Question 1 i) With regard to CO 2
​
transport we talk about "The chloride shift". Explain this term by clearly describing CO 2
​
transport in the form of bicarbonate, including the importance of carbonic anhydrase. Your answer must also include the part of the respiratory/circulatory system where this occurs and include which state hemoglobin is in when this process occurs (8 marks). ii) In addition to bicarbonate, how else is CO 2
​
carried in the blood and what proportions are carried in each form? (2 marks) Question 2 i) When a person exercises, ventilation increases. After exercise, ventilation does not return to basal levels until the O 2
​
debt has been repaid. Explain what " O 2
​
debt" is, including how it comes about and how long it takes to repay, and what the stimulus for the continued high ventilation is. ii) With exercise, expiration becomes active. Explain how this forced expiration allows for more CO 2
​
to be expelled from the lungs?

i) The chloride shift is a term used to describe the movement of chloride ions (Cl-) in and out of red blood cells during the transport of carbon dioxide (CO2) in the form of bicarbonate (HCO3-). This process occurs in the systemic capillaries.

When CO2 is produced as a waste product of cellular respiration, it diffuses into the red blood cells. Inside the red blood cells, the enzyme carbonic anhydrase catalyzes the reaction between CO2 and water (H2O), forming carbonic acid (H2CO3). Carbonic acid then dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+).

The chloride shift occurs to maintain the electrochemical balance within the red blood cells. As bicarbonate ions are formed, they move out of the red blood cells in exchange for chloride ions from the plasma. This exchange of ions helps to prevent the accumulation of negative charges inside the red blood cells, maintaining electrical neutrality.

During this process, hemoglobin in the red blood cells is in the deoxygenated state, meaning it has released oxygen molecules and is ready to bind with CO2 and H+.

ii) Apart from being carried in the form of bicarbonate, CO2 is also carried in the blood in two other forms:

Dissolved CO2: A small portion of CO2 dissolves directly in the plasma as a dissolved gas.

Carbaminohemoglobin: Some CO2 binds directly to the amino acids of hemoglobin molecules to form carbaminohemoglobin. This form accounts for a minor proportion of CO2 transport in the blood.

Approximately 70% of CO2 is transported in the form of bicarbonate ions, while dissolved CO2 and carbaminohemoglobin account for about 7% and 23%, respectively.

2) The term "O2 debt" refers to the oxygen that the body needs to replenish following intense exercise. During exercise, the demand for oxygen increases to support the increased energy production. However, the oxygen supply may not be sufficient to meet the elevated demand, resulting in an oxygen debt.

The oxygen debt occurs due to several factors:

During intense exercise, the muscles rely on anaerobic metabolism, which produces lactic acid as a byproduct. The accumulation of lactic acid leads to a decreased pH, causing fatigue. Repaying the oxygen debt helps restore normal pH levels by converting lactic acid back into glucose through a process called the Cori cycle.

Oxygen is also needed to restore depleted ATP (adenosine triphosphate) stores and replenish phosphocreatine levels, which are essential for muscle contraction.

Oxygen is required for the recovery of various physiological systems, including elevated heart and breathing rates, and the restoration of normal body temperature.

The repayment of the oxygen debt depends on the individual and the intensity of exercise. It can take several minutes to several hours for the oxygen debt to be fully repaid, depending on factors such as fitness level, recovery time, and the extent of anaerobic metabolism during exercise. During this repayment period, ventilation remains elevated to supply the increased oxygen demand.

ii) During forced expiration with exercise, the active contraction of expiratory muscles, such as the internal intercostals and abdominal muscles, helps to increase the pressure within the thoracic cavity. This increased pressure facilitates the forceful expulsion of air from the lungs.

The increased expiration pressure aids in the rapid elimination of CO2 from the lungs. As the pressure in the thoracic cavity rises, it compresses the airways, narrowing them and increasing resistance to airflow. This increased resistance helps to slow down the rate of airflow during expiration, allowing more time for gas exchange to occur. Consequently, more CO2 can be expelled from the lungs, aiding in the removal of metabolic waste products generated during exercise.

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During CO₂ transport as bicarbonate, "the chloride shift" involves the movement of chloride ions in and out of red blood cells to maintain electrical neutrality. Carbonic anhydrase facilitates the conversion of CO₂ to bicarbonate in peripheral tissues, with hemoglobin in the deoxygenated state (T-state). In addition to bicarbonate, CO₂ is carried in the blood as dissolved CO₂ (5-10%) and bound to hemoglobin as carbaminohemoglobin (20-30%). During exercise, the temporary oxygen deficit known as "O₂ debt" is repaid through increased ventilation to replenish ATP, convert lactic acid to glucose, and restore oxygen levels. Forced expiration during exercise expels more CO₂ from the lungs by increasing thoracic pressure through muscle contraction.

i) "The chloride shift" refers to the movement of chloride ions (Cl-) in and out of red blood cells (RBCs) to maintain electrical neutrality during the transport of carbon dioxide (CO₂) in the form of bicarbonate (HCO₃⁻) ions. CO₂ is converted to HCO₃⁻ by an enzyme called carbonic anhydrase, which catalyzes the reversible reaction between CO₂ and water. In the tissues, CO₂ diffuses into RBCs and combines with water to form carbonic acid (H2CO₃), which quickly dissociates into bicarbonate ions and hydrogen ions. To maintain electrical balance, chloride ions move into RBCs to replace the bicarbonate ions leaving the cell. This occurs in the peripheral tissues where CO₂ is produced. Hemoglobin in the RBCs is in the deoxygenated state (T-state) during this process.

ii) Apart from being carried as bicarbonate ions, CO₂ is also transported in the blood by physically dissolving in plasma and by binding to hemoglobin. Approximately 5-10% of CO₂ is carried in the dissolved form, while around 20-30% of CO₂ binds directly to hemoglobin, forming carbaminohemoglobin. The majority, about 60-70% of CO₂, is transported as bicarbonate ions.

Question 2:

i) "O₂ debt" refers to the additional oxygen consumption that occurs after exercise to repay the oxygen deficit accumulated during strenuous activity. During exercise, the demand for oxygen exceeds the supply, leading to a temporary oxygen deficit. After exercise, ventilation remains elevated to repay this debt. The repayment of the oxygen debt involves replenishing depleted ATP stores, converting lactic acid back to glucose, and restoring oxygen levels in the blood and tissues. The duration to repay the oxygen debt varies depending on the intensity and duration of exercise.

ii) During forced expiration in exercise, the contraction of the abdominal and internal intercostal muscles increases the pressure in the thoracic cavity, aiding in the expulsion of more CO₂ from the lungs. This active expiration assists in forcefully pushing air out of the respiratory system, allowing for more efficient removal of CO₂, which is produced as a byproduct of metabolism during exercise.

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