Problem 10 - Biology: The Dynamic Science

Question

The majority of \(\mathrm{CO}_{2}\) in the blood: a. is in the form of carbonic acid and bicarbonate ions. b. dissociates to add \(\mathrm{H}^{+}\) to the blood to raise its \(\mathrm{pH}\) to 7.4 c. has a lower \(P_{C O_{2}}\) than the \(P_{C O_{2}}\) in the alveolar air. d. increases in the lung capillaries, which have a higher \(\mathrm{pH}\) than the tissue capillaries. e. can be displaced on the hemoglobin molecule by \(\mathrm{CO}\) if \(\mathrm{CO}\) is inhaled.

Step-by-Step Solution

Answer

The correct statements are: the majority of \(\mathrm{CO}_{2}\) in the blood is in the form of carbonic acid and bicarbonate ions, \(\mathrm{CO}_{2}\) has a lower \(P_{CO_2}\) than the \(P_{CO_2}\) in the alveolar air, and \(\mathrm{CO}_{2}\) can be displaced on the hemoglobin molecule by \(\mathrm{CO}\) if \(\mathrm{CO}\) is inhaled.

Step 1: Analyzing the first statement

The first statement, 'the majority of \(\mathrm{CO}_{2}\) in the blood is in the form of carbonic acid and bicarbonate ions', is correct. When \(\mathrm{CO}_{2}\) is released into the bloodstream, it can readily combine with water to form carbonic acid, which quickly dissociates to form a hydroxonium ion (\(H^+\)) and a bicarbonate ion (\(HCO3^-\)). This represents the largest mechanism for \(\mathrm{CO}_{2}\) transport in the blood.

Step 2: Analyzing the second statement

The second statement, '\(\mathrm{CO}_{2}\) dissociates to add \(H^+\) to the blood to raise its pH to 7.4', is incorrect. \(\mathrm{CO}_{2}\) does dissociate to form \(H^+\), but this actually decreases the pH (makes the blood more acidic), not increase it.

Step 3: Analyzing the third statement

The third statement, '\(\mathrm{CO}_{2}\) has a lower \(P_{CO_2}\) than the \(P_{CO_2}\) in the alveolar air', is correct. The partial pressure of \(\mathrm{CO}_{2}\) (\(P_{CO_2}\)) in the bloodstream is generally lower than in the alveolar air to allow for its diffusion out of the blood and into the alveoli, where it can be exhaled.

Step 4: Analyzing the fourth statement

The fourth statement, '\(\mathrm{CO}_{2}\) increases in the lung capillaries, which have a higher pH than the tissue capillaries', is incorrect. The lung capillaries, being located close to the alveoli, have a lower \(\mathrm{CO}_{2}\) concentration and a higher pH as \(\mathrm{CO}_{2}\) is continuously removed and exhaled. Moreover, it's the tissue capillaries where \(\mathrm{CO}_{2}\) levels increase due to CO2 release from the metabolizing tissues.

Step 5: Analyzing the fifth statement

The fifth statement, '\(\mathrm{CO}_{2}\) can be displaced on the hemoglobin molecule by \(\mathrm{CO}\) if \(\mathrm{CO}\) is inhaled', is correct. Carbon monoxide (\(\mathrm{CO}\)) has a higher affinity for the hemoglobin molecule than \(\mathrm{CO}_{2}\), so if \(\mathrm{CO}\) is inhaled, it can displace \(\mathrm{CO}_{2}\) from hemoglobin, causing carbon monoxide poisoning.

Key Concepts

Carbonic Acid

Carbonic acid plays a crucial role in how carbon dioxide ( CO_2 ) is transported in the blood. When CO_2 is absorbed into the bloodstream, it can rapidly react with water to form carbonic acid. This reaction is facilitated by the enzyme carbonic anhydrase, which is especially abundant in red blood cells. The chemical reaction can be represented as:\[CO_2 + H_2O \rightleftharpoons H_2CO_3\]Although carbonic acid (H_2CO_3 ) is a weak acid, it does not stay intact for long. It quickly breaks down into hydrogen ions (H^+ ) and bicarbonate ions (HCO_3^- ). This process is critical to efficiently moving carbon dioxide from the tissues, where it is produced as a waste product, to the lungs, where it can be exhaled. The ability to convert CO_2 into a soluble form is vital, as it allows the blood to carry large amounts of this otherwise potentially harmful gas.

Bicarbonate Ions

Bicarbonate ions (HCO_3^- ) are responsible for carrying the majority of transported carbon dioxide in the blood. After the formation of carbonic acid, its dissociation yields bicarbonate ions via the reaction:\[H_2CO_3 \rightarrow H^+ + HCO_3^-\]These ions travel freely in the plasma, serving as a conveyor for CO_2 that can safely be shuttled back to the lungs. In the lungs, the process reverses, allowing CO_2 to diffuse back into the alveoli and be expelled. The exchange of bicarbonate for chloride ions in red blood cells, known as the chloride shift, enhances the transport capacity further. This efficient system for transporting CO_2 ensures that tissues don't accumulate it excessively, maintaining balance and proper gas exchange.

Hemoglobin Affinity

Hemoglobin's relationship with carbon dioxide is a fascinating aspect of blood chemistry. While hemoglobin primarily carries oxygen, it also has binding sites for carbon dioxide. However, its affinity for CO_2 is markedly less than for oxygen or carbon monoxide ( CO ). This difference in binding affinity is crucial for respiration. When blood reaches the tissues, carbon dioxide concentration is higher, facilitating its binding to hemoglobin and formation into carbaminohemoglobin. However, in the lungs, the high oxygen concentration promotes oxygen binding, releasing CO_2 for exhalation. This interplay is known as the Haldane effect, which maximizes efficiency in gas exchange and transport. It's important to note that exposure to carbon monoxide disrupts this delicate balance, as CO has an extremely high affinity for hemoglobin and can displace both oxygen and carbon dioxide from their sites, leading to toxicity and impaired oxygen transport.

Blood pH

Blood pH is a critical aspect of overall health and function, determined by the balance of various ions in the blood, including hydrogen ions from carbonic acid dissociation. When carbonic acid dissociates, it releases hydrogen ions, which, if abundant, can lower blood pH and lead to acidosis. The formula:\[H_2CO_3 \rightleftharpoons H^+ + HCO_3^-\]illustrates how carbon dioxide's presence can impact pH by increasing hydrogen ion concentration. Maintaining a stable pH (around 7.4) is essential for enzyme function and metabolic processes. The body uses buffer systems, such as bicarbonate and other mechanisms, to regulate pH around the normal range. Lungs and kidneys also play a significant role. Lungs can adjust breathing rates to control CO_2 levels, while kidneys excrete excess hydrogen ions. Together, these systems maintain homeostasis and the delicate chemistry of blood.