201. Assertion: In a right-shifted oxygen dissociation curve, haemoglobin saturation is lower at the same $P_{O_2}$ than before. Reason: A right shift indicates decreased affinity of haemoglobin for oxygen.
ⓐ. Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion
ⓑ. Both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion
ⓒ. Assertion is true, but the Reason is false
ⓓ. Assertion is false, but the Reason is true
Correct Answer: Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion
Explanation: A right shift means haemoglobin has become less strongly attached to oxygen at a given partial pressure. Because its affinity is reduced, haemoglobin will hold less oxygen at the same $P_{O_2}$ than it would before the shift. That is why saturation becomes lower at identical oxygen pressure values. This makes oxygen unloading easier in tissues that are metabolically active. The reason directly explains the assertion because lower affinity is exactly what causes the lower saturation at the same $P_{O_2}$. This is one of the most important interpretations of a shifted dissociation curve.
202. A student says, “The highest possible affinity of haemoglobin for oxygen would always be best for the body.” Which response is most accurate?
ⓐ. The statement is correct because tissues do not need haemoglobin to release oxygen
ⓑ. The statement is correct because oxygen is transported mainly in plasma, not by haemoglobin
ⓒ. The statement is incorrect because very high affinity could reduce oxygen unloading in tissues
ⓓ. The statement is incorrect because haemoglobin should not bind oxygen in the lungs
Correct Answer: The statement is incorrect because very high affinity could reduce oxygen unloading in tissues
Explanation: Haemoglobin must do two things well: bind oxygen in the lungs and release it in the tissues. If its affinity were always extremely high, it might load oxygen effectively but would not let go of it easily where cells need it. That would reduce oxygen delivery to metabolically active tissues. So the best physiological situation is not simply the strongest possible binding. Instead, haemoglobin must change its behavior according to conditions in lungs and tissues. This is why factors such as carbon dioxide, hydrogen ions, and temperature are important in regulating oxygen release.
203. In human blood, the largest proportion of carbon dioxide is transported as:
ⓐ. dissolved carbon dioxide in plasma
ⓑ. carbon monoxide bound to haemoglobin
ⓒ. bicarbonate ions in plasma
ⓓ. free gas bubbles inside red blood cells
Correct Answer: bicarbonate ions in plasma
Explanation: Most carbon dioxide produced by the tissues is not carried simply in dissolved form or directly attached to haemoglobin. Instead, a large part is converted into bicarbonate ions after carbon dioxide enters red blood cells. These bicarbonate ions then move into the plasma, where they are transported through the blood. This method allows the body to carry a much larger amount of carbon dioxide efficiently. It is therefore the main transport form of carbon dioxide in humans. Understanding this is essential for the whole topic of carbon dioxide transport.
204. Which statement correctly describes dissolved carbon dioxide in blood?
ⓐ. It is a small fraction of transported carbon dioxide that is carried directly in solution
ⓑ. It is the only form in which carbon dioxide can travel through plasma
ⓒ. It is the main form responsible for chloride shift in all cases
ⓓ. It is carbon dioxide permanently attached to haemoglobin
Correct Answer: It is a small fraction of transported carbon dioxide that is carried directly in solution
Explanation: A small amount of carbon dioxide is transported simply by dissolving in the plasma portion of blood. This dissolved fraction is important, but it is not the major form of transport. Most carbon dioxide is instead carried as bicarbonate ions, while another portion is transported as carbaminohaemoglobin. The dissolved form is the simplest transport form because no chemical conversion is required. Even so, its contribution is limited compared with bicarbonate transport. This distinction helps students compare the three main forms of carbon dioxide carriage.
205. Carbaminohaemoglobin is formed when:
ⓐ. oxygen binds irreversibly to the heme iron of haemoglobin
ⓑ. bicarbonate ions attach permanently to plasma proteins
ⓒ. chloride ions combine with haemoglobin inside alveoli
ⓓ. carbon dioxide binds with haemoglobin in the blood
Correct Answer: carbon dioxide binds with haemoglobin in the blood
Explanation: Carbaminohaemoglobin is the compound formed when carbon dioxide combines with haemoglobin. This is one of the important ways in which carbon dioxide is carried from tissues to lungs. It is different from oxyhaemoglobin, which is formed when oxygen binds with haemoglobin. Carbon dioxide does not need to be carried only in dissolved form because haemoglobin can also help transport it. This makes haemoglobin important in the carriage of both respiratory gases, though in different ways. Carbaminohaemoglobin is therefore a key concept in understanding carbon dioxide transport.
206. In red blood cells, carbon dioxide is converted into bicarbonate mainly through the reaction:
ⓐ. $CO_2 + O_2 \rightarrow CO_3$
ⓑ. $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$
ⓒ. $CO_2 + Cl^- \rightarrow HCl + O_2$
ⓓ. $CO_2 + Hb \rightarrow O_2 + H_2O$
Correct Answer: $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$
Explanation: When carbon dioxide enters red blood cells, it reacts with water to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate ions. This conversion is central to the major transport pathway of carbon dioxide in blood. The bicarbonate ions can then move into the plasma and be carried through circulation. This process helps blood transport much more carbon dioxide than would be possible by simple dissolution alone. It also links carbon dioxide transport with acid-base balance in the body. The reaction is therefore one of the most important chemical events in respiratory physiology.
207. What is meant by chloride shift in tissue capillaries?
ⓐ. The movement of oxygen into red blood cells as carbon dioxide leaves
ⓑ. The movement of sodium ions out of plasma during inspiration
ⓒ. The movement of chloride ions into red blood cells as bicarbonate ions move out
ⓓ. The movement of carbon dioxide from plasma directly into alveoli
Correct Answer: The movement of chloride ions into red blood cells as bicarbonate ions move out
Explanation: When bicarbonate ions are formed inside red blood cells, many of them move out into the plasma for transport. To maintain electrical balance, chloride ions move into the red blood cells. This exchange of bicarbonate and chloride ions is called the chloride shift. It is a very important part of carbon dioxide transport because it supports the continued movement of bicarbonate into plasma. Without such ion exchange, charge balance would be disturbed. So the chloride shift is a transport-supporting mechanism closely linked with bicarbonate formation.
208. The main importance of chloride shift is that it helps:
ⓐ. maintain ionic balance between red blood cells and plasma
ⓑ. convert all bicarbonate directly into oxygen
ⓒ. increase the number of alveoli in the lungs
ⓓ. pump carbon dioxide actively through the pleura
Correct Answer: maintain ionic balance between red blood cells and plasma
Explanation: As bicarbonate ions leave red blood cells, negative charge would be lost from the cell unless another negative ion enters. Chloride ions move in to balance this charge difference, and this keeps the ionic conditions stable. This is why chloride shift is so important in carbon dioxide transport. It is not mainly about oxygen formation or alveolar structure. Instead, it is a balancing mechanism that allows bicarbonate movement to continue smoothly. By maintaining electrical neutrality, chloride shift supports efficient transport of carbon dioxide in the blood.
209. Which option correctly compares the three main forms of carbon dioxide transport in human blood?
ⓐ. Most carbon dioxide is dissolved, less is as bicarbonate, and none binds to haemoglobin
ⓑ. Carbon dioxide is transported only as bicarbonate ions and never in any other form
ⓒ. Most carbon dioxide binds directly to platelets, with only a little as bicarbonate
ⓓ. Carbon dioxide is transported in dissolved form, as carbaminohaemoglobin, and mainly as bicarbonate ions
Correct Answer: Carbon dioxide is transported in dissolved form, as carbaminohaemoglobin, and mainly as bicarbonate ions
Explanation: Human blood carries carbon dioxide in three principal forms. A small portion remains dissolved directly in plasma, another portion combines with haemoglobin to form carbaminohaemoglobin, and the largest fraction is transported as bicarbonate ions. It shows that carbon dioxide transport is chemically more varied than oxygen transport. The bicarbonate pathway is dominant, but the other two forms also contribute. Knowing all three forms helps build a complete picture of carbon dioxide carriage.
210. Why is conversion of carbon dioxide into bicarbonate especially useful for transport in blood?
ⓐ. Because bicarbonate ions can enter the pleural cavity and be stored there
ⓑ. Because this form allows a large amount of carbon dioxide to be carried efficiently in plasma
ⓒ. Because bicarbonate ions replace haemoglobin in oxygen transport
ⓓ. Because dissolved carbon dioxide cannot exist at all in blood
Correct Answer: Because this form allows a large amount of carbon dioxide to be carried efficiently in plasma
Explanation: The body produces a large amount of carbon dioxide, and simple dissolution in plasma would not be sufficient for efficient transport. By converting carbon dioxide into bicarbonate ions, blood can carry much more of it in a chemically manageable form. These bicarbonate ions move into the plasma and circulate to the lungs. This is why bicarbonate transport becomes the major route for carbon dioxide carriage. It is an effective way of expanding transport capacity without relying only on dissolved gas. The usefulness of bicarbonate therefore lies in efficiency and quantity of transport.
211. In systemic tissue capillaries, which sequence best represents the usual handling of carbon dioxide in red blood cells?
ⓐ. Chloride leaves the red blood cell, bicarbonate enters, and oxygen is destroyed
ⓑ. Carbon dioxide leaves the blood, bicarbonate becomes oxygen, and chloride disappears
ⓒ. Carbon dioxide enters the red blood cell, bicarbonate forms and moves into plasma, and chloride enters the cell
ⓓ. Oxygen enters the red blood cell, carbon dioxide is trapped in plasma, and chloride moves into alveoli
Correct Answer: Carbon dioxide enters the red blood cell, bicarbonate forms and moves into plasma, and chloride enters the cell
Explanation: At the tissues, carbon dioxide produced by cells diffuses into the blood and then into red blood cells. Inside the red blood cells, much of it is converted into bicarbonate ions. These bicarbonate ions then move out into the plasma so they can be transported through the circulation. As this happens, chloride ions enter the red blood cells to maintain ionic balance. This sequence combines the key ideas of bicarbonate formation and chloride shift. It is one of the most important transport patterns on the tissue side of circulation.
212. What usually happens in the lungs during the reverse chloride shift?
ⓐ. Chloride enters red blood cells while bicarbonate leaves them for tissue delivery
ⓑ. Carbon dioxide becomes permanently fixed in plasma and cannot be exhaled
ⓒ. Oxygen leaves alveoli and is converted directly into bicarbonate ions
ⓓ. Bicarbonate re-enters red blood cells, chloride moves out, and carbon dioxide is regenerated for exhalation
Correct Answer: Bicarbonate re-enters red blood cells, chloride moves out, and carbon dioxide is regenerated for exhalation
Explanation: In the lungs, the process linked with chloride shift works in the reverse direction compared with tissue capillaries. Bicarbonate ions move back into red blood cells, while chloride ions move out. The bicarbonate is then converted back into carbon dioxide, which diffuses into the alveoli and is exhaled. This reverse movement is essential because transported carbon dioxide must ultimately be released from the body. It shows that chloride shift is not a one-way event but a reversible part of respiratory transport. The lung side therefore completes the transport cycle that began in the tissues.
213. A tissue begins producing unusually large amounts of $CO_2$ during rapid metabolism. Which outcome is most likely in the blood leaving that tissue?
ⓐ. Most of the added $CO_2$ will be transported mainly as bicarbonate ions after conversion in red blood cells
ⓑ. Most of the added $CO_2$ will remain as free gas bubbles inside plasma
ⓒ. Most of the added $CO_2$ will be carried only by platelets because haemoglobin cannot interact with it
ⓓ. Most of the added $CO_2$ will stay permanently dissolved without any chemical conversion
Correct Answer: Most of the added $CO_2$ will be transported mainly as bicarbonate ions after conversion in red blood cells
Explanation: When tissues produce more $CO_2$, that gas diffuses into the blood and then into red blood cells. Although some of it remains dissolved and some combines with haemoglobin, the largest share is converted into bicarbonate ions. This makes bicarbonate the principal transport form of carbon dioxide in circulating blood. The process greatly increases the blood’s ability to carry large quantities of $CO_2$ from tissues to lungs. It also shows that carbon dioxide transport is not limited to simple dissolution. In a metabolically active tissue, this bicarbonate pathway becomes especially important for efficient transport.
214. Which statement correctly compares oxyhaemoglobin with carbaminohaemoglobin?
ⓐ. Oxyhaemoglobin and carbaminohaemoglobin are both formed when oxygen binds to haemoglobin
ⓑ. Oxyhaemoglobin carries carbon dioxide, whereas carbaminohaemoglobin carries bicarbonate ions
ⓒ. Oxyhaemoglobin is formed by oxygen binding to haemoglobin, whereas carbaminohaemoglobin is formed by carbon dioxide binding to haemoglobin
ⓓ. Oxyhaemoglobin and carbaminohaemoglobin are identical names for the same respiratory compound
Correct Answer: Oxyhaemoglobin is formed by oxygen binding to haemoglobin, whereas carbaminohaemoglobin is formed by carbon dioxide binding to haemoglobin
Explanation: Haemoglobin can participate in transport of both major respiratory gases, but it forms different compounds with them. When oxygen binds with haemoglobin, the product is oxyhaemoglobin, which is central to oxygen transport. When carbon dioxide binds with haemoglobin, the product is carbaminohaemoglobin, which contributes to carbon dioxide transport. The two compounds therefore differ in the gas attached to haemoglobin and in their transport role. This distinction is important because students often confuse the names due to their similar structure. Understanding the difference helps separate oxygen transport from one of the secondary forms of carbon dioxide transport.
215. Assertion: Chloride shift in tissue capillaries supports continued transport of carbon dioxide in bicarbonate form. Reason: As bicarbonate ions move out of red blood cells, chloride ions move in to maintain electrical balance.
ⓐ. Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion
ⓑ. Both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion
ⓒ. Assertion is true, but the Reason is false
ⓓ. Assertion is false, but the Reason is true
Correct Answer: Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion
Explanation: In tissue capillaries, much of the incoming carbon dioxide is converted into bicarbonate inside red blood cells. For large amounts of bicarbonate to leave those cells and enter plasma, electrical neutrality must be preserved. Chloride ions therefore enter the red blood cells as bicarbonate ions leave them. This ion exchange is the chloride shift, and it directly supports efficient bicarbonate transport of carbon dioxide. The assertion is true because bicarbonate movement is a major transport pathway. The reason is also true and correctly explains why chloride shift is necessary for that pathway to continue smoothly.
216. If bicarbonate ions formed inside red blood cells could not move out into plasma in tissue capillaries, what would be the most direct consequence?
ⓐ. Carbon dioxide transport would become more efficient because all bicarbonate would stay concentrated inside red blood cells
ⓑ. The major transport pathway of carbon dioxide would be hindered because bicarbonate carriage in plasma would be reduced
ⓒ. Oxygen binding to haemoglobin would stop immediately in every body tissue
ⓓ. Chloride ions would move into the alveoli instead of into blood cells
Correct Answer: The major transport pathway of carbon dioxide would be hindered because bicarbonate carriage in plasma would be reduced
Explanation: The largest fraction of carbon dioxide is transported in blood as bicarbonate ions in plasma. If bicarbonate formed inside red blood cells could not move out, this main transport route would be disrupted. Carbon dioxide could still be carried to some extent in dissolved form and as carbaminohaemoglobin, but the dominant pathway would be limited. This would reduce the overall efficiency of carbon dioxide transport from tissues to lungs. That is why exchange mechanisms linked with chloride shift are so important.
217. A student says, “Chloride shift means chloride always moves into red blood cells in every part of circulation.” Which correction is most accurate?
ⓐ. The statement is correct because bicarbonate never enters red blood cells after it is formed
ⓑ. The statement is incorrect because in the lungs the direction reverses: bicarbonate enters red blood cells and chloride moves out
ⓒ. The statement is incorrect because chloride shift occurs only in plasma and never involves red blood cells
ⓓ. The statement is correct because chloride is the main form in which carbon dioxide is transported
Correct Answer: The statement is incorrect because in the lungs the direction reverses: bicarbonate enters red blood cells and chloride moves out
Explanation: In tissue capillaries, chloride enters red blood cells as bicarbonate leaves, and this is the usual chloride shift described during carbon dioxide loading. However, the process does not stay in the same direction throughout circulation. In the lungs, the reverse occurs: bicarbonate re-enters red blood cells, chloride moves out, and carbon dioxide is regenerated for exhalation. This reversal is necessary to complete the transport cycle. So chloride shift is not a fixed one-way movement in all parts of the body. The student’s statement is therefore a misconception that ignores the reverse process in pulmonary capillaries.
218. Which event best indicates that blood has reached the lungs and is preparing to release transported carbon dioxide?
ⓐ. Bicarbonate begins moving from plasma into red blood cells, helping regenerate $CO_2$ for exhalation
ⓑ. Chloride enters red blood cells continuously while more bicarbonate leaves them for tissue delivery
ⓒ. Carbon dioxide is converted permanently into bicarbonate so that none can be exhaled
ⓓ. Haemoglobin starts transporting oxygen only after all chloride ions have disappeared from blood
Correct Answer: Bicarbonate begins moving from plasma into red blood cells, helping regenerate $CO_2$ for exhalation
Explanation: The lung side of carbon dioxide transport requires reversal of the tissue-side changes. Bicarbonate ions that were carried in plasma move back into red blood cells. There they participate in reactions that regenerate carbon dioxide, which can then diffuse into alveoli and be exhaled. At the same time, chloride movement also reverses compared with tissue capillaries. This pattern shows that transport is a cycle, not a one-step event. The return of bicarbonate into red blood cells is therefore a strong sign that blood has reached the pulmonary exchange region and is preparing to unload carbon dioxide.
219. Which option lists the three main forms of carbon dioxide transport in the most appropriate order from greatest to least contribution under normal conditions?
ⓐ. Dissolved in plasma, carbaminohaemoglobin, bicarbonate ions
ⓑ. Bicarbonate ions, carbaminohaemoglobin, dissolved carbon dioxide
ⓒ. Carbaminohaemoglobin, dissolved carbon dioxide, bicarbonate ions
ⓓ. Bicarbonate ions, dissolved carbon dioxide, carbaminohaemoglobin
Correct Answer: Bicarbonate ions, carbaminohaemoglobin, dissolved carbon dioxide
Explanation: Carbon dioxide is transported in blood mainly as bicarbonate ions, which contribute the largest share. A smaller portion is carried in combination with haemoglobin as carbaminohaemoglobin. The smallest of the three principal fractions is the portion simply dissolved in plasma. This order is important because it shows that carbon dioxide transport relies most heavily on chemical conversion rather than simple solution. It also highlights that haemoglobin contributes to carbon dioxide transport, though not as much as the bicarbonate pathway. Remembering the order from greatest to least helps organize the topic clearly and avoids confusion among the three transport forms.
220. Which part of the brain is primarily responsible for generating the basic respiratory rhythm in humans?
ⓐ. Cerebellum
ⓑ. Hypothalamus
ⓒ. Respiratory rhythm centre in the medulla
ⓓ. Visual cortex
Correct Answer: Respiratory rhythm centre in the medulla
Explanation: The basic rhythm of breathing is generated by the respiratory rhythm centre located in the medulla oblongata. This centre produces the regular pattern of inspiration and expiration that continues automatically without conscious effort. It is part of the brainstem and is essential for maintaining normal ventilation. Other regions may influence breathing, but they do not create the primary rhythmic pattern in the same way. This is why normal breathing continues even when we are not thinking about it. The medullary rhythm centre is therefore the core controller of respiratory rhythm.
