Structure of the respiratory system.
             The respiratory surface of a mammal consists of many air sacs called alveoli inside a pair of lungs. The lungs are situated next to the heart in the thoracic cavity and are connected to the atmosphere by tubes. Air passes into the lungs through these tubes. Twelve pairs of bony ribs surround and protect the lungs and heart. Intercostals muscles are attached to the ribs, and a large diaphragm separates the thorax from the abdomen. These are involved in the ventilation mechanism.
             Air enters the body through two nostrils, each of which possesses a boarder of large hairs, which trap particles in the air and filter them out of the system. While passing through the nasal passages the air is warmed and moistened and its odour detected. Air passes from the nasal passages, through the pharynx and into the trachea. This is a tube, which lies in front of the oesophagus and extends into the thoracic cavity. The wall of the tube is strengthened and held open by horizontally arranged C-shaped bands of cartilage. The open section of the C is next to the oesophagus. The cartilage prevents collapse of the tube during inspiration (breathing in).
             At its lower end the trachea splits into two bronchi. Within the lungs each bronchus subdivides many times into much smaller tubes called bronchioles. These in turn branch into finer and finer tubes, ending with the alveolar ducts, which lead into sacs called alveolar sacs. Into each alveolar sac opens a group of alveoli.
             The walls of most of the respiratory passage are lined with ciliated epithelial cells and goblet cells, which secrete mucus. Mucus traps any particles, such as dust and bacteria that have managed to pass through the hairs of the nostrils. The beating of the cilia then carries the trapped particles to the back of the buccal cavity where the mucus is swallowed. Mucus also moistens the incoming air.
             The alveoli form the gas exchange surface. There a...