A 23-year-old woman is brought to the emergency department by her friends because she thinks that she is having an allergic reaction. The patient is visibly distressed and insists on immediate attention as she feels like she is going to die. Her friends say they were discussing future plans while dining at a new seafood restaurant when her symptoms started. The patient has no history of allergies. She takes no medications and has no significant medical history. Her vitals include: pulse 98/min, respiratory rate 30/min, and blood pressure 120/80 mm Hg. On physical examination, she is tachypneic and in distress. Cardiopulmonary examination is unremarkable. No rash is seen on the body and examination of the lips and tongue reveals no findings. Which of the following would most likely present in this patient?
Q12
A 74-year-old woman presents with severe and progressively worsening shortness of breath. She says that her breathing has been difficult for many years but now it is troubling her a lot. She reports a 50-pack-year smoking history and drinks at least 2 alcoholic beverages daily. On physical examination, the patient is leaning forward in her seat and breathing with pursed lips. Which of the following mechanisms best explains the benefit of oxygen supplementation in this patient?
Q13
A 14-year-old boy is brought to the emergency department by his mom after she found him complaining of headaches, nausea, lightheadedness, and muscle pain. He has had type I diabetes for 3 years with very well managed blood sugars, and he is otherwise healthy. He recently returned from a boy scout skiing trip where he drank from a mountain stream, ate unusual foods, and lived in a lodge with a wood-fired fireplace and cooking stove. On physical exam he has a diffuse redness of his skin. Which of the following changes to this patient's pulmonary system would cause oxygen to exhibit similar tissue hypoxia effects as the most likely cause of this patient's symptoms?
Q14
A 60-year-old man presents with breathlessness for the past 3 months. His symptoms have been getting progressively worse during this time. He denies any history of cough, fever, or chest pain. He works at a local shipyard and is responsible for installing the plumbing aboard the vessels. His past medical history is significant for hypertension for which he takes metoprolol every day. He denies smoking and any illicit drug use. His pulse is 74/min, respiratory rate is 14/min, blood pressure is 130/76 mm Hg, and temperature is 36.8°C (98.2°F). Physical examination is significant for fine bibasilar crackles at the end of inspiration without digital clubbing. Which of the following additional findings would most likely be present in this patient?
Q15
A 25-year-old man is brought by his wife to the emergency department due to sudden onset confusion that started 40 minutes ago. The patient’s wife says that he came home from work complaining of pain in his arms and legs. While resting on the couch, he mentioned feeling nauseous and then became quite confused. He has no previous medical history and takes no medications. He does not smoke and only drinks alcohol occasionally. His vital signs include pulse 80/min, respiratory rate 12/min, blood pressure 120/84 mm Hg, and SaO2 99% on room air. On physical examination, the patient is oriented x 0 and unable to answer questions or follow commands. Generalized pallor is present. There are also multiple scratches on the face and neck due to constant itching. Assuming this patient’s symptoms are due to his employment, he most likely works as which of the following?
Q16
A 27-year-old man presents to the emergency department following a motor vehicle accident. Having been found as a restrained driver, he did not suffer from any chest injuries; nevertheless, his legs were pinned in position by the front of the highly damaged vehicle. After a prolonged extrication, the man sustained multiple fractures on his left femur and tibia. That same night, he underwent surgery to address his left leg fractures. In the next morning, the man suddenly developed severe dyspnea. Upon examination, he is noted to have a diffuse petechial rash. His vital signs are the following: blood pressure is 111/67 mm Hg, pulse rate is 107/min, respiratory rate is 27/min, oxygen saturation level is 82%, and his body temperature is normal. What is the most likely mechanism of his respiratory distress?
Q17
An investigator is studying the affinity of hemoglobin for oxygen in different clinical settings. An illustration of an oxygen-hemoglobin dissociation curve is shown. Curve A shows the test results of one of the research participants and curve B shows a normal oxygen-hemoglobin dissociation curve. Which of the following is most likely present in this research participant?
Q18
A 32-year-old female with Crohn's disease diagnosed in her early 20s comes to your office for a follow-up appointment. She is complaining of headaches and fatigue. Which of the following arterial blood gas findings might you expect?
Q19
A research scientist attempts to understand the influence of carbon dioxide content in blood on its oxygen binding. The scientist adds carbon dioxide to dog blood and measures the uptake of oxygen in the blood versus oxygen pressure in the peripheral tissue. He notes in one dog that with the addition of carbon dioxide with a pressure of 90 mmHg, the oxygen pressure in the peripheral tissue rose from 26 to 33 mmHg. How can this phenomenon be explained?
Gas exchange US Medical PG Practice Questions and MCQs
Question 11: A 23-year-old woman is brought to the emergency department by her friends because she thinks that she is having an allergic reaction. The patient is visibly distressed and insists on immediate attention as she feels like she is going to die. Her friends say they were discussing future plans while dining at a new seafood restaurant when her symptoms started. The patient has no history of allergies. She takes no medications and has no significant medical history. Her vitals include: pulse 98/min, respiratory rate 30/min, and blood pressure 120/80 mm Hg. On physical examination, she is tachypneic and in distress. Cardiopulmonary examination is unremarkable. No rash is seen on the body and examination of the lips and tongue reveals no findings. Which of the following would most likely present in this patient?
A. Increased alveolar pCO2 and decreased alveolar pO2
B. Decreased alveolar pCO2 and unchanged alveolar pO2
C. Decreased alveolar pCO2 and increased alveolar pO2 (Correct Answer)
D. Increased alveolar pCO2 and increased alveolar pO2
E. Decreased alveolar pCO2 and decreased alveolar pO2
Explanation: ***Decreased alveolar pCO2 and increased alveolar pO2***
- The patient's **tachypnea** (respiratory rate 30/min) due to anxiety or a possible **panic attack** leads to hyperventilation.
- **Hyperventilation** causes increased expulsion of CO2 from the lungs, leading to decreased alveolar pCO2, and simultaneously increases the oxygen concentration in the alveoli, resulting in increased alveolar pO2.
*Increased alveolar pCO2 and decreased alveolar pO2*
- This pattern would be seen in **hypoventilation**, where there's insufficient air exchange, leading to CO2 retention and reduced oxygen uptake.
- The patient is actively tachypneic, indicating increased minute ventilation, which is the opposite of hypoventilation.
*Decreased alveolar pCO2 and unchanged alveolar pO2*
- While alveolar pCO2 would decrease with hyperventilation, it is unlikely for alveolar pO2 to remain completely unchanged, as increased ventilation would also lead to a higher oxygen concentration in the alveoli.
- This scenario suggests a significant mismatch in CO2 and O2 exchange that doesn't align with simple hyperventilation.
*Increased alveolar pCO2 and increased alveolar pO2*
- It is physiologically unlikely to have both increased alveolar pCO2 and increased alveolar pO2 simultaneously under normal or panic conditions.
- Increased pCO2 typically indicates hypoventilation, which would lead to decreased, not increased, pO2.
*Decreased alveolar pCO2 and decreased alveolar pO2*
- While hyperventilation would decrease alveolar pCO2, it would typically lead to an **increase** in alveolar pO2 due to improved gas exchange and washout of residual gases when fresh air in taken in.
- A decrease in both would suggest a complex ventilation-perfusion mismatch or severe diffusion impairment, which is inconsistent with the presented symptoms of anxiety-induced tachypnea.
Question 12: A 74-year-old woman presents with severe and progressively worsening shortness of breath. She says that her breathing has been difficult for many years but now it is troubling her a lot. She reports a 50-pack-year smoking history and drinks at least 2 alcoholic beverages daily. On physical examination, the patient is leaning forward in her seat and breathing with pursed lips. Which of the following mechanisms best explains the benefit of oxygen supplementation in this patient?
A. Aids in smoking cessation
B. Increased oxygen diffusion into capillary (Correct Answer)
C. Decreases respiratory rate and work of breathing
D. Free radical formation killing pathogens
E. Better binding of oxygen to hemoglobin
Explanation: ***Increased oxygen diffusion into capillary***
- Oxygen supplementation increases the **partial pressure of oxygen (PO2)** in the alveoli, creating a steeper gradient for **oxygen diffusion** across the alveolar-capillary membrane.
- In patients with emphysema, destruction of alveolar walls reduces the surface area for gas exchange, making this increased gradient crucial for maintaining adequate **oxygen saturation**.
*Aids in smoking cessation*
- Oxygen therapy does not directly aid in **smoking cessation**; while quitting smoking is vital for managing COPD, oxygen's primary role is to treat the hypoxemia.
- **Smoking cessation** is achieved through behavioral interventions, counseling, and pharmacotherapy, not supplemental oxygen.
*Decreases respiratory rate and work of breathing*
- While improved oxygenation can reduce the drive for hyperventilation, its primary benefit is not directly reducing **respiratory rate** or **work of breathing** by altering the respiratory pattern.
- The sensation of dyspnea and work of breathing may decrease indirectly due to better tissue oxygenation rather than a direct mechanism on respiratory control.
*Free radical formation killing pathogens*
- Oxygen supplementation does not involve a mechanism of **free radical formation** to kill pathogens; high concentrations of oxygen can be toxic but are not used therapeutically for antimicrobial effects.
- **Free radicals** are harmful byproducts of metabolism and oxidative stress, not a therapeutic mechanism for pathogen killing.
*Better binding of oxygen to hemoglobin*
- Oxygen supplementation impacts the **amount of oxygen dissolved in plasma** and the **partial pressure gradient**, but it does not directly alter the affinity of hemoglobin for oxygen.
- Hemoglobin's affinity for oxygen is primarily influenced by factors like **pH**, **temperature**, **2,3-BPG**, and **PCO2** (Bohr effect), not the concentration of supplemental oxygen.
Question 13: A 14-year-old boy is brought to the emergency department by his mom after she found him complaining of headaches, nausea, lightheadedness, and muscle pain. He has had type I diabetes for 3 years with very well managed blood sugars, and he is otherwise healthy. He recently returned from a boy scout skiing trip where he drank from a mountain stream, ate unusual foods, and lived in a lodge with a wood-fired fireplace and cooking stove. On physical exam he has a diffuse redness of his skin. Which of the following changes to this patient's pulmonary system would cause oxygen to exhibit similar tissue hypoxia effects as the most likely cause of this patient's symptoms?
A. Increasing capillary length
B. Nitrous oxide administration
C. Increasing capillary transit time
D. Interstitial thinning
E. Interstitial fibrosis (Correct Answer)
Explanation: ***Interstitial fibrosis***
- Carbon monoxide poisoning causes tissue hypoxia by reducing **oxygen-carrying capacity** (CO binds hemoglobin with high affinity, maintaining normal PaO2 but severely reducing oxygen content and delivery to tissues).
- Among the pulmonary changes listed, interstitial fibrosis most closely produces **tissue hypoxia** by impairing oxygen transfer across the thickened alveolar-capillary membrane, resulting in **hypoxemia and reduced tissue oxygen delivery**.
- While the mechanisms differ (CO affects carrying capacity vs. fibrosis affects diffusion), both ultimately result in inadequate oxygen delivery to meet tissue metabolic demands, manifesting as tissue hypoxia.
- Interstitial fibrosis creates a **diffusion barrier** that worsens with increased oxygen demand (exercise), similar to how CO poisoning impairs the ability to meet tissue oxygen requirements.
*Increasing capillary length*
- Increasing capillary length would **improve gas exchange** by providing more surface area and time for oxygen diffusion across the alveolar-capillary membrane.
- This adaptation enhances oxygen delivery to tissues, which is the opposite of the tissue hypoxia seen in CO poisoning.
*Nitrous oxide administration*
- Nitrous oxide is an anesthetic gas that acts primarily on the **central nervous system** and does not significantly impair oxygen transport or binding to hemoglobin.
- While it can displace oxygen at very high concentrations, its mechanism does not mimic the impaired oxygen delivery characteristic of CO poisoning.
*Increasing capillary transit time*
- Increased capillary transit time allows **more time for oxygen equilibration** between alveolar gas and capillary blood, thereby improving oxygenation.
- This would enhance tissue oxygen delivery rather than cause tissue hypoxia, opposite to the effect of CO poisoning.
*Interstitial thinning*
- Interstitial thinning **decreases the diffusion distance** for oxygen, facilitating more efficient gas exchange across the alveolar-capillary membrane.
- This would improve oxygen delivery to tissues and is the opposite of what occurs in CO poisoning.
Question 14: A 60-year-old man presents with breathlessness for the past 3 months. His symptoms have been getting progressively worse during this time. He denies any history of cough, fever, or chest pain. He works at a local shipyard and is responsible for installing the plumbing aboard the vessels. His past medical history is significant for hypertension for which he takes metoprolol every day. He denies smoking and any illicit drug use. His pulse is 74/min, respiratory rate is 14/min, blood pressure is 130/76 mm Hg, and temperature is 36.8°C (98.2°F). Physical examination is significant for fine bibasilar crackles at the end of inspiration without digital clubbing. Which of the following additional findings would most likely be present in this patient?
A. Increased pulmonary capillary wedge pressure
B. Increased residual lung volume
C. Reduced FEV1/FVC ratio
D. Decreased diffusing capacity of CO (Correct Answer)
E. Decreased pulmonary arterial pressure
Explanation: ***Decreased diffusing capacity of CO***
- This patient's occupation at a **shipyard**, progressive dyspnea, and bibasilar crackles without clubbing, along with normal vital signs, are highly suggestive of **asbestosis**, a type of **interstitial lung disease (ILD)**.
- ILDs cause **fibrosis of the alveolar-capillary membrane**, leading to impaired gas exchange and a characteristic **reduction in DLCO (diffusing capacity of the lung for carbon monoxide)**. This is a hallmark of parenchymal lung disease.
*Increased pulmonary capillary wedge pressure*
- An elevated **pulmonary capillary wedge pressure (PCWP)** indicates **left-sided heart failure** or **pulmonary venous hypertension**.
- While dyspnea can be a symptom of heart failure, the patient's normal blood pressure and absence of cardiac-specific symptoms or signs point away from primary cardiac pathology.
*Increased residual lung volume*
- **Increased residual lung volume** is a characteristic finding in **obstructive lung diseases** such as **COPD** and **asthma**, where there is air trapping due to airflow limitation.
- The patient's presentation with progressive dyspnea and bibasilar crackles is more consistent with a **restrictive lung disorder** like asbestosis, which typically causes **decreased lung volumes**.
*Reduced FEV1/FVC ratio*
- A **reduced FEV1/FVC ratio** is the hallmark of **obstructive lung diseases**, indicating airflow limitation.
- In **restrictive lung diseases** like asbestosis, both FEV1 and FVC are typically reduced proportionally, often resulting in a **normal or even increased FEV1/FVC ratio**.
*Decreased pulmonary arterial pressure*
- **Pulmonary arterial pressure (PAP)** is typically **normal or increased** in patients with interstitial lung disease due to **hypoxic vasoconstriction** and vascular remodeling.
- A decreased PAP would be an unusual and atypical finding in such a patient and is not associated with this clinical picture.
Question 15: A 25-year-old man is brought by his wife to the emergency department due to sudden onset confusion that started 40 minutes ago. The patient’s wife says that he came home from work complaining of pain in his arms and legs. While resting on the couch, he mentioned feeling nauseous and then became quite confused. He has no previous medical history and takes no medications. He does not smoke and only drinks alcohol occasionally. His vital signs include pulse 80/min, respiratory rate 12/min, blood pressure 120/84 mm Hg, and SaO2 99% on room air. On physical examination, the patient is oriented x 0 and unable to answer questions or follow commands. Generalized pallor is present. There are also multiple scratches on the face and neck due to constant itching. Assuming this patient’s symptoms are due to his employment, he most likely works as which of the following?
A. Farmer
B. Bird keeper
C. Shipyard worker
D. Fireman
E. Diving instructor (Correct Answer)
Explanation: ***Diving instructor***
- This presentation is classic for **decompression sickness (DCS)**, often seen in divers who surface too quickly, leading to **nitrogen bubble formation** in tissues.
- The symptoms of **confusion, musculoskeletal pain (arms and legs), nausea**, and possibly **pruritus (itching/scratches)** are all consistent with neurological and cutaneous manifestations of DCS.
*Farmer*
- Farmers may be exposed to **pesticides**, which can cause neurological symptoms, but the **sudden onset, musculoskeletal pain, and itching** pattern is less typical for acute pesticide poisoning.
- Pesticide exposure often presents with cholinergic symptoms like **salivation, lacrimation, urination, and defecation (SLUDGE syndrome)**, which are absent here.
*Bird keeper*
- Bird keepers are at risk for **psittacosis (ornithosis)**, a bacterial infection that can cause pneumonia-like symptoms, fever, and headache.
- While it can cause confusion, the **sudden onset, severe pain in limbs, and prominent itching** are not characteristic features of psittacosis.
*Shipyard worker*
- Shipyard workers can be exposed to **asbestos**, causing chronic lung diseases like **asbestosis** or **mesothelioma**, but not acute confused states.
- Acute exposures might involve **solvents or heavy metals**, which could cause neurological symptoms, but the specific cluster of symptoms (pain, nausea, itching) is not typical.
*Fireman*
- Firemen are at risk for **smoke inhalation**, leading to respiratory distress, carbon monoxide poisoning, or cyanide poisoning.
- **Carbon monoxide poisoning** can cause confusion and headaches, but the predominant musculoskeletal pain and generalized itching are not standard features.
Question 16: A 27-year-old man presents to the emergency department following a motor vehicle accident. Having been found as a restrained driver, he did not suffer from any chest injuries; nevertheless, his legs were pinned in position by the front of the highly damaged vehicle. After a prolonged extrication, the man sustained multiple fractures on his left femur and tibia. That same night, he underwent surgery to address his left leg fractures. In the next morning, the man suddenly developed severe dyspnea. Upon examination, he is noted to have a diffuse petechial rash. His vital signs are the following: blood pressure is 111/67 mm Hg, pulse rate is 107/min, respiratory rate is 27/min, oxygen saturation level is 82%, and his body temperature is normal. What is the most likely mechanism of his respiratory distress?
A. Fat embolism (Correct Answer)
B. Pulmonary edema
C. Spontaneous pneumothorax
D. Cardiac tamponade
E. Bacterial pneumonia
Explanation: ***Fat embolism***
- The combination of **severe dyspnea**, **hypoxemia**, **tachycardia**, and a **petechial rash** in a patient with recent long bone fractures (femur and tibia) is highly characteristic of a fat embolism.
- Fat emboli typically occur 24-72 hours after trauma, leading to **ARDS-like symptoms** and neurological changes due to fat globules entering the bloodstream and obstructing capillaries in the lungs and other organs.
*Pulmonary edema*
- While pulmonary edema causes dyspnea and hypoxemia, it is typically associated with **rales on auscultation** and often **cardiac dysfunction** or fluid overload.
- Pulmonary edema does not cause a petechial rash, which is a key distinguishing feature in this case.
*Spontaneous pneumothorax*
- A pneumothorax causes sudden dyspnea and hypoxemia, often with **unilateral chest pain** and diminished breath sounds on the affected side.
- It would not typically present with a diffuse petechial rash and is less likely following trauma to the lower extremities without direct chest injury.
*Cardiac tamponade*
- Cardiac tamponade involves the accumulation of fluid around the heart, leading to **Beck's triad** (hypotension, jugular venous distension, muffled heart sounds) and pulsus paradoxus.
- While it can cause dyspnea due to reduced cardiac output, it does not explain the petechial rash and the respiratory distress is primarily due to hemodynamic compromise rather than pulmonary pathology.
*Bacterial pneumonia*
- Bacterial pneumonia typically presents with **fever**, productive cough, and localized lung findings (e.g., **consolidation on CXR**), developing over a longer period.
- A petechial rash is not a typical symptom of bacterial pneumonia, and the acute onset after surgery points away from an infectious process, especially with normal temperature.
Question 17: An investigator is studying the affinity of hemoglobin for oxygen in different clinical settings. An illustration of an oxygen-hemoglobin dissociation curve is shown. Curve A shows the test results of one of the research participants and curve B shows a normal oxygen-hemoglobin dissociation curve. Which of the following is most likely present in this research participant?
A. Temperature of 39.1°C (102.4°F)
B. Serum pH of 7.1
C. Polycythemia (Correct Answer)
D. Neutrophilia
E. Sickled red blood cells
Explanation: ***Polycythemia***
- Curve A shows a **left shift** compared to the normal curve B, indicating **increased affinity of hemoglobin for oxygen**.
- **Polycythemia** (increased red blood cell count) is often a **compensatory response to chronic hypoxia**, such as living at **high altitude** or in conditions like **chronic lung disease**.
- At high altitude, the physiological adaptation includes:
- **Decreased 2,3-BPG initially** (causing left shift and increased O₂ affinity to help load oxygen in low PO₂ environments)
- **Increased red blood cell production** (polycythemia) to maximize oxygen-carrying capacity
- This combination allows better oxygen uptake in the lungs despite low atmospheric oxygen, though it makes tissue unloading slightly less efficient (compensated by increased RBC count).
*Temperature of 39.1°C (102.4°F)*
- An **increased temperature** causes a **right shift** in the oxygen-hemoglobin dissociation curve, indicating **decreased affinity of hemoglobin for oxygen**.
- This facilitates oxygen unloading to tissues during fever or increased metabolic activity.
- This does not match the left shift seen in Curve A.
*Serum pH of 7.1*
- A **decreased pH** (acidosis) causes a **right shift** via the **Bohr effect**, indicating **decreased affinity of hemoglobin for oxygen**.
- This facilitates oxygen release to metabolically active, acidotic tissues.
- This does not match the left shift seen in Curve A.
*Neutrophilia*
- **Neutrophilia** (increased neutrophil count) indicates infection or inflammation.
- It does **not affect the oxygen-hemoglobin dissociation curve**.
- This relates to immune response, not oxygen transport physiology.
*Sickled red blood cells*
- **Sickle cell disease** causes a **right shift** due to decreased hemoglobin-oxygen affinity (HbS) and increased 2,3-BPG levels.
- This facilitates oxygen unloading but impairs oxygen uptake.
- This does not match the left shift seen in Curve A.
Question 18: A 32-year-old female with Crohn's disease diagnosed in her early 20s comes to your office for a follow-up appointment. She is complaining of headaches and fatigue. Which of the following arterial blood gas findings might you expect?
A. High PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)
C. Normal PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)
D. Normal PaO2, normal O2 saturation (SaO2), low O2 content (CaO2) (Correct Answer)
E. Low PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)
Explanation: ***Normal PaO2, normal O2 saturation (SaO2), low O2 content (CaO2)***
- Patients with **Crohn's disease** are prone to developing **iron deficiency anemia** due to chronic inflammation, malabsorption, and blood loss, leading to reduced hemoglobin levels.
- While PaO2 and SaO2 measure oxygen *tension* and *percentage saturation* of available hemoglobin, respectively, **O2 content (CaO2)** directly reflects the *total amount* of oxygen delivered to tissues, which is primarily dependent on hemoglobin concentration. Therefore, with anemia, CaO2 will be low despite normal PaO2 and SaO2 because there is less hemoglobin to carry oxygen.
*High PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)*
- High PaO2 would indicate **hyperoxygenation**, which is not an expected complication of Crohn's disease or its associated anemia.
- Normal O2 content is inconsistent with the presence of anemia, which significantly reduces the body's total oxygen-carrying capacity.
*Low PaO2, low O2 saturation (SaO2), low O2 content (CaO2)*
- Low PaO2 and SaO2 suggest a primary **respiratory problem** or severe hypoxemia, which is not directly linked to Crohn's disease or the typical presentation of iron deficiency anemia.
- While low O2 content is correct for anemia, the accompanying low PaO2 and SaO2 indicate a different underlying pathology for oxygen transport issues.
*Normal PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)*
- This finding would indicate **normal oxygenation** and oxygen-carrying capacity, which is contrary to the clinical scenario of a patient with Crohn's likely complicated by anemia.
- The patient's symptoms of headaches and fatigue are consistent with poor tissue oxygenation, which would not occur if all these parameters were normal.
*Low PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)*
- A low PaO2 with a normal SaO2 is physiologically unlikely unless there is a **left shift of the oxygen dissociation curve** with adequate hemoglobin, which doesn't fit the expected anemic state.
- Normal O2 content would rule out the presence of anemia as a cause for the symptoms, which is a common complication in Crohn's disease.
Question 19: A research scientist attempts to understand the influence of carbon dioxide content in blood on its oxygen binding. The scientist adds carbon dioxide to dog blood and measures the uptake of oxygen in the blood versus oxygen pressure in the peripheral tissue. He notes in one dog that with the addition of carbon dioxide with a pressure of 90 mmHg, the oxygen pressure in the peripheral tissue rose from 26 to 33 mmHg. How can this phenomenon be explained?
A. High partial pressure of CO2 in tissues decreases peripheral blood volume
B. Binding of O2 to hemoglobin in lungs drives release of CO2 from hemoglobin
C. High partial pressure of CO2 in tissues causes alkalemia, which is necessary for O2 unloading
D. High partial pressure of CO2 in tissues facilitates O2 unloading in peripheral tissues (Correct Answer)
E. The sum of the partial pressures of CO2 and O2 cannot exceed a known threshold in blood
Explanation: **High partial pressure of CO2 in tissues facilitates O2 unloading in peripheral tissues**
- An increase in **PCO2** leads to a decrease in pH (acidosis) in the tissues, which **decreases hemoglobin's affinity for oxygen**, promoting oxygen release.
- This phenomenon is known as the **Bohr effect**, where an acidic environment (from CO2) shifts the oxygen dissociation curve to the right, enhancing O2 unloading to meet tissue metabolic demands.
*High partial pressure of CO2 in tissues decreases peripheral blood volume*
- **Increased CO2** generally causes vasodilation in peripheral tissues, which would lead to an **increase**, not a decrease, in peripheral blood flow.
- Decreased blood volume is typically associated with conditions like hypovolemia or intense vasoconstriction, not elevated tissue CO2.
*Binding of O2 to hemoglobin in lungs drives release of CO2 from hemoglobin*
- This statement describes the **Haldane effect**, which occurs primarily in the lungs, where oxygen binding to hemoglobin facilitates the release of CO2.
- While true, it does not explain the **increased oxygen pressure in peripheral tissue** observed with added CO2, which is related to O2 unloading.
*High partial pressure of CO2 in tissues causes alkalemia, which is necessary for O2 unloading*
- High **PCO2** in tissues leads to the formation of carbonic acid and H+ ions, resulting in a **decrease in pH (acidosis)**, not alkalemia.
- **Acidosis** facilitates O2 unloading (Bohr effect), whereas alkalemia would increase hemoglobin's affinity for O2, inhibiting unloading.
*The sum of the partial pressures of CO2 and O2 cannot exceed a known threshold in blood*
- There is **no fixed threshold** for the sum of partial pressures of CO2 and O2 in the blood; these gases are independently regulated and their partial pressures fluctuate with metabolic activity.
- The partial pressure of a gas reflects its concentration and does not have an upper limit when considering the sum of different gases.
