Question

Choose the right sequential phenomena during the passage of \(\mathrm{CO}_{2}\) from blood to tissues: (P) Absorption of \(\mathrm{CO}_{2}\) by blood. (Q) Reaction of \(\mathrm{CO}_{2}\) with water forming \(\mathrm{H}_{2} \mathrm{CO}_{3}\) inside \(\mathrm{RBCs}\) and then \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}^{+}\) ions. (R) Reaction of \(\mathrm{CO}_{2}\) with water forming \(\mathrm{H}_{2} \mathrm{CO}_{3}\) inside plasma followed by conversion into \(\mathrm{H}^{+}\) and \(\mathrm{HCO}_{3}^{-}\) ions. (S) Combination of \(\mathrm{H}^{+}\) with haeme part of \(\mathrm{HbO}_{2}\) to release \(\mathrm{O}_{2}\). (T) Combination of \(\mathrm{HCO}_{3}\) with haeme part of \(\mathrm{HbO}_{2}\) to form reduced haemoglobin and release of \(\mathrm{O}_{2}\) (a) \(\mathrm{P}, \mathrm{R}, \mathrm{S}\) (b) P, Q, T (c) \(\mathrm{P}, \mathrm{Q}, \mathrm{S}\) (d) \(P, R, T\)

Short answer

Therefore, based on the understanding of the sequential phenomena in the process of CO2 transport, the correct sequence would be P, Q, S making the answer - (c) P, Q, S.

Step-by-step solution

Absorption of CO2 by Blood

The correct sequence will start with the absorption of CO2 (P) by the blood from the tissues. This happens when the carbon dioxide produced during cellular metabolism diffuses out of cells and into the capillaries, where it's absorbed by the blood.

Conversion of CO2

After its absorption, CO2 then reacts with water to form carbonic acid (H2CO3). This reaction primarily happens inside the red blood cells (RBCs) aided by an enzyme called carbonic anhydrase. Hence the next step should be Q.

Formation of ions

The formed carbonic acid then spontaneously separates to form hydrogen (H+) and bicarbonate (HCO3-) ions. These ions are formed inside both RBCs and the plasma, but to maintain ionic balance, the bicarbonate ions move out of the RBCs into the plasma.

Combination of ions

Inside the RBCs, some of the released H+ ions combine with the haem part of haemoglobin, releasing oxygen (O2). Hence the next step should be S. The remaining bicarbonate ions do not react with the haeme part of haemoglobin, which rules out T.

Key concepts

Red Blood Cells

In the process of carbon dioxide transport, red blood cells (RBCs) play a crucial role. These cells absorb carbon dioxide as it diffuses from body tissues into the bloodstream.
Once inside the RBCs, carbon dioxide undergoes a series of reactions. The unique environment of RBCs, combined with the presence of enzymes, facilitates these reactions efficiently.
Red blood cells not only transport oxygen but also help in removing carbon dioxide, thereby maintaining a balance in the body’s respiratory processes. They act as mini converters, transforming CO2 into transportable forms.

Carbonic Acid Formation

When carbon dioxide enters the red blood cells, it combines with water. This reaction forms carbonic acid ( H_2CO_3 ), catalyzed by the enzyme carbonic anhydrase.
Carbonic acid is a weak acid and does not stay stable for long. It quickly dissociates into bicarbonate ( HCO_3^- ) and hydrogen ions ( H^+ ).
This conversion is crucial as it allows for the effective transport of carbon dioxide in a more soluble form, helping maintain the acid-base balance in the body.

Bicarbonate Ions

The dissociation of carbonic acid results in the formation of bicarbonate ions. These ions play an essential role in transporting carbon dioxide in the blood.
Bicarbonate ions move from the red blood cells into the plasma in exchange for chloride ions, a process known as the chloride shift. This maintains ionic equilibrium across the cell membrane.
Most carbon dioxide in the blood is carried in this form, making it an efficient way for the body to manage CO2 transport and maintain pH levels.

Hemoglobin Interaction

Hemoglobin, found in red blood cells, facilitates oxygen release while interacting with hydrogen ions. As hydrogen ions bind to hemoglobin, oxygen is released.
This interaction ensures that oxygen is delivered where it’s needed, while helping regulate carbon dioxide transport.
Hemoglobin acts as a buffer, stabilizing pH levels and assisting in the efficient transport of gases throughout the body.