CO2 and O2 transport questions: respiratory physiology
1
The oxygen-haemoglobin dissociation curve is shifted to the right in which of the following scenarios?
2
A 44-year-old man recieves a large volume transfusion of whole blood. The whole blood is two weeks old. Which one of the following best describes its handling of oxygen?
3
A 55-year-old man with COPD presents with increasing breathlessness. His arterial blood gas shows PaO₂ of 55 mmHg and SaO₂ of 89%. What is the best explanation for his relatively preserved oxygen saturation at this PaO₂?
4
A 24-year-old woman has severe sepsis and lactic acidosis. Her blood pH is 7.1. What is the most likely effect on her O₂-Hb dissociation curve, and why is this advantageous?
5
A 28-year-old man is brought to the emergency department after being rescued at sea after a sailing accident. He is currently unresponsive.
His heart rate is 110 BPM, his BP is 110/76mmHg, his oxygen saturation is 93%, and his temperature is 34.8 ºC. An ECG is unremarkable and venous blood is suggestive of type 2 respiratory failure. His oxygen dissociation curve shows a leftward shift.
What has caused this shift in the patient's oxygen dissociation curve?
6
A 45-year-old man presents to the emergency department with worsening breathlessness over the past two months. He has a known diagnosis of COPD. He reports smokes 10 cigarettes daily and has 10 units of alcohol per week.
His obs show: temperature 36.6ºC, heart rate 88bpm, blood pressure 128/78 mmHg, oxygen saturation at 96%, and respiratory rate of 18/min.
What aspect of this man's history will cause a rightward shift in his oxygen dissociation curve?
7
A 72-year-old man is admitted to the respiratory ward with an exacerbation of COPD. He has had worsening breathlessness and a productive cough for the last week. He is day three following initiation of his rescue medication of amoxicillin and prednisolone. His previous discharge summary notes that this patient is known to retain carbon dioxide. He is currently being treated with controlled oxygen through a 28% venturi mask. What should his target oxygen saturations be?
8
An 18-year-old man is brought to the emergency department with a headache, confusion, vertigo, and weakness. Paramedics report he was found asleep in his garage with the car's engine running. On examination, he has pink skin and mucus membranes, and pulse oximetry shows a saturation of 97%. Carbon monoxide toxicity is suspected and 100% oxygen by the mask is commenced.
What is the most likely explanation for this presentation?
9
Myoglobin (Mb), found in skeletal and cardiac muscle, has a distinct O2 dissociation curve compared to haemoglobin (Hb). Which statement correctly describes the key difference and its physiological consequence?
10
A 28-year-old pregnant woman is concerned about how her developing foetus receives enough oxygen during gestation. You explain that foetal haemoglobin plays a key role in oxygen transfer from the maternal to foetal circulation. Which of the following statements best explains the role of foetal haemoglobin in enhancing oxygen uptake in the foetus?
Feedback
The curve is shifted to the right when there is an increased oxygen requirement by the tissue. This includes:
Increased temperature
Acidosis
Increased DPG:
DPG is found in erythrocytes and is increased during glycolysis. It binds to the Hb molecule, thereby releasing oxygen to tissues. DPG is increased in conditions associated with poor oxygen delivery to tissues, such as anaemia and high altitude.
Stored blood will have an increased affinity for oxygen. This occurs due to the depletion of 2,3-diphosphoglycerate (2,3-DPG) during storage. 2,3-DPG is an important allosteric modulator of haemoglobin that promotes the release of oxygen to tissues by binding to deoxyhaemoglobin. During storage, red blood cells metabolise 2,3-DPG, and levels are significantly decreased after approximately one week of storage. This results in a left shift of the oxygen-haemoglobin dissociation curve, meaning the haemoglobin has a higher affinity for oxygen and releases it less readily to tissues.
Cooperative binding of haemoglobin gives the oxygen-haemoglobin dissociation curve its sigmoidal shape, so even at lower PaO₂ (like 55 mmHg), a relatively high saturation (~89%) can be maintained until the steep portion is reached.
Acidosis (low pH) causes a right shift (Bohr effect), facilitating O₂ release to metabolically active tissues, an adaptive mechanism.
The correct answer is hypothermia because lower temperatures cause a leftward shift in the oxygen dissociation curve, increasing haemoglobin's affinity for oxygen. This effect promotes the binding of oxygen in the lungs and is the opposite of the rightward shift induced by heat generation in tissues during aerobic respiration, which facilitates oxygen offloading.
The correct answer is paCO2 level, indicating hypercapnia. The oxygen dissociation curve plots haemoglobin saturation against the partial pressure of oxygen in the blood. Factors that decrease haemoglobin's affinity for oxygen, shifting the curve to the right and increasing tissue oxygen delivery, include hypercapnia.
In this case, the patient's COPD and smoking history have likely caused an elevated paCO2 level. This increase raises the hydrogen ion concentration, causing acidosis, which in turn reduces haemoglobin's oxygen affinity and prompts a rightward shift of the curve.
CO reduces oxygen carrying capacity is the correct answer. Carbon monoxide has a high affinity for haemoglobin and it can also replace oxygen already bound to haemoglobin. When one molecule of CO binds to haemoglobin, the remaining binding spots show an increased affinity for oxygen, which causes a left shift in the oxygen dissociation curve.
Myoglobin has a higher affinity for O2 than haemoglobin. It lacks cooperative binding, resulting in a hyperbolic O2 dissociation curve. This high affinity allows it to bind O2 even at low PaO2, enabling it to function as an O2 storage mechanism in muscles at rest and for O2 delivery during intense activity or hypoxia.
Foetal haemoglobin (HbF) has a higher affinity for oxygen compared to adult haemoglobin (HbA). This increased affinity is due to HbF's decreased ability to bind 2,3-diphosphoglycerate (2,3-DPG). As a result, the oxygen dissociation curve for HbF is shifted to the left of that for HbA. This leftward shift means that at any given partial pressure of oxygen, HbF is more saturated with oxygen than HbA. This property allows the foetus to effectively extract oxygen from the maternal blood across the placenta, ensuring adequate oxygen supply for development.
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