Chapter 14 — Breathing and Exchange of Gases

Chapter 14 of the Class 11 Biology NCERT textbook, "Breathing and Exchange of Gases", explains this topic. It is the process by which oxygen from the atmosphere is continuously supplied to cells and carbon dioxide produced during metabolism is released, involving respiratory organs, gas diffusion across alveolar membranes, transport of gases by blood, and cellular utilization of oxygen.

• The respiratory pathway — The chapter traces air from the nostrils through the nasal passage, pharynx, larynx, trachea, and bronchial tree to the alveoli, where the thin alveolar-capillary membrane makes gas exchange possible.
• Breathing as a pressure-driven mechanism — It explains breathing as two mechanical stages—inspiration, where the diaphragm and intercostal muscles enlarge the chest and lower pressure to draw air in, and expiration, where relaxation reverses this to expel air.
• Gas transport and rhythm control — The chapter shows how oxygen travels mainly as oxyhaemoglobin and carbon dioxide chiefly as bicarbonate, and how the medulla, pons, and chemosensitive areas sensitive to CO2 regulate the respiratory rhythm.

1. 01Respiration involves breathing, gas diffusion across alveolar membranes, gas transport by blood, tissue diffusion, and cellular utilization of oxygen
2. 02Inspiration increases thoracic volume via diaphragm contraction and external intercostal muscle action, decreasing intra-pulmonary pressure below atmospheric pressure
3. 03Expiration occurs when diaphragm and intercostal muscles relax, increasing intra-pulmonary pressure to release air from lungs
4. 04Oxygen (97%) is transported primarily as oxyhaemoglobin; carbon dioxide (70%) is transported as bicarbonate with carbonic anhydrase enzyme
5. 05Gas exchange occurs by simple diffusion across the thin alveolar-capillary diffusion membrane (~0.1 mm) following partial pressure gradients
6. 06Respiratory rate (~12-16 breaths/minute in healthy humans) is regulated by neural centres in the medulla and modulated by CO2 and hydrogen ion concentration