Breathing and Exchange of GasesNEET Zoology · Class 11 · NCERT Chapter 14

You can expect 1 question from Breathing and Exchange of Gases in NEET 2027. The most reliable scoring areas are: respiratory volumes and capacities (tidal volume, vital capacity, total lung capacity with their values), the partial pressure values of O2 and CO2 in alveoli and blood, oxyhaemoglobin formation and the sigmoid dissociation curve, how CO2 is mainly transported as bicarbonate (about 70%), and the Bohr effect.

Breathing (also called external respiration) is the physical process of moving air in and out of the lungs. It takes place in the respiratory system and is controlled by the nervous system. Cellular respiration is a biochemical process: it is the breakdown of glucose and other organic molecules inside cells to produce ATP, CO2 and water. Breathing delivers the O2 that cellular respiration needs and removes the CO2 it produces. The two processes are related but completely different in nature.

Tidal volume (TV): about 500 mL per breath at rest. Inspiratory reserve volume (IRV): about 2500 to 3000 mL extra air that can be inhaled after a normal breath. Expiratory reserve volume (ERV): about 1000 to 1100 mL extra air that can be pushed out after a normal breath. Residual volume (RV): about 1100 to 1200 mL of air that always stays in the lungs, even after maximum exhalation. Vital capacity (VC) = TV + IRV + ERV = about 3500 to 4500 mL. Total lung capacity (TLC) = VC + RV = about 6000 mL.

The Bohr effect is the shift of the oxygen-haemoglobin dissociation curve caused by changes in CO2 concentration, H+ (pH) or temperature. When CO2 is high, pH is low (more acidic), or temperature is high (as in active tissues), the curve shifts to the right. This means haemoglobin has a lower affinity for O2, so it releases more O2 to the tissues that need it most. When CO2 is low, pH is high, and temperature is lower (as in the lungs), the curve shifts to the left, meaning haemoglobin picks up O2 more easily in the lungs. This is a vital way the body adjusts O2 delivery to match demand.

CO2 is transported in three forms. About 7 percent is dissolved directly in plasma. About 23 percent is carried as carbamino-haemoglobin, which is CO2 bound directly to haemoglobin (at the amino groups, not at the haem iron). About 70 percent is transported as bicarbonate ions (HCO3-) in the plasma. When CO2 enters red blood cells, the enzyme carbonic anhydrase converts it to carbonic acid (H2CO3), which immediately breaks down into H+ and HCO3-. The HCO3- moves into the plasma in exchange for Cl- (the chloride shift). In the lungs, this process runs in reverse.

Inspiration: the diaphragm contracts and flattens downward. The external intercostal muscles contract and pull the ribs upward and outward. Both actions increase the volume of the thoracic cavity. The lungs expand, intra-pulmonary pressure drops below atmospheric pressure, and air rushes in. Expiration: the diaphragm relaxes and domes upward. The internal intercostal muscles contract (in forced expiration) or the ribs fall back (in relaxed expiration). Thoracic volume decreases, pressure rises above atmospheric, and air is pushed out.

Asthma: a condition where the bronchioles become inflamed and the smooth muscle contracts, making the airways narrow. Symptoms include wheezing and difficulty breathing. Emphysema: the walls of the alveoli are damaged, reducing the total surface area for gas exchange. Caused mainly by smoking. Occupational respiratory disorders: caused by long-term exposure to dust particles in certain jobs (like coal dust causing pneumoconiosis, or asbestos fibres causing asbestosis). All of these reduce effective gas exchange and cause breathlessness.