When blood circulates through the body, it picks up oxygen from the lungs and delivers it to the tissues. At the same time, blood collects carbon dioxide from the tissues and transports it back to the lungs, where it is exhaled.

This process is essential for human life.

In this guide, we will discuss how oxygen and carbon dioxide are transported in the blood. We will also describe how these gases are monitored and regulated by the body.

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How are Oxygen and Carbon Dioxide Transported in the Blood?

The primary goal of the lungs is to take in oxygen and remove carbon dioxide during a breathing cycle. This process is referred to as gas exchange.

During gas exchange, oxygen and carbon dioxide molecules must be transported to and from the lungs in the blood. First, let’s look at how oxygen is transported.

Oxygen Transport

The majority of oxygen molecules are transported from the lungs to other parts of the body via red blood cells. They contain hemoglobin which picks up oxygen molecules so that they can move through arteries to the tissues of the body.

One hemoglobin molecule is able to carry four oxygen molecules during transport. When oxygen is bound to hemoglobin, this gives arterial blood its bright-red appearance.

Oxygen transport bound to hemoglobin in red blood cells

Carbon Dioxide Transport

The majority of carbon dioxide molecules are transported in the blood dissolved as plasma. Specifically, there are three ways CO2 is transported:

  1. Dissolved Carbon Dioxide
  2. Bicarbonate
  3. Carbaminohemoglobin

Bicarbonate and dissolved CO2 are transported via blood plasma. Carbaminohemoglobin is formed when carbon dioxide binds with hemoglobin. This process is similar to how oxygen is transported.

Approximately 10% of carbon dioxide is transported by dissolving and diffusing into the blood plasma. Approximately 70% of carbon dioxide is transported as bicarbonate. Approximately 20% of carbon dioxide is bound to hemoglobin and transported via red blood cells.

Carbon dioxide transport bound to hemoglobin in red blood cells

Transport of Oxygen and Carbon Dioxide Practice Questions:

1. How is oxygen transported in the blood?
By binding to hemoglobin in the erythrocytes to form oxyhemoglobin

2. What oxygen is bound to in the blood?
Hemoglobin 97%; dissolved 3%

3. What is carbon dioxide bound to in the blood?
It is bound to hemoglobin and moves as bicarbonate (HCO3).

4. How long does it take oxygen to equilibrate in the capillaries?
0.25 seconds

5. What is the red blood cell safety factor in the capillary?
0.75 seconds

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6. What happens to the PO2 diffusion time in a damaged lung?
The RBC can take the entire 0.75 seconds, meaning that it has no safety time left. It will take longer to oxygenate the RBC.

7. What is systemic venous blood?
It refers to the blood in your body that is being transported to the lungs. This blood is de-oxygenated and needs to be re-oxygenated.

8. What is the standard diffusion gradient of CO2?
5 mmHg

9. Why does CO2 require a smaller pressure gradient than O2?
Carbon dioxide is 20 times more soluble than oxygen.

10. What is the partial pressure of carbon dioxide (PCO2) when entering the capillaries of the lungs?
45 mmHg

11. Why is the Bohr effect beneficial?
Respiring tissues have a high concentration of carbon dioxide as a by-product of respiration. As a result, more oxygen will be released in these areas due to the Bohr effect. This is beneficial because it provides more oxygen to the respiring tissues.

12. What happens if you increase blood flow to the peripheral tissue?
The PO2 level increases, and the PCO2 level decreases.

13. What happens to partial pressure of blood if metabolism is increased?
The PCO2 level increases, and the PO2 level decreases.

14. What does a shift to the right mean on the oxyhemoglobin dissociation curve?
A shift to the right means that the PO2 level has decreased, and oxygen needs to be unloaded. An increase in homeostasis, CO2, temperature, and BPG will cause a shift to the right.

15. What are the three ways to increase oxygen delivery to the peripheral tissue?
Increase blood flow, increase O2 extraction at the tissue, and shift the oxyhemoglobin dissociation curve to the right

16. How does carbon monoxide bind to hemoglobin?
Carbon monoxide (CO) binds to hemoglobin with 20 times more affinity than oxygen.

17. What are the signs of carbon monoxide poisoning?
Shortness of breath, nausea, headache, dizziness, and confusion

18. How is carbon dioxide transported in the blood?
70% as bicarbonate (HCO3-), 23% in hemoglobin, and 7% dissolved

19. What is hypoxia?
Low levels of oxygen in the tissues of the body

20. What are the four categories of hypoxia?
Hypoxemic hypoxia, anemic hypoxia, circulatory hypoxia, and histotoxic hypoxia

21. What is hypoxemia?
Low levels of oxygen in the blood

22. What level of reduced hemoglobin will cyanosis appear?
5%

23. What is the value of a normal anatomic shunt?
3%

24. What is the function of carbonic anhydrase?
It speeds up the chemical reactions regarding carbon dioxide.

25. What hematocrit values indicate polycythemia?
For men: more than 52%; For women: more than 48%

26. What hemoglobin values indicate polycythemia?
For men: greater than 18.5 g/dL; For women: greater than 16.5 g/dL

27. What are normal hematocrit levels in men and women?
For men: 45%; For women: 42%

28. What are the causes of capillary shunting?
Atelectasis, alveolar fluid accumulation, and alveolar consolidation

29. What is a true shunt?
Anatomic shunt + capillary shunt

30. What is the definition of a shunt?
Perfused alveoli without ventilation

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31. What factors shift the oxyhemoglobin curve to the right?
Decreased pH, increased PCO2, increased temperature, and increased BPG

32. What factors shift the oxyhemoglobin curve to the left?
Increased pH, decreased PCO2, decreased temperature, and decreased BPG

33. How is the majority of CO2 transported?
Bicarbonate

34. What percentage of CO2 is transported by bicarbonate?
70%

35. How is CO2 transported in plasma?
Carbamino compound (bound to protein), bicarbonate, and dissolved

36. What can occur as a result of venous admixture?
Hypoxemia

37. Which part of the dissociation curve illustrates the safety zone for oxygen loading?
The flat portion between 90-100%

38. How does the Bohr effect change the shape of the hemoglobin dissociation curve?
The hemoglobin dissociation curve shifts downward and to the right, which indicates that hemoglobin has a lower saturation of oxygen.

39. What happens to P50 when the curve shifts to the right?
It increases

40. What happens to P50 when the curve shifts to the left?
It decreases

41. What is a normal P50 reading?
27 torr

42. What is oxygen affinity?
The tendency of hemoglobin to associate with oxygen

43. Why is hemoglobin a good vehicle for oxygen?
It has a high affinity for oxygen.

44. What is the structure of hemoglobin?
Four sub-units consisting of a polypeptide chain and a heme prosthetic group, and each heme group contains an Fe²⁺ ion

45. How many oxygen molecules can one molecule of hemoglobin associate with?
Four

46. How does hemoglobin enable fast diffusion of oxygen into erythrocytes?
Once bound to the hemoglobin, the oxygen molecules are taken out of the solution, which keeps the concentration gradient steep enough for oxygen molecules to diffuse into the erythrocytes quickly.

47. What is the word equation for the association of hemoglobin with oxygen?
hemoglobin + oxygen ⇌ oxyhemoglobin

48. What is PO2?
Partial pressure of oxygen

49. What is PO2 measured in?
kPa

50. What is the general trend of the hemoglobin dissociation curve?
As the PO2 increases, so does the level of hemoglobin saturation

51. What is hemoglobin saturation?
The amount of oxygen bound to hemoglobin in proportion to the amount of oxygen the hemoglobin can carry.

52. How is hemoglobin saturation expressed?
% (Percentage)

53. What happens to hemoglobin at a high PO2?
Hemoglobin associates with oxygen to form oxyhemoglobin

54. Why does hemoglobin in the lung capillaries have almost 100% oxygen saturation?
The lungs have a very high PO2 because they take in oxygen from the atmosphere.

55. What happens to oxyhemoglobin at a low PO2?
Oxyhemoglobin dissociates into hemoglobin and oxygen

56. Why does hemoglobin in the respiring tissues have a very low oxygen saturation?
The respiring tissues have a very low PO2 because they use more oxygen during respiration. They also have a high CO2 concentration, which induces the Bohr effect.

57. What shape is the hemoglobin dissociation curve?
It is shaped like an “S.” The curve is shallow at first, then steep, then shallow.

58. Why is the hemoglobin dissociation curve shallow at a low PO2?
At a low PO2, most of the hemoglobin molecules have not yet associated with oxygen molecules, as there is not much oxygen to go around. It is more difficult for hemoglobin to associate with the first oxygen molecule because the heme groups are still in the center. The saturation level remains low as most hemoglobin molecules will not readily associate with oxygen, and there is not much oxygen to go around anyway.

59. Why does the hemoglobin dissociation curve steepen as the PO2 rises?
At a higher PO2, more hemoglobin molecules have associated with oxygen, as there is more oxygen to go around.

60. Why is the hemoglobin dissociation curve shallow at the highest PO2 levels?
Once the hemoglobin approaches 100% saturation, there are increasingly fewer unsaturated hemoglobin molecules around to associate with oxygen.

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61. How does oxygen bind to hemoglobin?
Each of the Fe²⁺ ions in the four heme groups associates with one oxygen molecule

62. How does fetal hemoglobin differ from adult hemoglobin?
It has a higher affinity for oxygen.

63. Why is the fact that fetal hemoglobin has a higher oxygen affinity useful for the fetus?
Oxygenated blood from the mother is delivered through the placenta, which has a low PO2. In these conditions, the mother’s adult oxyhemoglobin dissociates and releases oxygen. Meanwhile, the fetal hemoglobin will more readily associate with oxygen.

64. How is the fetal hemoglobin dissociation curve different from an adult’s?
It shifts to the left.

65. Why does carbon dioxide need to be transported?
To prevent harm caused by CO₂ build-up in the body’s respiring tissues

66. Why does the pH of the erythrocytes’ cytoplasm become more acidic in high concentrations of carbon dioxide?
Because carbon dioxide reacts with water to form carbonic acid, which itself dissociates into H⁺ ions and HCO₃⁻ ions. It is the H⁺ ions that lower the pH of the erythrocytes’ cytoplasm.

67. What does the Bohr effect do?
It lowers hemoglobin’s affinity for oxygen, causing oxyhemoglobin to dissociate and release oxygen.

68. What does the movement of gases between the lungs and the tissues depend on?
Diffusion

69. The PACO2 varies directly with CO2 production and inversely with what?
Alveolar ventilation

70. How is PAO2 computed?
It is computed using the alveolar air equation.

71. PAO2 varies inversely with what?
At a constant FiO2, PAO2 varies inversely with PACO2.

72. What is the normal PAO2?
100 mmHg

73. What is the normal PACO2?
40 mmHg

74. What is hemoglobin saturation?
It is the ratio of oxyhemoglobin to total hemoglobin, expressed as a percentage.

75. What is the arteriovenous oxygen content difference?
It is the amount of oxygen given up by every 100 mL of blood on each pass through the tissues.

76. When does the hemoglobin affinity for oxygen increase?
It increases with a high PO2, high pH, low temperature, and low levels of 2,3-DPG.

77. Whenever the oxygen dissociation curve is modified due to changes in CO2 levels, this is known as what?
Bohr effect

78. What can cause hypoxia?
It can occur if the arterial blood oxygen content is decreased, blood flow is decreased, or abnormal cellular function prevents the proper uptake of oxygen.

79. What can cause decreased PaO2 levels?
Low ambient PO2, hypoventilation, impaired diffusion, V/Q imbalances, and right-to-left anatomic or physiologic shunting

80. What can cause a decrease in alveolar ventilation?
Inadequate minute ventilation, increased dead space ventilation, and V/Q imbalance

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FAQ

What is Gaseous Diffusion?

Gaseous diffusion is the process by which oxygen and carbon dioxide move across the alveolar membranes. This process is driven by the difference in partial pressure of the gases on either side of the membrane.

Gaseous diffusion is an important process because it allows oxygen and carbon dioxide to be exchanged between the lungs and the blood.

What is Hypoxemia?

Hypoxemia is a condition in which the blood oxygen levels are lower than normal. It is caused by a variety of conditions, including heart and lung disease.

Symptoms of hypoxemia include shortness of breath, tachypnea, cyanosis, and fatigue. It can be treated with supplemental oxygen.

What is Hypercapnia?

Hypercapnia is a condition in which the blood carbon dioxide levels are higher than normal. It is caused by a variety of conditions, including respiratory infections and COPD.

Symptoms of hypercapnia include headache, dizziness, and shortness of breath. It can be treated with supplemental oxygen and positive pressure ventilation.

What is Sickle Cell Hemoglobin?

Sickle cell hemoglobin is a type of hemoglobin that is found in people with sickle cell disease. This condition is a hereditary blood disorder that affects red blood cells.

People with sickle cell disease have abnormal hemoglobin molecules that can cause the red blood cells to become misshapen and break down. This can lead to anemia, a condition in which the body does not have enough healthy red blood cells to carry oxygen throughout the body.

People with sickle cell disease often experience episodes of pain, called sickle cell crises, when the red blood cells block blood flow to the tissues.

What is Acute Chest Syndrome?

Acute chest syndrome (ACS) is a serious complication of sickle cell disease. It occurs when blood flow to the lungs is blocked and oxygen levels in the blood drop. This can cause chest pain, coughing, and difficulty breathing.

ACS is a medical emergency and requires immediate treatment. Treatment typically includes hospitalization and intravenous fluids to prevent dehydration.

In some cases, blood transfusions may also be necessary. ACS can be fatal, but early treatment can improve the chances of a full recovery.

What is Anatomic Dead Space?

Anatomic dead space is the volume of air that is inhaled that does not participate in gas exchange. This volume includes the volume of air in the mouth and throat, as well as the volume of air in the bronchi and bronchioles.

Anatomic dead space in adults is typically 1 mL per pound of body weight.

What is Alveolar Dead Space?

Alveolar dead space is the volume of air in the lungs that is not participating in gas exchange. This volume includes the volume of air in the alveoli that are not perfused with blood, as well as the volume of air in the alveoli that are perfused but not ventilated.

Alveolar dead space can be caused by a variety of conditions, including respiratory infections and COPD.

What is an Alveolar Shunt?

An alveolar shunt is a condition in which blood bypasses the alveoli and does not participate in gas exchange. This can occur if there is a hole in the heart or if the lungs are not functioning properly.

Alveolar shunts can be dangerous because they can lead to hypoxemia, a condition in which the blood oxygen levels are lower than normal.

What is the Oxyhemoglobin Dissociation Curve?

The oxyhemoglobin dissociation curve is a graph that shows the percentage of hemoglobin that is bound to oxygen at various oxygen levels.

The curve is shifted to the right in conditions that cause the body to release oxygen to the tissues, such as exercise and high altitude. The curve is shifted to the left in conditions that cause the body to retain oxygen, such as carbon monoxide poisoning.

What is the Bohr Effect?

The Bohr effect is a shift in the oxygen-hemoglobin dissociation curve in response to changes in pH. This effect occurs because hemoglobin is more likely to bind to oxygen at lower pH levels.

The Bohr effect is important because it allows the body to release oxygen to the tissues when they need it most.

What is the Haldane Effect?

The Haldane effect is the shift in the oxygen-hemoglobin dissociation curve in response to changes in carbon dioxide levels. This effect occurs because hemoglobin is more likely to bind to oxygen at higher carbon dioxide levels.

The Haldane effect is important because it allows carbon dioxide to be released from hemoglobin for removal.

What is the Hamburger Phenomenon?

The hamburger phenomenon, also known as a chloride shift, is a process in the cardiovascular system that involves the exchange of bicarbonate and chloride molecules across the red blood cell membrane.

Chloride ions (Cl-) from the plasma move into the red blood cells, and bicarbonate ions (HCO3-) from the red blood cells move into the plasma.

The net result is a decrease in the concentration of chloride ions in the plasma and an increase in the concentration of bicarbonate ions in the plasma. This phenomenon is important because it helps to maintain the electrical neutrality of the blood.

What is Carboxyhemoglobin?

Carboxyhemoglobin is a type of hemoglobin that is bound to carbon dioxide. This molecule is produced when red blood cells are exposed to carbon monoxide.

Carboxyhemoglobin can be dangerous because it reduces the amount of oxygen that the red blood cells can carry.

Symptoms of carboxyhemoglobin poisoning include headache, dizziness, and shortness of breath. Carboxyhemoglobin poisoning can be treated with supplemental oxygen, positive pressure ventilation, and hyperbaric oxygen therapy.

What is Methemoglobin?

Methemoglobin is a type of hemoglobin that is bound to iron. This molecule is produced when red blood cells are exposed to certain chemicals, such as nitrites.

Methemoglobin can be problematic because it cannot bind to oxygen, which results in less oxygen being delivered to the tissues of the body.

What is the Respiratory Exchange Ratio?

The respiratory exchange ratio (RER) is a measure of the amount of carbon dioxide that is produced for every oxygen molecule that is consumed. The RER can be used to assess the efficiency of gas exchange in the lungs.

A normal RER is between 0.8 and 1.0. An RER that is lower than 0.8 indicates that the body is not using oxygen efficiently, while an RER that is higher than 1.0 indicates that the body is producing too much carbon dioxide.

What is Venous Admixture?

Venous admixture is the mixing of oxygenated and non-oxygenated blood in the veins. This can occur due to a shunt or V/Q mismatch.

It leads to hypoxemia, which can result in shortness of breath and difficulty breathing.

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Final Thoughts

The transport of oxygen and carbon dioxide in the blood is a complex process that is essential for maintaining homeostasis. This process can be affected by various factors, such as pH levels, carbon dioxide levels, and the respiratory exchange ratio.

Understanding how oxygen and carbon dioxide are transported is important for diagnosing and treating pulmonary conditions. This is why this is an important topic for respiratory therapists.

We have a similar guide about gas exchange that I think you’ll find helpful. Thanks for reading and, as always, breathe easy, my friend.

Medical Disclaimer: This content is for educational and informational purposes only. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Please consult with a physician with any questions that you may have regarding a medical condition. Never disregard professional medical advice or delay seeking it because of something you read in this article. We strive for 100% accuracy, but errors may occur, and medications, protocols, and treatment methods may change over time.

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]
  • Jardins, Des Terry. Cardiopulmonary Anatomy & Physiology: Essentials of Respiratory Care. 7th ed., Cengage Learning, 2019. [Link]
  • “Modeling Oxygen and Carbon Dioxide Transport and Exchange Using a Closed Loop Circulatory System.” National Center for Biotechnology Information, 2008, www.ncbi.nlm.nih.gov/pmc/articles/PMC3377501.
  • “Physiology, Oxygen Transport And Carbon Dioxide Dissociation Curve.” National Center for Biotechnology Information, 1 June 2020, www.ncbi.nlm.nih.gov/books/NBK539815.
  • “Physiology, Carbon Dioxide Transport.” National Center for Biotechnology Information, 1 Sept. 2020, www.ncbi.nlm.nih.gov/books/NBK532988.
  • “Regulation of Tissue Oxygenation.” National Center for Biotechnology Information, 2011, www.ncbi.nlm.nih.gov/books/NBK54103.
  • “The Active Transport of Oxygen and Carbon Dioxide into the Swim-Bladder of Fish.” PubMed Central (PMC), 1 July 1966, www.ncbi.nlm.nih.gov/pmc/articles/PMC3328323.

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Medical Disclaimer: The information provided by Respiratory Therapy Zone is for educational and informational purposes only. It should not be used as a substitute for professional medical advice, diagnosis, or treatment. Please consult with a physician with any questions that you may have regarding a medical condition.