Free Respiration MCQs with Answers

1. The correct path taken by inhaled air after it leaves the larynx is

• A. trachea, alveoli, bronchi, bronchioles
• B. bronchi, trachea, bronchioles, alveoli
• C. trachea, bronchi, bronchioles, alveoli
• D. bronchioles, bronchi, trachea, alveoli

Explanation: Air passes down the single trachea, which divides into two bronchi, one to each lung, and these branch repeatedly into ever finer bronchioles ending in the alveolar sacs where exchange occurs. The sequence runs from wide and few to narrow and numerous, which is the quick way to check any order given in an option. Cartilage supports the trachea and bronchi but disappears in the smallest bronchioles.

2. During swallowing, entry of food into the trachea is prevented by the

• A. epiglottis
• B. uvula
• C. soft palate
• D. glottis

Explanation: The epiglottis is a flap of cartilage that folds back over the opening of the larynx as the larynx rises during swallowing, so the bolus is directed into the oesophagus behind it. The soft palate closes the passage to the nose at the same moment, which is why it is the tempting alternative. The glottis is the opening itself, not the structure that covers it.

3. The rings of cartilage in the wall of the trachea are C shaped rather than complete because

• A. complete rings would restrict the flow of air
• B. cartilage is too expensive for the body to make
• C. the gap allows nerves to reach the lung
• D. the incomplete side lies against the oesophagus and lets it expand during swallowing

Explanation: The cartilage keeps the airway permanently open so it cannot collapse when pressure falls during inspiration, while the soft muscular gap at the back allows a swallowed bolus to bulge into the space as it passes down the oesophagus behind. Complete rings would give the same support but would obstruct swallowing. The rings do not restrict airflow in any way.

4. Which feature of the alveoli is the most important adaptation for efficient gaseous exchange?

• A. Their supporting rings of cartilage
• B. Their walls, one cell thick, giving an enormous surface area with a dense capillary network
• C. The cilia lining their inner surface
• D. The mucus that coats them

Explanation: Around 300 million alveoli give a surface of roughly 70 square metres, and a wall a single cell thick lying against an equally thin capillary means oxygen diffuses across a distance well under a micrometre. Continual blood flow and ventilation keep the diffusion gradient steep. Alveoli contain no cartilage or cilia, and a coat of mucus would in fact lengthen the diffusion path.

5. During inspiration the diaphragm

• A. contracts and flattens, increasing the volume of the thorax
• B. relaxes and domes upwards, increasing the volume of the thorax
• C. contracts and domes upwards, decreasing the volume of the thorax
• D. does not move at all

Explanation: A flattening diaphragm and the external intercostals swinging the ribs up and out together enlarge the thoracic cavity, so the pressure inside the lungs falls below atmospheric and air flows in. The diaphragm is a muscle, so contraction shortens and flattens the dome rather than raising it. Air movement is always the result of a pressure difference created by these volume changes.

6. Quiet expiration at rest is described as a passive process because

• A. no air actually leaves the lungs
• B. the internal intercostal muscles contract forcefully
• C. it results from the elastic recoil of the lungs and relaxation of the inspiratory muscles
• D. the diaphragm contracts to push air out

Explanation: When the diaphragm and external intercostals simply relax, the stretched elastic tissue of the lungs recoils and the thoracic volume falls without any muscle actively pulling it down, so no extra energy is spent. Forced expiration during exercise or coughing does use the internal intercostals and the abdominal muscles, which is the active case. Loss of this elasticity in emphysema makes breathing out difficult.

7. The volume of air moved in or out of the lungs during one normal quiet breath is called the

• A. vital capacity
• B. tidal volume
• C. residual volume
• D. total lung capacity

Explanation: The tidal volume of an adult at rest is about 500 cm3, of which roughly 150 cm3 stays in the airways as dead space and never reaches the alveoli. Vital capacity is the much larger volume, around 4,500 cm3, that can be moved between the deepest possible inspiration and the fullest expiration. The two are commonly confused because both are measured on the same spirometer trace.

8. Vital capacity is defined as

• A. the air left in the lungs after the deepest possible expiration
• B. the volume of one quiet breath
• C. the volume of air in the dead space
• D. the maximum volume of air that can be expelled after the deepest possible inspiration

Explanation: Vital capacity is the sum of the tidal volume and the inspiratory and expiratory reserve volumes, so it measures the usable range of the lungs and falls in diseases such as emphysema and fibrosis. The air remaining after the fullest expiration is the residual volume, about 1,200 cm3, which cannot be expelled and keeps the alveoli from collapsing. Total lung capacity is vital capacity plus residual volume.

9. Most of the carbon dioxide carried in the blood travels as

• A. hydrogen carbonate ions dissolved in the plasma
• B. carbaminohaemoglobin
• C. carbon dioxide dissolved in the plasma
• D. carbon monoxide

Explanation: About seventy per cent of carbon dioxide enters the red cells, where carbonic anhydrase converts it with water into carbonic acid, which dissociates into hydrogen and hydrogen carbonate ions, and the latter diffuse out into the plasma. Roughly twenty three per cent is carried on haemoglobin as carbaminohaemoglobin and only about seven per cent stays simply dissolved. The reactions reverse in the lungs.

10. One molecule of haemoglobin can combine with

• A. one molecule of oxygen
• B. four molecules of oxygen
• C. two molecules of oxygen
• D. an unlimited number of oxygen molecules

Explanation: Haemoglobin has four polypeptide chains, each with an iron containing haem group that binds one oxygen molecule, giving a maximum of four. Binding is cooperative, so the attachment of the first oxygen makes the next ones easier, and this is what gives the oxygen dissociation curve its S shape. Full saturation is reached in the lung capillaries within a fraction of a second.