The transport of oxygen and carbon dioxide in the blood is a critical process that supports life by ensuring that oxygen reaches tissues for energy production while carbon dioxide, a byproduct of metabolism, is efficiently removed.
This delicate balance is achieved through the intricate interactions between the lungs, blood, and tissues. Oxygen is primarily carried by hemoglobin in red blood cells, while carbon dioxide is transported in various forms, including as bicarbonate ions.
Understanding the mechanisms behind these transport processes is essential for anyone studying respiratory physiology or preparing for exams related to this topic.
Transport of Oxygen and Carbon Dioxide in Blood
The transport of oxygen (O2) and carbon dioxide (CO2) in the blood is a vital aspect of respiratory physiology, ensuring that tissues receive enough oxygen for metabolic processes and that waste products, such as carbon dioxide, are efficiently removed from the body.
This process involves the lungs, blood, and various molecules to facilitate gas exchange and transport.
Oxygen Transport in Blood
Oxygen is transported in the blood in two main ways:
- Bound to Hemoglobin: Approximately 98% of oxygen in the blood is bound to hemoglobin, a protein found in red blood cells. Hemoglobin has a high affinity for oxygen and forms oxyhemoglobin (HbO2) when oxygen binds to it. Each hemoglobin molecule can bind up to four oxygen molecules, and this process occurs primarily in the pulmonary capillaries where oxygen concentrations are high. The amount of oxygen bound to hemoglobin is influenced by several factors, including partial pressure of oxygen (PaO2), pH, temperature, and levels of carbon dioxide (Bohr effect). In tissues with lower oxygen concentrations, hemoglobin releases oxygen, making it available for cells.
- Dissolved in Plasma: A small portion of oxygen, about 1.5-2%, is dissolved in the plasma. This form of oxygen is responsible for maintaining the partial pressure of oxygen in the blood (PaO2). Although this amount is small, it is crucial for gas exchange between the blood and tissues.
Carbon Dioxide Transport in Blood
Carbon dioxide is transported in the blood in three main ways:
- As Bicarbonate (HCO3-): The majority of CO2 (about 70%) is transported in the form of bicarbonate ions. When CO2 diffuses into red blood cells, it reacts with water under the influence of the enzyme carbonic anhydrase, forming carbonic acid (H₂CO₃). Carbonic acid quickly dissociates into bicarbonate (HCO3-) and hydrogen ions (H⁺). The bicarbonate is then transported in the plasma. This reaction is reversible, allowing CO2 to be released back into the lungs during exhalation.
- Bound to Hemoglobin: Around 20-25% of CO2 is transported bound to hemoglobin in the form of carbaminohemoglobin (HbCO2). Carbon dioxide binds to the globin part of hemoglobin (not the heme), and this binding is enhanced in tissues where the concentration of oxygen is low (Haldane effect).
- Dissolved in Plasma: A small portion of CO2, about 5-7%, is dissolved directly in the plasma. This form contributes to the partial pressure of CO2 in the blood (PaCO2) and is readily available for diffusion into the alveoli during gas exchange in the lungs.
The transport of oxygen and carbon dioxide in the blood is essential for maintaining homeostasis and ensuring that cells receive adequate oxygen for metabolism while removing carbon dioxide, a waste product.
Oxygen is primarily transported bound to hemoglobin, whereas carbon dioxide is mostly transported as bicarbonate. This complex but efficient system ensures the respiratory needs of the body are met under varying physiological conditions.
What is Gas Exchange?
Gas exchange is the biological process where oxygen (O2) is absorbed into the bloodstream and carbon dioxide (CO2) is released from the bloodstream into the lungs to be exhaled.
This occurs in the alveoli, tiny air sacs within the lungs, where oxygen from inhaled air diffuses across the thin alveolar walls into surrounding capillaries. At the same time, carbon dioxide, a waste product from cellular metabolism, moves from the blood into the alveoli to be expelled during exhalation.
The driving force behind gas exchange is the difference in partial pressures of oxygen and carbon dioxide between the alveoli and blood, a process called diffusion.
Oxygen, which has a higher concentration in the alveoli, moves to areas of lower concentration in the blood, while carbon dioxide, which is more concentrated in the blood, moves into the alveoli.
Note: This exchange is crucial for delivering oxygen to tissues and removing carbon dioxide from the body.
What is the Oxygen–Hemoglobin Dissociation Curve?
The oxygen–hemoglobin dissociation curve is a graphical representation that illustrates the relationship between the partial pressure of oxygen (PaO2) and the percentage of hemoglobin saturation with oxygen (SaO2).
It shows how readily hemoglobin in red blood cells binds to and releases oxygen depending on the surrounding oxygen concentration.
Image by CNX OpenStax, CC BY 4.0, via Wikimedia Commons.
The curve has a sigmoidal shape due to hemoglobin’s cooperative binding, meaning that once one oxygen molecule binds to hemoglobin, the remaining sites bind oxygen more easily.
In the lungs, where PaO2 is high, hemoglobin becomes almost fully saturated with oxygen. In tissues, where PaO2 is lower, hemoglobin releases oxygen for cellular use.
The curve can shift:
- Right shift: Indicates hemoglobin releases oxygen more easily (due to increased temperature, CO2, or acidity).
- Left shift: Indicates hemoglobin holds onto oxygen more tightly (due to lower temperature, CO2, or alkalinity).
Note: This curve is vital for understanding how oxygen is delivered to tissues under different physiological conditions.
Transport of Oxygen and Carbon Dioxide Practice Questions
1. How is oxygen transported in the blood?
Oxygen is primarily transported by binding to hemoglobin in red blood cells (erythrocytes) to form oxyhemoglobin.
2. What is oxygen bound to in the blood?
Approximately 97% of oxygen is bound to hemoglobin, while about 3% is dissolved in plasma.
3. What is carbon dioxide bound to in the blood?
Carbon dioxide is bound to hemoglobin and also transported as bicarbonate (HCO3-) in the blood.
4. How long does it take oxygen to equilibrate in the capillaries?
Oxygen equilibrates in the capillaries within 0.25 seconds.
5. What is the red blood cell safety factor in the capillary?
The red blood cell has a safety factor of 0.75 seconds within the capillary for gas exchange.
6. What happens to the PO2 diffusion time in a damaged lung?
In a damaged lung, the red blood cell may take the full 0.75 seconds for oxygenation, leaving no safety margin, which delays the oxygenation process.
7. What is systemic venous blood?
Systemic venous blood refers to deoxygenated blood that is returning to the lungs to be re-oxygenated.
8. What is the standard diffusion gradient of CO2?
The standard diffusion gradient for carbon dioxide is 5 mmHg.
9. Why does CO2 require a smaller pressure gradient than O2?
CO2 requires a smaller pressure gradient because it is 20 times more soluble in blood than oxygen.
10. What is the partial pressure of carbon dioxide (PCO2) when entering the capillaries of the lungs?
The partial pressure of carbon dioxide is 45 mmHg when entering the capillaries of the lungs.
11. Why is the Bohr effect beneficial?
The Bohr effect facilitates the release of oxygen in tissues with high CO2 concentrations, ensuring that more oxygen is available where it is needed for respiration.
12. What happens if you increase blood flow to peripheral tissues?
Increasing blood flow to peripheral tissues raises the PO2 and lowers the PCO2 levels, improving oxygen delivery.
13. What happens to the partial pressures of blood if metabolism is increased?
When metabolism increases, PCO2 levels rise, and PO2 levels drop as the body consumes more oxygen and produces more carbon dioxide.
14. What does a shift to the right on the oxyhemoglobin dissociation curve mean?
A rightward shift indicates that hemoglobin has a reduced affinity for oxygen, facilitating oxygen unloading. Factors like increased temperature, CO2, H⁺ (acidity), and 2,3-BPG contribute to this shift.
15. What are the three ways to increase oxygen delivery to peripheral tissues?
To increase oxygen delivery: (1) increase blood flow, (2) enhance oxygen extraction at the tissues, and (3) shift the oxyhemoglobin dissociation curve to the right.
16. How does carbon monoxide bind to hemoglobin?
Carbon monoxide (CO) binds to hemoglobin with an affinity 200 times stronger than oxygen, making it highly dangerous as it prevents oxygen from binding.
17. What are the signs of carbon monoxide poisoning?
Symptoms of carbon monoxide poisoning include shortness of breath, nausea, headache, dizziness, confusion, and even loss of consciousness in severe cases.
18. How is carbon dioxide transported in the blood?
Carbon dioxide is transported in three ways: 70% as bicarbonate (HCO3-), 23% bound to hemoglobin as carbaminohemoglobin, and 7% dissolved in plasma.
19. What is hypoxia?
Hypoxia refers to abnormally low levels of oxygen in the body’s tissues.
20. What are the four categories of hypoxia?
The four categories of hypoxia are: (1) hypoxemic hypoxia, (2) anemic hypoxia, (3) circulatory hypoxia, and (4) histotoxic hypoxia.
21. What is hypoxemia?
Hypoxemia refers to low levels of oxygen in the blood.
22. At what level of reduced hemoglobin does cyanosis appear?
Cyanosis appears when 5% of hemoglobin is reduced (unsaturated).
23. What is the value of a normal anatomic shunt?
A normal anatomic shunt is approximately 3% of cardiac output.
24. What is the function of carbonic anhydrase?
Carbonic anhydrase speeds up the conversion of carbon dioxide and water into carbonic acid (H₂CO₃), which dissociates into bicarbonate (HCO3-) and hydrogen ions (H⁺).
25. What hematocrit values indicate polycythemia?
Polycythemia is indicated by hematocrit levels greater than 52% in men and greater than 48% in women.
26. What hemoglobin values indicate polycythemia?
Polycythemia is indicated by hemoglobin levels greater than 18.5 g/dL in men and greater than 16.5 g/dL in women.
27. What are normal hematocrit levels in men and women?
Normal hematocrit levels are around 45% for men and 42% for women.
28. What are the causes of capillary shunting?
Capillary shunting can be caused by atelectasis, alveolar fluid accumulation, and alveolar consolidation.
29. What is a true shunt?
A true shunt is the sum of anatomic shunt and capillary shunt.
30. What is the definition of a shunt?
A shunt refers to perfused alveoli that are not ventilated, resulting in a lack of gas exchange.
31. What factors shift the oxyhemoglobin dissociation curve to the right?
Factors that shift the curve to the right include decreased pH (acidosis), increased PCO2, increased temperature, and increased 2,3-BPG levels.
32. What factors shift the oxyhemoglobin dissociation curve to the left?
Factors that shift the curve to the left include increased pH (alkalosis), decreased PCO2, decreased temperature, and decreased 2,3-BPG levels.
33. How is the majority of CO2 transported?
The majority of carbon dioxide is transported as bicarbonate (HCO3-).
34. What percentage of CO2 is transported by bicarbonate?
Approximately 70% of carbon dioxide is transported in the form of bicarbonate.
35. How is CO2 transported in plasma?
In plasma, carbon dioxide is transported as a carbamino compound (bound to proteins), bicarbonate, and dissolved CO2.
36. What can occur as a result of venous admixture?
Venous admixture can result in hypoxemia, where oxygen levels in the blood are abnormally low.
37. Which part of the dissociation curve illustrates the safety zone for oxygen loading?
The flat portion of the oxyhemoglobin dissociation curve between 90-100% saturation represents the safety zone for oxygen loading.
38. How does the Bohr effect change the shape of the hemoglobin dissociation curve?
The Bohr effect causes the hemoglobin dissociation curve to shift downward and to the right, indicating a decrease in hemoglobin’s affinity for oxygen, promoting oxygen release in tissues.
39. What happens to P50 when the curve shifts to the right?
When the curve shifts to the right, P50 increases, meaning that more oxygen is required to achieve 50% hemoglobin saturation.
40. What happens to P50 when the curve shifts to the left?
When the curve shifts to the left, P50 decreases, meaning less oxygen is needed to achieve 50% hemoglobin saturation.
41. What is a normal P50 reading?
A normal P50 reading is 27 torr.
42. What is oxygen affinity?
Oxygen affinity refers to the tendency of hemoglobin to bind with oxygen.
43. Why is hemoglobin a good vehicle for oxygen?
Hemoglobin is a good vehicle for oxygen because it has a high affinity for oxygen, allowing it to efficiently pick up and transport oxygen throughout the body.
44. What is the structure of hemoglobin?
Hemoglobin consists of four subunits, each containing a polypeptide chain and a heme prosthetic group. Each heme group contains an Fe²⁺ ion that binds to oxygen.
45. How many oxygen molecules can one molecule of hemoglobin associate with?
One molecule of hemoglobin can associate with four oxygen molecules.
46. How does hemoglobin enable fast diffusion of oxygen into erythrocytes?
Once oxygen binds to hemoglobin, it is removed from the solution, maintaining a steep concentration gradient that allows more oxygen to diffuse quickly into the erythrocytes.
47. What is the word equation for the association of hemoglobin with oxygen?
hemoglobin + oxygen ⇌ oxyhemoglobin
48. What is PO2?
PO2 refers to the partial pressure of oxygen.
49. What is PO2 measured in?
PO2 is measured in kPa (kilopascals).
50. What is the general trend of the hemoglobin dissociation curve?
As the PO2 increases, hemoglobin saturation also increases.
51. What is hemoglobin saturation?
Hemoglobin saturation is the amount of oxygen bound to hemoglobin relative to the total amount of oxygen that hemoglobin can carry.
52. How is hemoglobin saturation expressed?
Hemoglobin saturation is expressed as a percentage (%).
53. What happens to hemoglobin at a high PO2?
At a high PO2, hemoglobin binds to oxygen to form oxyhemoglobin.
54. Why does hemoglobin in the lung capillaries have almost 100% oxygen saturation?
Hemoglobin in lung capillaries has almost 100% oxygen saturation because the lungs have a high PO2 due to the intake of oxygen from the atmosphere.
55. What happens to oxyhemoglobin at a low PO2?
At a low PO2, oxyhemoglobin dissociates, releasing oxygen and converting back to hemoglobin.
56. Why does hemoglobin in the respiring tissues have a very low oxygen saturation?
Hemoglobin in the respiring tissues has low oxygen saturation because these tissues have a low PO2, as they consume more oxygen during respiration. Additionally, the high CO2 concentration in these tissues triggers the Bohr effect, promoting oxygen release.
57. What shape is the hemoglobin dissociation curve?
The hemoglobin dissociation curve has an “S” shape. It is initially shallow, then steep, and flattens again at higher PO2 levels.
58. Why is the hemoglobin dissociation curve shallow at a low PO2?
At low PO2, most hemoglobin molecules have not yet bound oxygen, making it harder for them to associate with the first oxygen molecule. The heme groups are still in the center, and the lack of oxygen keeps saturation levels low.
59. Why does the hemoglobin dissociation curve steepen as the PO2 rises?
As PO2 rises, more hemoglobin molecules bind to oxygen, and the increasing availability of oxygen makes it easier for hemoglobin to become saturated.
60. Why is the hemoglobin dissociation curve shallow at the highest PO2 levels?
At high PO2, hemoglobin approaches full saturation, leaving fewer unsaturated hemoglobin molecules to bind with oxygen, causing the curve to flatten.
61. How does oxygen bind to hemoglobin?
Each of the Fe²⁺ ions in the four heme groups binds with one oxygen molecule, allowing hemoglobin to carry up to four oxygen molecules.
62. How does fetal hemoglobin differ from adult hemoglobin?
Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin.
63. Why is the fact that fetal hemoglobin has a higher oxygen affinity useful for the fetus?
Fetal hemoglobin’s higher oxygen affinity allows it to effectively bind oxygen even in the low PO2 environment of the placenta. This enables the fetus to extract oxygen from the mother’s blood, where adult hemoglobin has released oxygen.
64. How is the fetal hemoglobin dissociation curve different from an adult’s?
The fetal hemoglobin dissociation curve is shifted to the left compared to an adult’s, reflecting its higher oxygen affinity.
65. Why does carbon dioxide need to be transported?
Carbon dioxide needs to be transported to prevent its accumulation in respiring tissues, which could lead to harmful effects such as acidosis.
66. Why does the pH of the erythrocytes’ cytoplasm become more acidic in high concentrations of carbon dioxide?
High concentrations of CO2 react with water to form carbonic acid (H₂CO₃), which dissociates into hydrogen ions (H⁺) and bicarbonate (HCO3-). The H⁺ ions cause the pH of the erythrocyte cytoplasm to become more acidic.
67. What does the Bohr effect do?
The Bohr effect lowers hemoglobin’s affinity for oxygen, promoting the release of oxygen in areas with high CO2 concentrations, such as actively respiring tissues.
68. What does the movement of gases between the lungs and the tissues depend on?
The movement of gases between the lungs and tissues depends on diffusion, driven by differences in partial pressures.
69. The PACO2 varies directly with CO2 production and inversely with what?
PACO2 varies inversely with alveolar ventilation.
70. How is PAO2 computed?
PAO2 is computed using the alveolar air equation: PAO2 = (FiO2 × (PB – PH₂O)) – (PACO2 ÷ RQ), where FiO2 is the fraction of inspired oxygen, PB is barometric pressure, PH₂O is water vapor pressure, and RQ is the respiratory quotient.
71. PAO2 varies inversely with what?
At a constant FiO2, PAO2 varies inversely with PACO2.
72. What is the normal PAO2?
Normal PAO2 is approximately 100 mmHg.
73. What is the normal PACO2?
Normal PACO2 is approximately 40 mmHg.
74. What is hemoglobin saturation?
Hemoglobin saturation is the percentage of hemoglobin molecules that are bound to oxygen in relation to the total amount of hemoglobin available.
75. What is the arteriovenous oxygen content difference?
The arteriovenous oxygen content difference represents the amount of oxygen extracted by tissues per 100 mL of blood as it passes through the body.
76. When does hemoglobin affinity for oxygen increase?
Hemoglobin’s affinity for oxygen increases in conditions of high PO2, high pH (alkalosis), 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?
This is known as the Bohr effect.
78. What can cause hypoxia?
Hypoxia can be caused by decreased arterial oxygen content, reduced blood flow (ischemia), or impaired cellular function, preventing adequate oxygen utilization.
79. What can cause decreased PaO2 levels?
Decreased PaO2 levels can result from low ambient PO2, hypoventilation, impaired diffusion, ventilation-perfusion (V/Q) imbalances, and right-to-left anatomic or physiologic shunting.
80. What can cause a decrease in alveolar ventilation?
A decrease in alveolar ventilation can be caused by inadequate minute ventilation, increased dead space ventilation, and ventilation-perfusion (V/Q) imbalance.
81. 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.
82. 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.
83. 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.
84. 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.
85. 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.
86. 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.
87. What is alveolar dead space?
Alveolar dead space is the volume of air in the lungs that does not participate 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.
88. 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.
89. 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.
90. 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.
91. 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.
92. 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.
93. 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.
94. 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.
95. 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.
96. How is most carbon dioxide produced in the body?
Most carbon dioxide is produced as a byproduct of cellular respiration during the metabolic breakdown of glucose and other nutrients.
97. How does temperature affect the hemoglobin dissociation curve?
An increase in temperature shifts the hemoglobin dissociation curve to the right, reducing hemoglobin’s affinity for oxygen and enhancing oxygen release to tissues.
98. What is the relationship between PaO2 and SaO2?
PaO2 refers to the partial pressure of oxygen in arterial blood, while SaO2 is the percentage of hemoglobin saturated with oxygen. As PaO2 increases, SaO2 also increases until hemoglobin approaches full saturation.
99. How does pH affect the hemoglobin dissociation curve?
A decrease in pH (acidosis) shifts the hemoglobin dissociation curve to the right, reducing hemoglobin’s affinity for oxygen, while an increase in pH (alkalosis) shifts it to the left, increasing oxygen affinity.
100. How does altitude affect oxygen transport in the blood?
At high altitudes, the partial pressure of oxygen is lower, which can reduce hemoglobin saturation and oxygen delivery to tissues, leading to symptoms of hypoxemia and altitude sickness.
Final Thoughts
The transport of oxygen and carbon dioxide in the blood exemplifies the body’s complex and efficient mechanisms to maintain homeostasis.
Oxygen delivery to tissues and carbon dioxide removal are fundamental to life, and disruptions in these processes can lead to significant health issues.
By exploring the pathways and forms of gas transport, students and healthcare professionals gain crucial insights into respiratory function.
Written by:
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.
References
- “Modeling Oxygen and Carbon Dioxide Transport and Exchange Using a Closed Loop Circulatory System.” National Center for Biotechnology Information, 2008.
- “Physiology, Oxygen Transport And Carbon Dioxide Dissociation Curve.” National Center for Biotechnology Information, 1 June 2020.
- “Physiology, Carbon Dioxide Transport.” National Center for Biotechnology Information, 1 Sept. 2020.