Question

Bulk of carbon dioxide \(\left(\mathrm{CO}_{2}\right)\) released from body tissues into blood is present as (a) Bicarbonate in blood plasma and RBCs (b) Free \(\mathrm{CO}_{2}\) in blood plasma. (c) 70 per cent carbaminohaemoglobin and 30 per cent as bicarbonate. (d) Carbaminohaemoglobin in RBCs.

Short answer

The correct option is (a) Bicarbonate in blood plasma and RBCs. Most of the CO2 released from body tissues into the blood is present as bicarbonate in blood plasma and red blood cells (RBCs).

Step-by-step solution

Understanding the Forms of CO2 in Blood

When carbon dioxide is produced in the cells during metabolic processes, it diffuses out of the cells and into the blood, where it's transported to the lungs for exhalation. In the blood, CO2 can be found in three forms: dissolved in plasma, as bicarbonate ions, or bound to hemoglobin (carbaminohaemoglobin).

Analyzing the Options

Let's examine each of the given options: (a) Bicarbonate in blood plasma and RBCs. (b) Free CO2 in blood plasma. (c) 70 per cent carbaminohaemoglobin and 30 per cent as bicarbonate. (d) Carbaminohaemoglobin in RBCs. Focus on the proportions and forms of CO2 in each option.

Identify the Correct Option

In the blood, about 10% of the carbon dioxide is dissolved directly into the plasma. About 30% binds to hemoglobin to form carbaminohaemoglobin. The majority of carbon dioxide (about 60%) forms bicarbonate ions in the red blood cells. So, the correct option is (a) Bicarbonate in blood plasma and RBCs.

Key concepts

Bicarbonate Ion

When carbon dioxide is released from body tissues, it plays a fascinating role in blood chemistry. A significant portion of this carbon dioxide is transformed into bicarbonate ions, which are critical in maintaining the body's pH balance. Here's how that transformation happens:

1. **Formation Process:** Inside red blood cells (RBCs), carbon dioxide reacts with water to form carbonic acid. This reaction is catalyzed by an enzyme called carbonic anhydrase. The carbonic acid then quickly dissociates into bicarbonate ions ( ext{HCO}_3^{-}) and hydrogen ions (H^+).
2. **Transport in Plasma:** The bicarbonate ions diffuse out of the RBCs into the blood plasma in exchange for chloride ions, a process known as the chloride shift. This movement ensures that bicarbonate ions are efficiently carried in the plasma.
3. **Buffer System:** Bicarbonate serves an essential function as part of the blood’s buffering system, helping to regulate the pH. This is crucial for maintaining homeostasis and ensuring that bodily functions run smoothly.
Thus, around 60% of carbon dioxide in the blood is transported in the form of bicarbonate ions, making it the primary method of CO2 transport.

Carbaminohaemoglobin

Carbaminohaemoglobin is another form in which carbon dioxide is transported in the blood. It represents about 30% of CO2 transport. When carbon dioxide enters the bloodstream, it can attach directly to hemoglobin molecules:

- **Binding Process:** Hemoglobin, which is more commonly known for carrying oxygen, can also bind to carbon dioxide. Instead of binding to the iron atom in heme like oxygen does, CO2 attaches to the amino groups in the globin protein part of the hemoglobin, forming carbaminohaemoglobin.
- **Significance in CO2 Transport:** Though less abundant than bicarbonate ions, carbaminohaemoglobin is crucial because it allows for a different route for CO2 to hitch a ride in the blood. This process also helps release oxygen from hemoglobin more effectively, an important part of the body's gas transport dynamics.
- **Reversibility:** At the lungs, carbaminohaemoglobin releases CO2 for exhalation. This reversibility helps maintain efficient gas exchange and ensures that carbon dioxide is expelled from the body.

The role of carbaminohaemoglobin highlights the versatility of hemoglobin and the body's efficiency in transporting and eliminating carbon dioxide.

Blood Plasma

Blood plasma is the liquid component of blood, and it plays a key role in carbon dioxide transport beyond just being a carrier medium.

- **CO2 Solubility:** Although only about 10% of carbon dioxide is dissolved directly in blood plasma, this small amount is vital for the rapid transport of CO2. The dissolved CO2 in plasma is a balance between what diffuses from tissues and what is released from the red blood cells.
- **Conduit for Bicarbonate:** Blood plasma acts as a conduit for the bicarbonate ions that originate from red blood cells. This role is crucial because the bicarbonate must be transported back to the lungs to complete the gas exchange cycle.
- **Maintaining Circulatory Efficiency:** The presence of dissolved CO2 and bicarbonate in plasma ensures that carbon dioxide is efficiently transported without disrupting the delicate balance of blood components. This contributes to an optimized circulatory system that supports regular metabolic activity.

Understanding the roles that blood plasma plays provides insight into why it is an indispensable part of carbon dioxide transport in the human body.