Solution:Concept: Breathing is the process of intake of fresh air from the environment and expulsion of foul air from the body. When we breathe, we intake oxygen from the air (inhalation) and give out carbon dioxide (exhalation) which is utilized by the plants.
Breathing mechanism involves– inspiration and expiration.
Inspiration is the process of intake of atmospheric air and involves contraction of diaphragm and external intercostal muscles. Intercostal muscles are muscles present between the ribs.
The internal intercostal muscles form the middle layer of intercostal muscles, while the external intercostal muscles originate on the interior surface of ribs. When the radial muscle of the diaphragm contracts, it becomes flattened therefore, the intrapulmonary volume increases and intrapulmonary pressure decreases. When external intercostal muscles contract, ribs and sternum move upwards and outwards respectively which also increases intrapulmonary volume and increases intrapulmonary pressure resulting in entry of air into the lungs through the respiratory tract. Expiration is the process of giving out of air. It involves relaxation of diaphragm and external intercostal muscles. When the radial muscles of the diaphragm relax, it becomes dome-shape hence the intrapulmonary volume decreases and intrapulmonary pressure increases.
When external intercostal muscles relax, ribs and sternum move downwards and inwards respectively. It also decreases intrapulmonary volume and increases intrapulmonary pressure resulting in expelling air out of the lungs through the respiratory tract.
Hence, expiration and inspiration are the correct answer.