UPPSC 2021 Prelims GS Paper-I · Question 79 of 150

Correct — D, Alveoli. Gaseous exchange in the lungs takes place in the alveoli — millions of tiny, balloon-like air sacs at the ends of the finest airways. Each alveolus has an extremely thin (one-cell-thick) wall wrapped in a dense network of pulmonary capillaries, giving an enormous surface area and a very short diffusion distance. Oxygen diffuses from the alveolar air into the blood while carbon dioxide diffuses from the blood into the alveolar air, down their partial-pressure gradients. This makes the alveoli the actual site of gas exchange in the lungs.

• (a)Tracheoles — Wrong — tracheoles are the fine terminal branches of the tracheal system of INSECTS, where insect gas exchange occurs. They are not present in mammalian lungs, so they cannot be the site of exchange in human lungs (a classic trap for those who confuse 'trachea' with 'tracheoles').
• (b)Bronchioles — Wrong — bronchioles are small conducting airways that carry air deeper into the lung towards the alveoli. They belong to the conducting (air-transport) zone; the bulk of actual gas exchange happens in the alveoli at the end of these passages, not in the bronchioles themselves.
• (c)Pulmonary vein — Wrong — the pulmonary vein is a blood vessel that carries oxygenated blood from the lungs back to the left atrium of the heart. It transports blood but is not a respiratory surface where gases are exchanged with air.

The human respiratory pathway runs nostrils → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. The trachea, bronchi and bronchioles form the conducting zone (they only move air); the alveoli form the respiratory zone where exchange occurs. An alveolus is a thin-walled air sac surrounded by capillaries, so O2 and CO2 cross the alveolar–capillary membrane by simple diffusion.

Eliminate by function. A pulmonary vein carries blood, not air, so it is not a gas-exchange surface. Bronchioles merely conduct air towards the sacs. Tracheoles belong to insects, not the human lung. What remains — and what is specifically built for diffusion (thin wall + huge surface area + surrounding capillary bed) — is the alveolus.

• Alveoli are the site of gaseous exchange in the lungs (O2 in, CO2 out, by diffusion).
• Alveolar walls are one cell thick and surrounded by pulmonary capillaries — large area, short diffusion path.
• Trachea, bronchi and bronchioles are conducting airways; alveoli are the respiratory (exchange) surface.
• Tracheoles are the gas-exchange endings of the insect tracheal system, not part of human lungs.

• Picking 'bronchioles' (which only conduct air) instead of the alveoli where exchange actually occurs.
• Confusing tracheoles (insect gas exchange) with parts of the human lung.

UPPSC and UPSC ask for the 'site of gas exchange', the sequence of the respiratory pathway, or how a particular animal group respires — anchor on alveoli = human gas-exchange surface; tracheoles = insects.

• practice — not a real PYQ

  Gaseous exchange between alveolar air and blood in the lungs occurs mainly by which process?

  • (a)Active transport
  • (b)Simple diffusion
  • (c)Osmosis
  • (d)Bulk flow

  Answer(b) Simple diffusion — O2 and CO2 move down their partial-pressure gradients across the thin alveolar–capillary membrane.

• practice — not a real PYQ

  In insects, gaseous exchange with the tissues takes place at the:

  • (a)Alveoli
  • (b)Gills
  • (c)Tracheoles
  • (d)Book lungs

  Answer(c) Tracheoles — the fine, blind endings of the insect tracheal system deliver air directly to the tissues.