Transport of Gases: Have you ever wondered about the air we breathe in, how it is transported in our body? Blood is the medium for the transport of oxygen and carbon dioxide in our bodies. A major amount of oxygen is transported by the red blood cells in the blood. The remaining amount of oxygen is carried in a dissolved state through the plasma. Carbon dioxide is transported by red blood cells as bicarbonate and is also carried in a dissolved state through plasma. Read this article to know more about the transport of gases and various factors responsible for the transport of gases in our bodies.

Transport of Oxygen

Oxygen is carried by the blood in the body. It is carried in two forms:

1. As dissolved gas in plasma
2. As oxyhemoglobin by red blood cells

1. As dissolved gas: Under normal conditions of temperature and pressure, oxygen is soluble in plasma to a very less amount. Hence, most of it is carried by red blood cells. Approximately 3% of oxygen is transported by blood in dissolved form in plasma.
2. As oxyhemoglobin: Red blood cells contain a protein called hemoglobin. It has four polypeptide chains. Therefore it is written as Hb₄. It contains four haem groups or four atoms of iron in ferrous form (Fe²⁺) attached to it. Thus it can combine with four molecules of oxygen to form Hb₄O₈. This is known as Oxyhemoglobin.

Reaction for the formation of oxyhemoglobin is:
Hb₄ ₊ 4O₂ ↔ Hb₄O₈
Hemoglobin + Oxygen     Oxyhemoglobin

Hemoglobin has a high affinity for oxygen, and this affinity is increased by a fall in the partial pressure of carbon dioxide of blood. At the alveolus in the lungs, venous blood has a low partial pressure of oxygen. Thus oxygen diffuses into red blood cells and forms oxyhemoglobin. The partial pressure of carbon dioxide is less in alveolus as compared with that of blood, so carbon dioxide diffuses from blood to alveoli. This results in a fall of the partial pressure of carbon dioxide, which further increases the uptake of oxygen.

Under high partial pressure of oxygen, oxygen easily binds with the hemoglobin molecule in the pulmonary capillaries. This oxygenated blood, when it reaches different tissues in the body, partial pressure of oxygen falls. This results in the breaking of the bond between oxygen and hemoglobin molecules. Hence, oxygen is released from the blood capillaries.

Oxygen- Hemoglobin Dissociation Curve

The amount of oxygen that binds with the hemoglobin is known as oxygen tension. This oxygen tension is expressed in terms of the partial pressure of oxygen. The percentage of hemoglobin that binds with oxygen is known as the percentage saturation of hemoglobin. The relationship between the partial pressure of oxygen and percentage saturation of the hemoglobin with oxygen is graphically proven by a curve called Oxygen- hemoglobin dissociation curve or oxygen dissociation curve. The partial pressure of the air within the alveoli is approximately 104mmHg. The partial pressure of oxygen leaving the alveoli is about 100 mmHg. This is because oxygen is not completely saturated with blood plasma. At blood pO₂ of 100 mmHg, 97% of hemoglobin in red blood cells is in the form of oxyhemoglobin. As blood flows into the capillaries, oxygen leaves the blood and diffuses into the tissues. The blood that leaves the tissue and enters into the veins has pO₂ about 40 mm Hg. At this point, the percentage saturation of hemoglobin with oxygen is only about 75%. The partial pressure of oxygen at which hemoglobin is 50% saturated is known as P50.

Under normal conditions, the oxygen hemoglobin dissociation curve is Sigmoid shape or S-shaped. The upper part of the curve indicates acceptance of oxygen by hemoglobin molecule, and the lower part of the curve indicates dissociation of the oxygen molecule from the hemoglobin molecule.

 A person who is at rest has only 22% of the oxyhemoglobin that has released its oxygen to the tissues, which means 1/5^th of oxygen is unloaded to tissue and 4/5^th of it is reserved. During heavy exercise, the muscles use more oxygen from capillary blood, thereby decreasing the partial pressure of oxygen in the venous blood. 

Every 100 ml of oxygenated blood can deliver around 5mL of O₂ to the tissues under normal physiological conditions. During exercise, oxygen reserve ensures blood has enough oxygen to maintain life for 4-5 minutes even after the heart stops pumping. Hemoglobin does not take up oxygen at a low partial pressure of oxygen, but as the oxygenation of pigment occurs, its affinity for oxygen increases. In hemoglobin, four subunits of oxygen are present; the addition of one molecule of oxygen increases the affinity of side heme for oxygen known as Cooperativity between active sites.  

1gm of hemoglobin transits 1.34 ml of oxygen. 100ml of blood normally contains 15 gm of hemoglobin, so 100 ml blood carries around 20 ml of oxygen.

Factors Affecting Oxygen Dissociation Curve

The oxygen hemoglobin dissociation curve is shifted either to the right or left by various factors.

Shift to Right: The oxygen hemoglobin dissociation curve is shifted to the right under the following conditions:

1. Decrease in partial pressure of oxygen.
2. Increase in partial pressure of carbon dioxide(Bohr effect).
3. Increase in hydrogen ion concentration and decrease in pH (acidity)
4. Increase in body temperature.
5. Excess of 2,3-DPG (2,3- Diphosphoglycerate- by-product in glycolysis process)

Shift to Left: The oxygen hemoglobin dissociation curve is shifted to the left under the following conditions:

1. In the fetal blood, because fetal hemoglobin has more affinity for oxygen than adult hemoglobin.
2. Decrease in hydrogen ion concentration and increase in pH (alkalinity)
3. Decrease in partial pressure of CO₂.

Bohr Effect: Shifting of the Oxygen- hemoglobin dissociation curve to the right by increasing partial pressure of carbon dioxide is also known as the Bohr effect.  This effect is seen at the level of tissue capillaries. The presence of carbon dioxide decreases the affinity of hemoglobin for oxygen and increases the release of oxygen to the tissues. The pH of the blood falls as carbon dioxide concentration increases. This occurs because the dissociation of carbonic acid causes the release of carbon dioxide and hydrogen ions. An increase in the concentration of hydrogen ions causes falls in the pH. As a result of this, when the partial pressure of carbon dioxide rises, the curve shifts to the right, and thus P₅₀ rises.

Factors influencing Bohr Effect: The factors which are responsible for shifting the oxygen hemoglobin dissociation curve to the right are responsible for increasing the Bohr effect.

Transport of Carbon dioxide

Carbon dioxide in gaseous form diffuses out of the cells into the capillaries, from where it is transported in 3 ways. These are as follows:

1. In dissolved form: Due to the high solubility of carbon dioxide, approximately 7 % of it gets dissolved in the blood plasma and is carried in solution to the lungs.
2. In the form of bicarbonate: The largest fraction of carbon dioxide is converted to bicarbonate (HCO₃ˉ) and is transported in plasma. When carbon dioxide diffuses into red blood cells, it combines with water forming carbonic acid (H₂CO₃). Carbonic acid (H₂CO₃) is unstable and dissociates quickly into H⁺ and HCO₃ˉ ions. Reaction for the formation of carbonic acid is as follows:
   CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃ˉ
   Carbon + Water ↔ Carbonic acid↔ Hydrogen + Bicarbonate
   Dioxide                                                  Ion                  Ion
   The given reaction is very slow in blood plasma but occurs rapidly inside red blood cells because of the presence of a zinc-containing enzyme called Carbonic Anhydrase.
3. In the form of Carbaminohemoglobin: Carbon dioxide combines with the amine radicals of hemoglobin to form Carbaminohemoglobin. About 23 of carbon dioxide is carried by the hemoglobin in this form:
   CO₂ + Hb ↔ HbCO₂

Fig: Gas Exchange at the Level of Tissues and Lung

At the tissue site, where the partial pressure of carbon dioxide is high due to catabolism, carbon dioxide diffuses into the blood and forms HCO₃ˉ and H⁺. Thus, carbon dioxide gets trapped as bicarbonate at the tissue level and then it is transported to the alveoli. It is then released out of the body when we exhale out. Every 100 ml of deoxygenated blood provides approximately 4ml of carbon dioxide to the alveoli.

Chloride shift: The most of HCO₃ˉ formed within red blood cells diffuse into the blood plasma along the concentration gradient. The exit of bicarbonate ions causes changes in the ionic balance between the plasma and the red blood cells. To restore the ionic balance, the chloride ion diffuses from the plasma back into the red blood cells. This movement of chloride ions is known as the Chloride shift or Hamburger’s Phenomenon.

Haldane Effect: This effect was put forward by the scientist named J.S. Haldane, and it is based on the fact that the combination of oxygen with hemoglobin causes the hemoglobin to become a strong acid. The binding of oxygen with hemoglobin causes the displacement of carbon dioxide from the blood. This effect is far more vital in promoting carbon dioxide transport than in the Bohr effects, which promote oxygen transport.

As carbon dioxide enters the bloodstream, more oxygen gets dissociated from hemoglobin. This, in turn, allows more carbon dioxide to combine with hemoglobin, and as a result of this, more bicarbonate ions are formed. In the pulmonary circulation, the situation is reversed. Uptake of oxygen facilitates the release of carbon dioxide. As hemoglobin becomes saturated with oxygen, the H⁺ released combine with HCO₃ˉ, helping to unload carbon dioxide from the pulmonary blood. The Haldane effect encourages carbon dioxide exchange in both the tissues and lungs.

Release of Carbon Dioxide in the Alveoli of Lung

Deoxygenated blood is carried via pulmonary arteries to the lung. This blood is rich in carbon dioxide. Carbon dioxide is present in the blood as dissolved in plasma, as bicarbonate ions, and as carbaminohemoglobin. Carbon dioxide is released into the alveoli of the lungs in the following manner:

1. Since carbon dioxide is less stable in arterial blood as compared with that of venous blood, it easily diffuses from the blood into the lung alveoli.
2. The Dissociation of carbonic acid causes the release of carbon dioxide and hydrogen ions into the blood.
3. The high partial pressure of oxygen in the blood causes the release of carbon dioxide from carbaminohemoglobin.

Summary

Blood is the medium for the transport of oxygen and carbon dioxide in our bodies. 97% of oxygen is transported by the red blood cells in the blood and the remaining 3% of oxygen is carried in a dissolved state through the plasma. 20-25% of carbon dioxide is transported by red blood cells, 70% as bicarbonate and approximately 7% is carried in a dissolved state through plasma. The relationship between the partial pressure of oxygen and percentage saturation of the hemoglobin with oxygen is graphically proven by a curve called Oxygen- hemoglobin dissociation curve. Various factors affect the saturation of oxygen with that of hemoglobin. Chloride shift and the Haldane effect are the two phenomena that are responsible for the release of carbon dioxide from the body.

Frequently asked questions (FAQs) on Transport of Gases

Q.1. Give the various ways in which transport of oxygen occurs in our body?
Ans: 97% of oxygen is transported by the red blood cells in the blood, and the remaining 3% of oxygen is carried in a dissolved state through the plasma.

Q.2. How is carbon dioxide transported in our bodies?
Ans: 20-25% of carbon dioxide is transported by red blood cells, 70% as bicarbonate, and approximately 7% is carried in a dissolved state through plasma.

Q.3. Explain the oxygen-hemoglobin dissociation curve.
Ans: The relationship between the partial pressure of oxygen and percentage saturation of the hemoglobin with oxygen is graphically proven by a curve called the Oxygen-hemoglobin dissociation curve.

Q.4. What is chloride shift?
Ans: The movement of chloride ions from the plasma back into the red blood cells in order to restore the ionic balance is known as chloride shift.

Q.5. What is the Haldane effect?
Ans: This effect is based on the fact that the combination of oxygen with hemoglobin causes hemoglobin to become a strong acid. The binding of oxygen with hemoglobin causes the displacement of carbon dioxide from the blood, thus releasing the carbon dioxide out of the body.

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