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In this article, we will describe the role of red blood cells in transporting oxygen and carbon dioxide with reference to the roles of haemoglobin, carbonic anhydrase, the formation of haemoglobin acid, and the formation of carbaminohaemoglobin. We will also describe the chloride shift and explain its importance of the chloride shift. Moreover, we will explain the role of plasma in the transport of carbon dioxide. So, let us get started.
Red Blood Cells: Haemoglobin and Oxygen
In this section, we will discuss the role of red blood cells in transporting oxygen and carbon dioxide with reference to the roles of haemoglobin. Before discussing the role of red blood cells with reference to the haemoglobin, first, let us see what haemoglobin is?
What is Haemoglobin?
Haemoglobin refers to the globular protein which is an oxygen-carrying pigment present in huge quantities in red blood cells. The main function of the haemoglobin is to bind oxygen in the lung and transport the oxygen to the tissue to be employed in metabolic pathways. Since haemoglobin is soluble in water, but oxygen is not, so oxygen can be transported more efficiently around the body when it is bound to the haemoglobin.
Role of Haemoglobin in Transporting Oxygen
Most of the oxygen that is transported around the body is bound to the protein known as haemoglobin in the red blood cells. Red blood cells are also referred to as erythrocytes.
Each haemoglobin molecule has four haem groups and each forms a bond with a single oxygen molecule. This implies that each haemoglobin molecule is able to carry four oxygen molecules or a total of eight oxygen atoms.
Oxyhaemoglobin is formed when oxygen binds to haemoglobin.
The chemical and word equations of this reaction are given below:
Oxygen + Haemoglobin 
Oxyhaemoglobin
+ Hb 
When the first oxygen molecule binds, a conformational change in the structure of haemoglobin occurs. This change makes it easier for each next oxygen molecule to bind. This is known as cooperative binding.
When oxygen dissociates in the tissues, the reverse of this process, i.e. cooperative binding occurs.
Solved Example
There is approximately 150 g of haemoglobin in a single dm3 of blood. At room temperature, in a healthy adult body, 1 g of haemoglobin combines with 1.3 cm3 of oxygen.
Calculate the amount of oxygen that can be carried in 1 dm3 of blood.
To solve this example, you must consider the information already present in the question. You do not require prior information to solve this example.
There are 150 g of haemoglobin in 1 dm3 of blood.
1.3 cm3 of oxygen can be carried in 1 g of haemoglobin
1.3 x 150 = 195
Hence, 150 g of haemoglobin can carry 195 cm3 of oxygen. This means that 195 cm3 is the right answer to this question.
In the next section of the article, we will discuss the chloride shift.
The Chloride Shift
The chloride shift refers to the movement of chloride ions into the red blood cells that take place when hydrogen carbonate ions are created.
The formation of hydrogen carbonate ions occurs by the following process:
- The diffusion of carbon dioxide into the red blood cells
- The enzyme known as carbonic anhydrase speeds up, i.e., catalyzes the combination of carbon dioxide and water to create carbonic acid (
)
The chemical equation of this reaction is given below:
Carbon dioxide + Water 
Carbonic acid
The dissociation of carbonic acid occurs and results in the formation of hydrogen carbonate ions and hydrogen ions. The chemical equation of this reaction is given below:
The negatively charged hydrogen carbonate ions created from the dissociation of carbonic acid are moved out of the red blood cells through a transport protein in the membrane.
The negatively charged chloride ions are moved into the red blood cells through the same transport protein in order to prevent electrical imbalance.
Remember that if this does not happen, then the red blood cells will become positively charged because of the buildup of hydrogen ions created from the dissociation of carbonic acid.
In the next section of the article, we will discuss the role of plasma in transporting carbon dioxide.
Plasma and Carbon Dioxide
Surplus carbon dioxide generated as a result of respiration diffuses from the tissues into the blood. This waste carbon dioxide is carried around the body in the following ways:
- In the blood plasma, it is carried in the form of hydrogen carbonate ions (
). Approximately 85% of the carbon dioxide is transported in this manner. - The dissolution of approximately 5% of carbon dioxide takes place directly in the blood plasma
- Since carbon dioxide is bound to the haemoglobin as carbaminohaemoglobin, hence this is around 10% of carbon dioxide transported in the blood
In the next section of the article, we will discuss the carbon dioxide in the blood plasma.
Carbon dioxide in the Blood Plasma
When carbon dioxide is released as a waste product from the respiring cells, it diffuses into the cytoplasm of the red blood cells. Carbon dioxide combines with water to create hydrogen carbonate, i.e. inside the red blood cells.
An enzyme known as carbonic anhydrase is present in the red blood cells which speed up, i.e. catalyzes the reaction that takes place between carbon dioxide and water.
Without the enzyme carbonic anhydrase, this reaction proceeds extremely slow. Since the plasma has a small amount of carbonic anhydrase, therefore creates more slowly in plasma than in the cytoplasm of the red blood cells.
The dissociation of carbonic acid results in the formation of hydrogen ions (
) and hydrogen carbonate ions ().
The combination of hydrogen ions and haemoglobin results in the formation of haemoglobin acid. It prevents the hydrogen ions from decreasing the pH of the red blood cell. Haemoglobin plays the role of buffer in this situation.
The hydrogen carbonate ions diffuse out of the red blood cells into the plasma so that they can be transported in the solution.
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