Introduction
The transport of oxygen and carbon dioxide is a vital physiological process that sustains life in animals. These gases are exchanged in the lungs (or gills) and transported via the circulatory system to and from tissues and cells. Oxygen is essential for cellular respiration, while carbon dioxide is a waste product that must be removed efficiently. A brief description of the transport of oxygen and carbon dioxide is given bellow-
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| Infographic showing oxygen binding to hemoglobin in lungs, traveling through arteries, releasing oxygen in tissues, and carbon dioxide being carried back to lungs as bicarbonate and carbaminohemoglobin. |
Oxygen Transport
1. Carriage of Oxygen in Blood
Oxygen is poorly soluble in plasma, so 98.5% of it binds with hemoglobin (Hb) in red blood cells, forming oxyhemoglobin:
Only about 1.5% is transported dissolved directly in plasma.
2. Role of Hemoglobin: Hemoglobin is a respiratory pigment found in vertebrates and some invertebrates. Each hemoglobin molecule can bind four oxygen molecules. Binding is influenced by partial pressure of oxygen (pO₂), temperature, pH, and CO₂ levels. The oxygen dissociation curve is sigmoid-shaped due to cooperative binding.
3. Oxygen Delivery to Tissues
When oxygen-rich blood reaches the tissues where pO₂ is lower, hemoglobin releases oxygen. Factors enhancing this release include- Increased CO₂ concentration (Bohr effect), Lower pH and Higher temperature
Carbon Dioxide Transport
Carbon dioxide is carried from tissues to the lungs by three main methods:
1. Dissolved in Plasma (7%)
A small percentage of CO₂ is transported directly dissolved in plasma.
2. As Carbamino Compounds (23%)
CO₂ binds with amino groups in hemoglobin to form carbaminohemoglobin:
Hb-NH2+CO2⇌Hb-NH-COOH
3. As Bicarbonate Ions (70%)
The majority of CO₂ is transported as bicarbonate (HCO₃⁻). In red blood cells, CO₂ reacts with water to form carbonic acid (H₂CO₃). The enzyme carbonic anhydrase catalyzes the reaction then H₂CO₃ dissociates into H⁺ and HCO₃⁻ and it diffuses into the plasma, while H⁺ binds to hemoglobin, buffering the pH.
Bohr and Haldane Effect
These mechanisms optimize gas exchange duing respiration and metabolism.
Bohr Effect
The Bohr Effect describes how increased levels of carbon dioxide (CO₂) and decreased pH (more acidic conditions) in the blood reduce the affinity of hemoglobin for oxygen.
Mechanism: In active tissues, CO₂ levels are high and pH is low due to metabolic activity. This causes hemoglobin to release oxygen more easily to meet the tissue’s oxygen demand. This shift is visualized as a rightward shift of the oxygen-hemoglobin dissociation curve.
Biological Significance: It enhances oxygen delivery to metabolically active tissues and also ensures that tissues get more oxygen when they need it the most.
Haldane Effect
The Haldane Effect describes how oxygenation of hemoglobin in the lungs promotes the release of carbon dioxide, while deoxygenated hemoglobin in tissues binds CO₂ more readily.
Mechanism: In the lungs, as oxygen binds to hemoglobin, its affinity for CO₂ decreases, allowing CO₂ to be released and exhaled. In tissues, deoxygenated hemoglobin binds CO₂ and H⁺ ions more effectively, helping carry CO₂ back to the lungs.
Biological Significance: It enhances CO₂ removal in the lungs and increases CO₂ uptake in the tissues.
These effects work together to make gas exchange more efficient by matching oxygen delivery with carbon dioxide removal where and when it's needed most.
Table : Comparision of Bohr Effect and Haldane Effect
|
Feature |
Bohr
Effect |
Haldane
Effect |
|
Trigger |
↑ CO₂ or ↓ pH |
↑ O₂ (oxygenation) |
|
Effect |
↓ Hemoglobin’s affinity for O₂ |
↓ Hemoglobin’s affinity for CO₂ |
|
Location Focus |
Active tissues (oxygen delivery) |
Lungs (CO₂ removal) |
|
Physiological Role |
Promotes O₂ release to tissues |
Promotes CO₂ release in lungs |
The efficient transport of oxygen and carbon dioxide is crucial for maintaining cellular metabolism and homeostasis in animals. The unique properties of hemoglobin, along with physiological adaptations like the Bohr and Haldane effects, ensure that tissues receive the oxygen they need while removing waste CO₂ effectively.
References
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Guyton, A. C., & Hall, J. E. (2016). Textbook of Medical Physiology (13th ed.). Elsevier.
Sherwood, L. (2012). Human Physiology: From Cells to Systems (8th ed.). Brooks Cole.
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Tortora, G. J., & Derrickson, B. (2017). Principles of Anatomy and Physiology (15th ed.). Wiley.
FAQs
Q1. Why is most oxygen transported by hemoglobin?
Answer: Oxygen has low solubility in plasma, so hemoglobin binds and carries most of it efficiently to tissues.
Q2. What is the main form of carbon dioxide transport?
Answer: Around 70% of carbon dioxide is carried as bicarbonate ions formed in red blood cells.
Q3. What role does carbonic anhydrase play?
Answer: It catalyzes the rapid conversion of CO₂ and water to carbonic acid, which dissociates to bicarbonate for transport.
Q4. How does the Bohr effect aid tissue oxygenation?
Answer: It causes hemoglobin to release more oxygen in tissues where CO₂ levels are high and pH is low.
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