Gas Exchange: Alveoli and Diffusion — NEET Biology
📚 Practice ConceptAlveoli are the primary sites of exchange of gases. Exchange of gases also occur between blood and tissues. O₂ and CO₂ are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient. Solubility of the gases as well as the thickness of the membranes involved in diffusion are also some important factors that can affect the rate of diffusion.
Which of the above statements are correct? S1: The rate of diffusion of gases across the alveolar membrane is directly proportional to the partial pressure gradient and solubility of the gas. S2: In patients with alveolar fibrosis, the total thickness of the diffusion membrane significantly decreases, thereby facilitating more efficient oxygen uptake. S3: A higher partial pressure of CO₂ in the blood, as seen in active tissues, shifts the oxygen dissociation curve to the right, promoting oxygen release from oxyhaemoglobin. S4: Under normal physiological conditions, every 100 ml of deoxygenated blood delivers approximately 5 ml of O₂ to the tissues. S5: The diffusion membrane is composed of three major layers: the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance between them. S6: Increasing the partial pressure of inspired oxygen, such as through oxygen therapy, helps to compensate for reduced oxygen diffusion in conditions of increased diffusion membrane thickness.
View solution & NCERT explanation
Correct answer: C — S1, S3, S5 and S6 are correct.
The core concept for gas exchange is diffusion, which is influenced by partial pressure gradients, solubility of gases, and the thickness of the diffusion membrane. S1: The rate of diffusion is indeed directly proportional to the partial pressure gradient and solubility of the gas. This is a fundamental principle of gas exchange mentioned in NCERT. Hence, S1 is correct. S2: Alveolar fibrosis is a disorder where alveolar walls are damaged, leading to the proliferation of fibrous tissues, which increases the thickness of the diffusion membrane. This increased thickness hinders, rather than facilitates, oxygen uptake. Hence, S2 is not correct. S3: High pCO₂, high H+ concentration (low pH), and higher temperature in tissues are conditions that favor the dissociation of oxygen from oxyhaemoglobin, shifting the oxygen dissociation curve to the right (Bohr effect). Hence, S3 is correct. S4: Under normal physiological conditions, every 100 ml of *oxygenated* blood can deliver around 5 ml of O₂ to the tissues. Deoxygenated blood, on the other hand, delivers approximately 4 ml of CO₂ to the alveoli. Hence, S4 is not correct. S5: The diffusion membrane is accurately described as being composed of the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance between these layers. Its total thickness is much less than a millimeter. Hence, S5 is correct. S6: In conditions where the diffusion membrane is thickened (e.g., fibrosis), the increased barrier reduces the rate of diffusion. By increasing the partial pressure of inspired oxygen (oxygen therapy), the partial pressure gradient for oxygen across the membrane is elevated, thereby helping to drive more oxygen across the thickened membrane and compensate for the reduced diffusion efficiency. Hence, S6 is correct. Therefore, statements S1, S3, S5, and S6 are correct.
What does NCERT say about Alveoli are primary sites?
Has this concept appeared in NEET?
Which chapter is this from?
Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.