CBSE Class 9 Answered

Birds possess a very unique and efficient respiratory system. They lack a diaphragm, have non-expandable lungs and a system of air sacs which extend into many of their bones. The nostrils or nares are usually located at the base of the beak.

Air enters the nares and moves in the nasal cavity. The nasal cavity filters inhaled airborne particles by means of mucus and small hair-like cilia. These particles enter the nares and are directed by the cilia through the choana and are then swallowed. Inhaled air is warmed and moistened by the nasal cavity.

The larynx is at the entrance of the trachea. The opening of the larynx is the glottis. The trachea is formed by a series of ring-shaped cartilages, which are completely closed in birds. The trachea bifurcates (divides) after entering the thoracic cavity. The syrinx or voice box is located at this bifurcation. The lungs are paired and are not divided into lobes as they are in mammals. They have very little ability to expand and are attached firmly to the ribs and dorsal body wall.

Birds do not have a diaphragm and therefore rely on pressure changes in their air sacs to move air through the lungs. Most birds have eight air sacs. These air sacs act as a bellows system to move air through the respiratory tract. Many of these air sacs extend into bones. These bones are described as pneumatic bones.