffusion?

Pulmonary diffusion refers to the process by which gases move across the respiratory membrane in the lungs, essentially serving as the core mechanism of gas exchange in humans. This occurs in the alveoli, the tiny sac-like structures in the lungs where the walls are extremely thin—only about one cell thick—and highly vascularized, meaning they have many tiny blood vessels, or capillaries.

The term “diffusion” in this context describes the passive movement of gases from an area of higher concentration to an area of lower concentration.

During inhalation, air rich in oxygen fills the alveoli. The concentration of oxygen is higher in the alveolar air than in the blood of the capillaries surrounding the alveoli.

As a result, oxygen diffuses across the alveolar membrane into the blood. Hemoglobin in the red blood cells binds to this oxygen and carries it throughout the body, delivering it to tissues where it is needed for cellular respiration.

Conversely, carbon dioxide, a waste product of cellular metabolism, is present in higher concentrations in the blood than in the alveolar air. This difference in concentration causes carbon dioxide to diffuse across the respiratory membrane from the blood into the alveoli, from where it is expelled during exhalation.

The efficiency of pulmonary diffusion can be influenced by a number of factors, including the thickness of the respiratory membrane, the surface area of the alveoli, and the concentration gradient of the gases.

Note: Any conditions that affect these factors, such as pulmonary fibrosis (which thickens the respiratory membrane) or emphysema (which reduces the surface area for gas exchange), can impair pulmonary diffusion and, consequently, the overall process of gas exchange.

Alveolar-Capillary Membrane

The alveolar-capillary membrane is the thin barrier that separates the air in the alveoli from the blood in the surrounding capillaries in the lungs. This membrane is crucial for the process of gas exchange and pulmonary diffusion.

The alveolar wall is made up of a single layer of epithelial cells, and the capillary wall is similarly thin, composed of a single layer of endothelial cells. These features make the membrane extremely permeable to gases like oxygen and carbon dioxide, enabling efficient diffusion.

The alveolar-capillary membrane allows oxygen to move from the alveoli into the blood and carbon dioxide to move from the blood into the alveoli.

The efficiency of this gas exchange process depends on several factors, including the thickness of the membrane, the surface area available for gas exchange, and the concentration gradients of the gases involved.

Diseases and conditions like pneumonia, pulmonary edema, or pulmonary fibrosis can cause the alveolar-capillary membrane to thicken, thereby decreasing the efficiency of gas exchange.

Emphysema, on the other hand, damages the alveoli, reducing the surface area available for gas exchange. Consequently, the health and integrity of the alveolar-capillary membrane are essential for effective respiratory function.

Alveolar-Capillary Membrane Vector Illustration

Gas Exchange Practice Questions

1. What are the two forms in which oxygen is transported?
Physically dissolved in plasma and chemically bound to hemoglobin

2. What are the additional costs of increased ventilation?
Increased work of breathing, increased oxygen consumption, and a higher burden of external ventilation

3. What is the alveolar air equation based on Dalton’s law?
PAO2=FiO2 x (PB-47)-(PACO2/0.8)

4. What must exist in order for gas exchange to occur between the alveoli and pulmonary capillaries?
A difference in partial pressures

5. What is an area with ventilation but no blood flow called?
Dead space

6. What is the formula for figuring out how much oxygen is dissolved in the plasma?
Henry’s Law, which states: Dissolved oxygen (mL/dL) = PO2 x 0.003

7. The oxyhemoglobin dissociation curve describes what?
It describes the relationship between PAO2 and SaO2.

8. What is hemoglobin saturation?
Hemoglobin saturation is the percentage of hemoglobin that is carrying oxygen compared to the total hemoglobin.

9. How many oxygen receptors does hemoglobin have?

10. How much oxygen is bound to hemoglobin?
1.34 mL of oxygen per gram of hemoglobin

11. What is the normal CO2 pressure in veins?
46 mmHg

12. What is the normal oxygen pressure in veins?
40 mmHg

13. What is respiration?
The process of moving oxygen to tissues for aerobic metabolism and removing CO2

14. What is a right-left shunt?
A malfunction in the septum that causes deoxygenated blood to travel from the RIGHT atrium to the LEFT atrium

15. What is the total O2 content of blood equation?
CaO2=(0.003 x PaO2) + (Hb x 1.34 x SaO2)

16. What disorders lead to alveolar deadspace?
Pulmonary emboli, partial obstruction of the pulmonary vasculature, destroyed pulmonary vasculature, and reduced cardiac output

17. What are the causes of a relative shunt?
COPD, restrictive disorders, and any condition resulting in hypoventilation

18. What is carboxyhemoglobin?
The combination of carbon monoxide bound to hemoglobin

19. What is deadspace ventilation?
When there is ventilation in excess of perfusion

20. What is methemoglobin?
Ferric iron that cannot bind with oxygen and alters the HbO2 affinity; it causes a left shift in the HbO2 curve

21. What is needed for the CaO2 to be adequate?
Sufficient hemoglobin

22. What is the P50?
It shows the PO2 when the hemoglobin is 50% saturated with oxygen.

23. What is a reduction in blood flow called?
Shock or ischemia

24. How can we analyze gas exchange between the lungs and the blood?
In arterial blood, we can analyze the oxygen and CO2 levels. In expired air, we can analyze the CO2 levels.

25. What is the best value to assess ventilation?

26. What is the formula for alveolar minute ventilation?
Frequency x (Tidal Volume – Physiological Deadspace)

27. What is the importance of the Bohr effect on oxygen transport?
A low pH shifts the HbO2 curve to the right, and a high pH shifts the curve to the left.

28. How is the majority of the oxygen in the body carried?
Bound to hemoglobin

29. What is the normal methemoglobin percentage reading?
1%-2% on a CO-oximeter

30. What leads to an increased PaCO2?
Decreased VA, increased VCO2, and increased VD

31. What is respiration?
The process of taking oxygen into the body for tissue utilization and removing CO2 into the atmosphere

32. What is the normal range for SaO2?

33. What causes carbon dioxide to build up in the tissues?

34. Carbon dioxide diffuses into what?
It diffuses into capillary blood before being carried to the lungs for removal.

35. Gas movement between the lungs and tissues occurs because of what?
Simple diffusion

36. What ratio represents a perfect V/Q balance?

37. What is the most common approach to analyzing gas exchange between the blood and tissues?
Measure oxygen levels in mixed venous blood

38. What is the difference between the oxygen and carbon dioxide diffusion gradients?
Oxygen has a cascade gradient moving from the atmosphere into the cell. Carbon dioxide moves from the cell into the atmosphere.

39. The carbon dioxide diffusion gradient cascade causes what?
It causes CO2 to move from tissues into venous blood, which is transported to the lungs. Then it is exhaled out into the atmosphere.

40. The alveolar partial pressure of carbon dioxide varies directly with what?
It varies with the body’s production of CO2 and inversely with alveolar ventilation.

41. What is the portion of the cardiac output that returns to the left heart without being oxygenated?
Anatomic shunt

42. Respiratory control mechanisms normally maintain the PaCO2 within a range of what?
35-45 mmHg

43. If carbon dioxide production increases, what happens to ventilation?
Ventilation automatically increases to maintain the PaCO2 within the normal range.

44. What is the most important factor in determining the alveolar partial pressure of oxygen?
The inspired partial pressure of oxygen

45. Oxygen in the lungs is diluted by what?
Water vapor and carbon dioxide

46. What is the alveolar air equation formula?
PAO2= FiO2 x (PB-47)-(PaCO2/0.8)

47. According to Dalton’s law, the partial pressure of alveolar nitrogen must be what?
It must be equal to the pressure it would exert if it alone were present.

48. What are the barriers to gas diffusion?
Alveolar epithelium, interstitial space and structures, capillary endothelium, and RBC membranes

49. When a patient is breathing room air, what is the sum of the alveolar PO2 and alveolar PCO2?
140 mmHg

50. If a patient is breathing room air at sea level, the respiratory therapist should or shouldn’t expect to see a PaO2 any higher than 120 mmHg during hyperventilation?
They should not expect to see a PaO2 higher than 120 mmHg.

51. A PO2 value that is higher than 120 mmHg indicates what?
It indicates that the patient is breathing supplemental oxygen.

52. What is diffusion?
The process of gas molecules moving from an area of high partial pressure to an area of low partial pressure

53. In order for oxygen to diffuse into and out of the lungs and tissues, oxygen and carbon dioxide must move through what?

54. What three barriers must be penetrated in order for oxygen and CO2 to move between the alveoli and pulmonary capillary blood?
Alveolar epithelium, interstitial space, and capillary endothelium

55. What is Fick’s first law of diffusion?
It states that the greater the surface area, diffusion constant, and pressure gradient, the more diffusion will occur.

56. Diffusion in the lungs mainly depends on what?
Gas pressure gradients

57. What is the pressure gradient for oxygen diffusion into the blood?
60 mmHg

58. Does carbon dioxide diffuse faster or slower across the alveolar-capillary membrane than oxygen?
It diffuses 20 times faster than oxygen because it has higher solubility in the plasma.

59. The diffusion time in the lungs depends on what?
The rate of pulmonary blood flow

60. How long does blood take to flow through the pulmonary capillary?
0.75 seconds

61. Low concentrations of what is used to measure the diffusion capacity of the lungs?
Carbon monoxide

62. The PaO2 of a healthy person breathing at sea level is approximately how much less than the calculated PaO2?
5-10 mmHg

63. What two factors account for a 5-10 mmHg difference in the calculated PaO2?
(1) Right-to-left shunts in pulmonary and cardiac circulation, and (2) Regional differences in pulmonary ventilation and blood flow

64. What does a right-to-left shunt cause poorly oxygenated venous blood to do?
It causes it to move directly into the arterial circulation, lowering the oxygen content of the arterial blood.

65. What is the ideal ventilation and perfusion ratio?

66. What does a high ventilation/perfusion ratio indicate?
It indicates that either ventilation is greater than normal, perfusion is less than normal, or both.

67. A pressure gradient must exist for gas to move between what?
The alveoli and pulmonary capillaries

68. Does perfusion increase or decrease as you move further down into the lung bases?
It increases because the bases of the lungs receive nearly 20 times as much blood flow as the apices.

69. Does ventilation increase or decrease as you move further down into the lung bases?
It increases; there are four times as much ventilation in the bases of the lungs compared to the apices.

70. Blood carries oxygen in what two forms?
(1) Dissolved in plasma and erythrocyte fluid, and (2) Combines with hemoglobin inside red blood cells

71. What is the formula for dissolved oxygen in the blood?
Dissolved oxygen = PO2 x 0.003

72. Deoxygenated hemoglobin serves as a what during transport?
An important blood buffer for hydrogen ions

73. What is oxyhemoglobin?
Oxygen molecules bound to hemoglobin

74. In whole blood, each gram of normal hemoglobin can carry how many mL of oxygen?
1.34 mL

75. Hemoglobin increases the oxygen-carrying capacity of the blood by how much?
70-fold as compared to plasma alone

76. What is the portion of inspired air that is exhaled without being exposed to perfused alveoli?

77. According to Fick’s equation, if the oxygen consumption remains constant, a decrease in cardiac output will do what to the arteriovenous oxygen content difference?
It will increase the C(a-v)O2

78. According to Fick’s equation, if the cardiac output increases and oxygen consumption remains constant, what will happen to the arteriovenous oxygen content difference?
The C(a-v)O2 will decrease

79. What factors other than the HbO2 curve will affect the loading and unloading of oxygen?
Blood pH, body temperature, erythrocyte concentration of certain organic phosphates, variations in the structure of hemoglobin, and chemical combinations of hemoglobin with substances other than oxygen

80. A low pH shifts the oxyhemoglobin curve to the?

81. A high pH shifts the oxyhemoglobin curve to the?

82. When blood pH drops and shifts the curve to the right, the hemoglobin saturation for a given PO2 will do what?
It will decrease

83. As blood pH increases and the curve shifts to the left, hemoglobin saturation for a given PO2 does what?
It increases

84. As blood inside the tissues picks up CO2, the pH decreases, and the HbO2 curve shifts in which direction?
It shifts to the right, therefore, decreasing the affinity of hemoglobin for oxygen.

85. Which way does the HbO2 shift when there is a drop in body temperature?
It shifts the curve to the left, which increases the hemoglobin affinity for oxygen.

86. As body temperature increases, the oxyhemoglobin curve shifts which way?
It shifts to the right, and the affinity of hemoglobin for oxygen decreases.

87. What happens to CO2 and PaO2 levels if you increase deadspace?
CO2 increases and PaO2 decreases

88. What is the normal level of carbon dioxide production for an adult?
200 mL/minute

89. Hemoglobin’s affinity for carbon monoxide is how much greater than it is for oxygen?
200 times greater

90. The combination of carbon monoxide and hemoglobin shifts the HbO2 curve in which direction?
It shifts it to the left, impeding oxygen delivery to the tissues.

91. What is the treatment for carbon monoxide poisoning?
You should provide the patient with as much oxygen as possible because oxygen reduces the half-life of carboxyhemoglobin.

92. What is a normal P50?
26.6 mmHg

93. Conditions that cause a decrease in hemoglobin affinity for oxygen cause a shift in which direction?
These conditions cause a shift to the right.

94. How much carbon dioxide is carried in the blood?
45-55 mL/dL

95. What are the three ways carbon dioxide is carried in the blood?
(1) Dissolved in a physical solution, (2) Chemically combined with protein, and (3) Ionized as bicarbonate

96. What is hydrolysis?
The chemical process in which a molecule is cleaved into two parts by the addition of a molecule of water

97. What does the hydrolysis of CO2 form?
Carbonic acid

98. What enzyme enhances the hydrolysis reaction?
Carbonic anhydrase

99. What are the three causes of hypoxia?
1) Arterial blood oxygen content is decreased, 2) Cardiac output or perfusion is decreased, and 3) Abnormal cellular function prevents the proper uptake of oxygen

100. When does hypoxemia occur?
When the partial pressure of oxygen in arterial blood (PaO2) is decreased

101. What causes a decreased PaO2?
Low ambient PO2, hypoventilation, impaired diffusion, V/Q imbalances, right-to-left anatomical or physiological shunting, aging, and altitude

102. What is the approximate PaO2 at the age of 60?
85 mmHg

103. What is the most common cause of hypoxemia in patients with lung diseases?
Ventilation/perfusion imbalances

104. What happens when ventilation is greater than perfusion?
There is wasted ventilation (i.e., alveolar dead space)

105. What happens when ventilation is less than perfusion?
The ventilation/perfusion ratio is decreased, and blood leaves the lungs with abnormally low oxygen content.

106. What does a ventilation/perfusion ratio of 0 represent?
It means that there is blood flow but no ventilation, which is equivalent to a right-to-left anatomical shunt.

107. How can you differentiate between hypoxemia caused by a V/Q imbalance and hypoxemia caused by shunting?
If the FiO2 is greater than 50% and the PaO2 is less than 50%, significant shunting is present. Otherwise, the hypoxemia would be caused by a V/Q imbalance.

108. For the arterial oxygen content to be adequate, there must also be what?
There must be enough normal hemoglobin in the blood

109. Can hypoxia occur if the hemoglobin is low, even when the PaO2 is normal?
Yes, because there is low oxygen content in arterial blood.

110. What can low hemoglobin content do to the oxygen-carrying capacity of the blood?
It can seriously impair the oxygen-carrying capacity of the blood.

111. What are two types of reduced blood flow?
1) Circulatory failure (shock) and 2) Local reduction in perfusion (ischemia)

112. Any disorder that lowers alveolar ventilation relative to metabolic need impairs what?
Carbon dioxide removal

113. What happens as a result of impaired CO2 removal by the lungs?
Hypercapnia and respiratory acidosis

114. When does a decrease in alveolar ventilation occur?
When the minute ventilation is inadequate, dead space ventilation per minute increases, or a ventilation/perfusion imbalance exists

115. Inadequate minute ventilation is caused by what?
Decreased tidal volume

116. Inadequate minute ventilation occurs in what type of conditions?
Restrictive conditions, such as atelectasis and neuromuscular disorders, and conditions that impede thoracic expansion (e.g., kyphoscoliosis)

117. What causes increased dead space ventilation?
Rapid shallow breathing or increased physiologic dead space

118. What does increased dead space ventilation result in?
The proportion of wasted deadspace increases, which lowers alveolar ventilation and impairs CO2 removal.

119. In theory, a V/Q imbalance should cause an increased PaCO2. However, many patients who are hypoxemic due to a V/Q imbalance have a low or normal PaCO2. What does this suggest?
It suggests that V/Q imbalances have a greater effect on oxygenation than carbon dioxide removal.

120. What must happen to compensate for a high PaCO2 value with V/Q imbalances?
The patient’s minute ventilation must increase.

121. When a patient is hypercapnic, what must happen for them to maintain a normal PaCO2?
They must sustain a higher than normal minute ventilation.

122. What disorders can lead to alveolar deadspace?
Pulmonary edema, partial obstruction of the pulmonary vasculature, destroyed pulmonary vasculature, and reduced cardiac output

123. How does gas move across the system?
Simple diffusion

124. How long is pulmonary blood exposed to alveolar gas?
0.75 seconds

125. How long is pulmonary blood exposed to alveolar gas during exercise?
0.25 seconds

126. How many times greater is ventilation at the bases?
Four times greater

127. How many times higher is blood flow at the bases?
20 times higher

128. What is ventilation that enters into the alveoli without any perfusion?
Alveolar deadspace

129. What are the normal values for the partial pressure of arterial blood?
O2 = 100 mmHg and CO2 = 40 mmHg

130. What are the normal values for the partial pressure of venous blood?
O2 = 40 mmHg and CO2 = 46 mmHg

FAQs About Gas Exchange

What is Pulmonary Ventilation?

Pulmonary ventilation is the process of moving air in and out of the lungs. This occurs through a series of muscular contractions and relaxations that create negative pressure within the thoracic cavity.

As the thoracic cavity expands, air is drawn into the lungs. The opposite occurs during expiration as the thoracic cavity contracts, and air is forced out of the lungs. Pulmonary ventilation is vital for gas exchange to take place.

Where Does Gas Exchange Occur in the Lungs?

Gas exchange occurs in the alveoli. These are small, sack-like structures that are lined with a thin layer of type I alveolar cells. The alveoli are surrounded by a network of capillaries.

These vessels are lined with endothelial cells that have small pores that allow gases to diffuse across them. Oxygen and carbon dioxide diffuse across the alveolar-capillary membrane in order to reach equilibrium.

What Factors Affect Gas Exchange?

There are several factors that can affect how well gas exchange occurs in the lungs. One factor is the surface area of the alveolar-capillary membrane. This is the area over which diffusion can take place.

The surface area of the alveolar-capillary membrane is increased by having a larger number of alveoli and a greater density of capillaries.

Another factor that affects gas exchange is ventilation. This is the rate at which air moves in and out of the lungs. Increasing ventilation will increase the rate of gas exchange. However, too much ventilation can also be detrimental as it can cause a decrease in the partial pressure of oxygen in the alveoli.

Finally, the thickness of the alveolar-capillary membrane can also affect gas exchange. A thicker membrane will decrease the rate of diffusion.

What is the Main Function of Gas Exchange?

The main function of gas exchange is to ensure that the blood is supplied with oxygen while helping get rid of carbon dioxide.

Oxygen is essential for cellular respiration, which is the process that cells use to produce energy. Carbon dioxide is a waste product of cellular respiration and must be removed from the body.

Gas exchange ensures that these important gases are exchanged between the lungs and the blood.

What is Impaired Gas Exchange?

There are several conditions that can lead to impaired gas exchange. One condition is hypoventilation. This is when ventilation is decreased, which can lead to a build-up of carbon dioxide in the blood.

Another condition is atelectasis. This is when the alveoli collapse, which prevents oxygen from diffusing into the blood. A third condition is pneumonia, which is an infection of the lungs that can cause the alveoli to fill with fluid. This makes it difficult for oxygen to diffuse into the blood.

Finally, chronic obstructive pulmonary disease (COPD) is a condition that makes it difficult to breathe and can also lead to impaired gas exchange.

What is the Difference Between External and Internal Respiration?

External respiration is the process of exchanging gases between the lungs and the blood. Internal respiration is the process of exchanging gases between the blood and the cells. Both of these processes are necessary for gas exchange to occur.

External respiration is responsible for exchanging oxygen and carbon dioxide between the lungs and the blood. Internal respiration is responsible for exchanging oxygen and carbon dioxide between the blood and the cells.

Both of these processes are necessary for the body to function properly.

Final Thoughts

Gas exchange is a cornerstone of human physiology that allows for the uptake of oxygen and the removal of carbon dioxide. This process is highly regulated and can be affected by numerous factors, including lung function, blood flow, and external environment.

Malfunctions in this system can lead to serious health complications, such as hypoxia or hypercapnia, underscoring the need for ongoing research and advanced medical interventions.

As our understanding of gas exchange deepens, it opens avenues for the development of more effective treatments for respiratory diseases and improves our overall grasp of human health.

John Landry, BS, RRT

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.


  • Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
  • Jardins, Des Terry. Cardiopulmonary Anatomy & Physiology: Essentials of Respiratory Care. 7th ed., Cengage Learning, 2019.
  • Powers KA, Dhamoon AS. Physiology, Pulmonary Ventilation and Perfusion. [Updated 2023 Jan 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.
  • Butler JP, Tsuda A. Transport of gases between the environment and alveoli–theoretical foundations. Compr Physiol. 2011.

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