Class 11 Biology Short Notes 2026-27

14.1 RESPIRATORY ORGANS

Living cells continuously break down food molecules such as glucose and fatty acids in order to release the energy needed for their activities. Oxygen is consumed in these reactions and carbon dioxide, which becomes harmful if allowed to accumulate, is produced as a waste. An organism must therefore have some arrangement by which oxygen from the surroundings is supplied to its cells and carbon dioxide is removed from them. The taking in of oxygen-rich air and the giving out of air rich in carbon dioxide is termed breathing.

The structures used for breathing are not the same in all animals. Lower invertebrates exchange gases over the general body surface, insects possess a network of tubes called tracheae, most aquatic animals use gills, and land vertebrates breathe with the help of lungs. In human beings, air enters through a pair of nostrils into the nasal chamber and passes into the pharynx, which serves as a common passage for food and air. From the pharynx air moves through the larynx, the voice box, into the windpipe or trachea.

The trachea divides into right and left primary bronchi, which branch repeatedly into finer tubes ending in small air sacs called alveoli. The two lungs are covered by a double-layered membrane with fluid between the layers.