Respiratory: MRCEM Primary questions with answers

This Respiratory topic explains how breathing is controlled, measured and disturbed. Physiology carries 60 of the 180 questions in MRCEM Primary. Lessons cover central control of breathing, lung receptors, lung volumes, compliance, surfactant, airway resistance, dead space, gas exchange, diffusion, the oxygen haemoglobin dissociation curve, the alveolar arterial gradient and ventilation perfusion mismatch. Emergency physicians use these principles to interpret blood gases and manage hypoxia.

5 free questions below, each with the answer and the full explanation · 118 in the full bank · Last updated after the 27 October 2026 MRCEM Primary sitting.

1. Question 1 · Lung Volumes and Capacities

   Which lung capacity is most likely lowered during the Valsalva maneuver?

   1. AVital Capacity
   2. BTidal Volume
   3. CResidual Volume
   4. DFunctional residual capacity
   5. EInspiratory Capacity
   Show answer and explanation

   Answer: A. Valsalva raises intrathoracic pressure, compressing the lungs and reducing vital capacity.

   • B: Not the volume cited as reduced; vital capacity is.
   • C: Not the volume cited as reduced; vital capacity is.
   • D: Not the capacity cited as reduced; vital capacity is.
   • E: Not the capacity cited as reduced; vital capacity is.

   Full explanation: Lung Volumes and Capacities

   Lung volume and capacities

   Tidal Volume (TV): The volume of air that is inhaled and then exhaled with each normal breath. 

   • Normal tidal volume is 500ml or 7ml/kg.

   Inspiratory Reserve Volume (IRV): The maximum volume of air that can be inhaled following and above a normal tidal inspiration. 

   • Normal Inspiratory reserve volume (IRV) is 3300ml. 

   Expiratory Reserve Volume (ERV): The maximum volume of air that can be exhaled from the resting expiration of tidal volume. 

   • Normal Expiratory reserve volume (ERV) is 1700ml. 

   Residual Volume (RV):  The volume of air that remains in the lung following a maximum expiration. 

   • Residual Volume cannot be measured using spirometry.  
   • Normal residual volume is 1800ml.  

   Capacities

   Inspiratory Capacity (IC): is the maximum volume of air that can be inhaled at the end of resting expiration.  

   • Inspiratory capacity is sum of IC= TV+ IRV 

   Functional Residual Capacity (FRC): the volume of air remaining in the lungs at the resting end-expiratory level or Volume remaining in the lungs after tidal volume is expired in air. 

   • FRC is sum of: FRC= ERV+ RV 
   • FRC include Residual volume so FRC cannot be measured using spirometry.  
   • Normal FRC is 3500ml. 

   Factors affecting FRC. 

   • Factors increasing FRC: Emphysema, Air trapping in asthma, Ageing (due to loss of elastic properties) and Increasing height of patient. 
   • Factors decreasing FRC: Restrictive ventilatory defects e.g., pulmonary fibrosis, Posture – lying supine, Increased intra-abdominal pressure (e.g., obesity, pregnancy, ascites) and Reduced muscle tone of diaphragm e.g., muscle relaxants in anesthesia, neuromuscular disease. 

   Vital Capacity (VC) is the maximum volume of air that may be exhaled following a maximum inspiration OR inhaled following a maximum expiration. 

   • VC= IRV+TV+ ERV OR VC= IC+ERV 
   • Normal VC in young adult (70kg) is 5500ml.  

   Total Lung Capacity (TLC) is the volume of air contained within the lungs following a maximum inspiration. 

   • TLC include Residual volume so it cannot be measured using spirometry.  
   • TLC= IRV + TV+ ERV + RV OR TLC= IC + FRC Or TLC = VC+ RV 
   • Normal TLC in young adult (70kg) is 7300ml. 

   Forced expiratory volume (FEV1)  

   • FEV1 is the volume of air that can be expired in the first second of a forced maximal expiration.  
   • FEV1 is normally 80% of the forced vital capacity, which is expressed as 
   • FEV1/ FVC = 0.8
   • In obstructive lung disease such as asthma and chronic obstructive pulmonary disease (COPD), both FEV1 and FVC are reduced, but FEV1 is reduced more than FVC. FEV1/FVC ratio is less than 0.7. 
   • In restrictive lung disease such as fibrosis, both FEV1 and FVC are reduced, but FEV1 is reduced less than FVC is thus, Fev1 /FVC increased.  

   Lung volume measurements and Peak Expiratory flow rate: 

   • Lung volume can be measured with Spirometry. 
   • Residual volume cannot be measured using spirometry. 
   • Residual volume and thus (FRC and TLC) can be measured using helium dilution. 

   Peak Expiratory flow rate:

   The easiest and quickest measurement is the peak expiratory flow rate (PEFR) 

   • Airway resistance and lung compliance can be assessed indirectly by measuring the forced expiratory flows and volumes. 
   • Lung volumes can be measured with spirometry. 
   •  PEFR is reduced if airway resistance is increased in obstructive disease and is commonly used to monitor asthma. 

2. Question 2 · Oxygen Haemoglobin Dissociation Curve

   During your physiology lecture you ask the medical students which of the following causes a right shift in the oxygen dissociation curve?

   1. ADecrease PCO2
   2. BDecrease H+
   3. CFetal Hb
   4. DDecrease PH
   5. EIncrease PH
   Show answer and explanation

   Answer: D. A fall in pH (rise in H+) reduces haemoglobin's oxygen affinity, shifting the curve right.

   • A: Decreased PCO2 shifts the curve left.
   • B: Decreased H+ means higher pH, shifting left.
   • C: Fetal haemoglobin has higher affinity, shifting left.
   • E: Raised pH shifts the curve left.

   Full explanation: Oxygen Haemoglobin Dissociation Curve

   Oxygen-Haemoglobin Dissociation Curve

   The Haemoglobin oxygen Dissociation Curve is a plot of percent saturation of haemoglobin as a function of po2. 

   Changes in the hemoglobin-02 dissociation curve 

   1. Shifts to the right:

   • occur when the affinity of hemoglobin for O2 is decreased.  
   • The P50 is increased, and unloading of O2 from arterial blood to the tissues is facilitated. 

   Causes of Right ward shift 

   Decreased affinity for oxygen (More willing to give up oxygen) 

   • ️Decreased pH 
   • Increased pCO2 
   • Increased H+ 
   • Increased Temperature 
   • Increased 2-3DPG. 
   • Pregnancy (oxygen to the fetus) 

   1. Shifts to the left: 

   • occur when the affinity of hemoglobin for 02 is increased.
   • The P50 is decreased, and unloading of O2 from arterial blood into the tissues is more difficult. 

   Causes of left ward shift 

   Increased affinity for oxygen (Less willing to give up oxygen) 

   • Increased pH 
   • Decreased pCO2. 
   • Decreased temperature. 
   • Decreased H+ 
   • Decreased 2-3DPG. 
   • CO poisoning 
   • Methemoglobinemia 
   • Fetal hemoglobin  

   Note:  

   • Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin. Maternal and Fetal blood never mix during pregnancy, but they come close to each other in the placenta.  Oxygen diffuses easily to fetal hemoglobin here. 
   • The reason fetal hemoglobin has a higher affinity for 02. It is composed of two alpha and two gamma subunits, while adult hemoglobin is composed of two alpha and two beta subunits. 
   • 2,3DPG present in RBCs enhances the ability of RBCs to release o2 near tissue and 2,3DPG only binds with beta globulin chains, fetal hemoglobin (HbF) lack of beta globulin chains that’s why fetal hemoglobin has higher affinity for O2 

    

3. Question 3 · Respiratory

   A male aged 55, a known case of COPD, came complaining of breathlessness. His observations include BP=100/70 Pulse=102bpm and Sa02 is 78%. You place the patient on 02. What is the targeted 02 saturation prior to discharge?

   1. AUnless SaO2 becomes 100%
   2. B88 – 92%
   3. C80-84%
   4. DUnless SaO2 becomes 98%
   5. EUnless SaO2 becomes 94%
   Show answer and explanation

   Answer: B. Titrate oxygen to 88–92% in COPD exacerbations and other chronic respiratory failure, to avoid hypercapnia from excess oxygen.

   • A: Targeting 100% risks oxygen-induced hypercapnia in COPD.
   • C: 80–84% is too low, risking hypoxia.
   • D: Targeting 98% is the general target and risks hypercapnia in COPD.
   • E: Targeting 94% is above the COPD range and risks hypercapnia.

   Full explanation

   COPD and conditions associated with chronic respiratory failure. In the treatment of exacerbations of chronic obstructive pulmonary disease (COPD), oxygen should be titrated to achieve a target oxygen saturation range of 88–92%.

4. Question 4 · Ventilation Perfusion Mismatch

   A woman aged 25 presents with shortness of breath and right-sided chest pain. You suspect pulmonary embolism.

   Which parameter is most likely to be reduced?

   1. AVentilation to perfusion ratio
   2. BPo2 in pulmonary venous blood
   3. CLeft atrial pressure
   4. DPulmonary vascular resistance
   5. ERight atrial pressure
   Show answer and explanation

   Answer: B. PE causes V/Q mismatch and hypoxaemia, so pulmonary venous PO2 falls.

   • A: Unperfused ventilated lung raises, not lowers, the V/Q ratio.
   • C: Not the parameter cited as decreased; pulmonary venous PO2 is.
   • D: Pulmonary vascular resistance rises with obstruction and vasoconstriction.
   • E: Right atrial pressure rises with raised pulmonary vascular resistance.

   Full explanation: Ventilation Perfusion Mismatch

   Ventilation- Perfusion mismatch

   The ratio of alveolar ventilation to pulmonary blood flow is V/Q ratio. 

   • For the lung at rest, it is about 0.8.  

   Ventilation-Perfusion at base and apex of lung: 

   • Both ventilation and perfusion are highest towards the lung base because of gravitational effects and low at the apex of lung 
   • Gravitational effects are greater on perfusion than ventilation, so V/Q ratio is high at apex and low at base 

   Abnormal V/Q: 

   Ventilation perfusion mismatch (V/Q) most common cause of Hypoxemia 

   1: Ventilation defect: Alveoli perfused but not ventilated 

   • Produce intrapulmonary shunting. In large ventilation defect, V/Q is zero, which is called a shunt. 
   •  There is no gas exchange, increased A-a gradient. 
   • Partial pressure of pulmonary CO2 increases while that of oxygen decreases because normal amount of oxygen for ventilation is not available. 
   • V/Q ratio zero-pulmonary shunt: No ventilation but perfusion remains normal Causes: ARDS, Compression atelectasis due to hydrothorax or pneumothorax. 
   • V/Q ratio <1: Low ventilation relative to its perfusion such as in partial obstruction of airways. Causes: Low compliance e.g., pulmonary fibrosis, High airway resistance e.g., COPD, asthma   

   2: Perfusion defect (Ventilation normal but perfusion decreased) 

   • If blood flow to a lung is completely blocked (e.g., by an embolism occluding a pulmonary artery), then blood flow to that lung is zero. If ventilation is normal, then V/Q is infinite, which is called dead space. 
   • Partial pressure of pulmonary CO2 decreases while that of oxygen increases. 
   • There is no gas exchange. ↑A-a gradient. As a result, partial pressure of oxygen and CO2 equal to inspired air. 

   V/Q ratio of infinity-dead space:  

   • No perfusion but ventilation remains normal. 
   • Causes: Embolism 

     

   Anatomic and physiologic shunt 

   Type	Characteristic	Examples
   Anatomic	Blood flow bypass the lungs.	Intracardiac shunt. Right to left shunt or left to right.
   Physiologic	Normal delivery of blood to lungs but not takes place in exchange.	Pneumonia or pulmonary edema, Interstitial fibrosis and Emphysema

   • The shunted blood will not have been oxygenated or been able to offload CO2 and thus its levels of PO2 and PCO2 are that of mixed venous blood. Changes in PCO2 and PO2 stimulate the chemoreceptors and increase ventilation, so that arterial PCO2 returns to normal.  
   • However, increased ventilation cannot increase blood O2 content, as the haemoglobin of the blood passing through the lungs is already close to saturation. Thus, right to left shunts commonly result in a low arterial PO2 but a normal or low PCO2. 

    

    

5. Question 5 · Dead Space

   Concerning dead space, which statement is INCORRECT:

   1. AIn a healthy adult physiological dead space is equivalent to anatomical dead space.
   2. BFowler's method is used to measure anatomical dead space.
   3. CThe Bohr equation can be used to calculate physiological dead space.
   4. DThe anatomical dead space is normally about 150 mL.
   5. EIn V/Q mismatch the alveolar dead space may increase to more than the physiological dead space.
   Show answer and explanation

   Answer: E. Alveolar dead space cannot exceed physiological dead space, which is anatomical plus alveolar dead space, so this is incorrect.

   • A: True: alveolar dead space is negligible in health.
   • B: True: Fowler's method measures anatomical dead space.
   • C: True: the Bohr equation gives physiological dead space.
   • D: True: anatomical dead space is about 150 mL.

   Full explanation: Dead Space

   Dead Space

   Anatomic dead space

   • Refers to volume of conducting airways.
   • Normally approximately 150 ml   

   Alveolar dead space:

   • Refers to alveoli incapable of gas exchange, in normal healthy individual it is negligible, but it may become significant in V/Q mismatch i.e., in pulmonary embolism when alveoli do not take part in gas exchange.    

   Physiologic dead space: sum of anatomical and alveolar dead space. 

   • Defined as the volume of the lungs that does not participate in gas exchange. is approximately equal to the anatomic dead space in normal lungs may be greater than the anatomic dead space in lung diseases in which the ventilation/perfusion (V/Q) defects.