Site of gaseous exchange in lungs is

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2021 Q79 — Science & Technology PYQ with Answer

State PCS 2021 Q79 — Science & Technology PYQ with Answer