During respiration in human beings, the actual place of gaseous exchange is

Correct — C, Alveoli. The nasal passage, trachea and bronchi are conducting tubes; they warm, moisten and filter the air and carry it in and out, but no gas crosses their walls into the blood. Exchange happens in the alveoli, the tiny air sacs at the ends of the finest bronchioles, where a single layer of flattened cells is separated from a capillary by a barrier under a micrometre thick. Oxygen diffuses across into the blood and carbon dioxide diffuses out.

• (a)Bronchi — The bronchi are the two large airways branching from the trachea into each lung. Their walls are cartilage-reinforced conducting tissue, not an exchange surface.
• (b)Nasal passage — The nasal passage conditions incoming air, warming it, adding moisture and trapping dust on its mucus lining. Nothing is absorbed into the bloodstream there.
• (d)Trachea — The trachea is the windpipe, held open by C-shaped rings of cartilage. It is a pipe and nothing more, which is why it is described as part of the dead space of the airway.

The human respiratory tract divides into a conducting portion and a respiratory portion. The conducting portion runs from the nostrils through the pharynx, larynx, trachea, bronchi and bronchioles, delivering air without exchanging any gas. The respiratory portion begins where the walls become thin enough for diffusion, at the alveolar ducts and alveoli.

The design problem the lung solves is area within a small volume. Roughly 300 million alveoli give a surface of the order of 70 square metres inside a chest cavity of a few litres, and each is wrapped in capillaries so that blood is spread in a thin sheet against the air. The barrier is thin, the area vast and the blood supply rich, which is the standard checklist for any efficient exchange surface. Emphysema destroys alveolar walls and merges small sacs into larger ones, cutting that area and leaving a patient breathless.

• Gaseous exchange in humans takes place in the alveoli, not anywhere in the conducting airways.
• The alveolar wall is one cell thick and lies against a capillary wall one cell thick, giving a diffusion barrier well under a micrometre.
• Around 300 million alveoli give a total exchange surface of the order of 70 square metres.
• Air travels nostrils to pharynx to larynx to trachea to bronchi to bronchioles to alveoli.
• Oxygen is carried onward bound to haemoglobin, while most carbon dioxide travels in the plasma as bicarbonate.

• Naming the lungs as a whole when the question asks for the actual site of exchange.
• Treating the trachea or bronchi as exchange surfaces because air passes through them.
• Forgetting that the conducting airways form dead space, air that is moved but never exchanged.

A precise-location item. Every wrong option is a real part of the same system, so a vague answer such as 'the lungs' does not survive.

No directly related past PYQ was found.

• practice — not a real PYQ

  The exchange of gases in the human lung is efficient mainly because the alveoli provide

  • (a)a thick muscular wall
  • (b)a very large surface area with a thin moist wall
  • (c)a cartilage-supported tube
  • (d)a store of oxygen

  Answer(b) a very large surface area with a thin moist wall — about 70 square metres of surface behind a barrier well under a micrometre thick.

• practice — not a real PYQ

  Which one of the following is NOT part of the conducting portion of the human respiratory tract?

  • (a)Trachea
  • (b)Bronchi
  • (c)Alveoli
  • (d)Bronchioles

  Answer(c) Alveoli — the conducting portion only moves air, while the alveoli are where it is exchanged.