Problem 35 - Anatomy and Physiology of Animals

Question

Explain how the \(\mathrm{CO}_{2}\) generated by cells and exhaled in the lungs is carried as bicarbonate in the blood.

Step-by-Step Solution

Answer

Cells produce CO2, which forms bicarbonate in the blood, and is eventually released as CO2 in the lungs.

Step 1: CO2 Diffusion into Blood

The CO2 generated by the cells as a byproduct of metabolism diffuses into the bloodstream from the tissues. This diffusion occurs because of a concentration gradient where CO2 is higher in cells and lower in blood.

Step 2: Formation of Carbonic Acid

When CO2 enters the blood, it reacts with water present in the blood plasma. This reaction is catalyzed by the enzyme carbonic anhydrase, leading to the formation of carbonic acid: \[ ext{CO}_2 + ext{H}_2 ext{O} \rightarrow ext{H}_2 ext{CO}_3\]

Step 3: Dissociation into Bicarbonate

Carbonic acid is a weak acid and readily dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+):\[ ext{H}_2 ext{CO}_3 \rightarrow ext{HCO}_3^- + ext{H}^+\]

Step 4: Transportation in the Bloodstream

Bicarbonate ions are then transported through the bloodstream to the lungs. This form of CO2 transportation assists in maintaining the pH balance of the blood by acting as a buffer.

Step 5: Conversion to CO2 in the Lungs

In the lungs, bicarbonate ions are reconverted into carbonic acid, and then into CO2 and water by the reverse action of carbonic anhydrase:\[ ext{HCO}_3^- + ext{H}^+ \rightarrow ext{H}_2 ext{CO}_3 \rightarrow ext{CO}_2 + ext{H}_2 ext{O}\] This CO2 is then exhaled.

Key Concepts

carbonic anhydrase

Carbonic anhydrase is a key enzyme that plays a crucial role in the transport of carbon dioxide ( CO_2 ) as bicarbonate in the blood.
Imagine it as the catalyst in the spotlight, speeding up the chemical reactions involving carbon dioxide and water. It facilitates the formation of carbonic acid from CO2 and water, an essential step in CO2 transport. Without carbonic anhydrase, the reaction would be too slow. The enzyme increases the reaction rate by converting CO2 and water into carbonic acid very quickly. This is important because it ensures that CO2 transport from the tissues to the lungs occurs efficiently.
This enzyme is present in high concentrations in red blood cells where it catalyzes:

• The conversion of CO2 to carbonic acid ( H_2 CO_3 ) (forward reaction)
• The breakdown of carbonic acid back to CO2 and water in the lungs (reverse reaction)

This dual function of carbonic anhydrase ensures that CO2 can be carried and released effectively during respiration.

carbonic acid

Carbonic acid ( H_2 CO_3 ) is an important player in CO2 transport in the blood. It's formed when carbon dioxide reacts with water, a reaction catalyzed by carbonic anhydrase. Carbonic acid is a weak acid, meaning it doesn't completely dissociate in solution.
This characteristic is helpful because it allows carbonic acid to easily break down into bicarbonate ions ( HCO_3^- ) and hydrogen ions ( H^+ ), facilitating CO2 transport in the blood. This reversible reaction is key for controlling how much CO2 is carried in each form:

• As carbonic acid, when in proximity with carbonic anhydrase
• As bicarbonate and hydrogen ions, for transportation through the blood

Carbonic acid also provides a buffer system to maintain stable conditions within our bodies. Its ability to interchange forms allows CO2 to be transported without disrupting blood pH.

blood pH balance

Maintaining a stable blood pH is essential for our body to function properly, and bicarbonate ions play a crucial role here.
They act as a buffer, which basically means they help keep the blood's acidity or alkalinity stable even when acids or bases are added. When CO2 is converted into bicarbonate ions in the blood, it helps to regulate pH levels. Here's how it works:

• Bicarbonate ions neutralize excess acids in the blood. If the blood becomes too acidic, bicarbonate ions can absorb excess hydrogen ions ( H^+ ), forming carbonic acid.
• Conversely, if the blood becomes too alkaline, carbonic acid can release hydrogen ions, thus increasing acidity.

This buffering action is vital because even a small change in blood pH can disrupt normal physiological functions. Bicarbonate thus helps to ensure that our body's internal environment remains consistent, allowing all systems to operate smoothly and efficiently.