GASEOUS EXCHANGE IN A MAMMAL

Structure of the respiratory system

The respiratory surface of a mammal consists of many air sacs called alveolar inside a pair of lungs. The lungs are situated next to the heart in the thoracic cavity and arc 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. intercontinental muscles are attached to the ribs, and a large diaphragm separates the thorax from the abdomen.

These are involved in the ventilation mechanism, as described in section. Air enters the body through two nostrils, each of which possesses a border 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 dour detected. Air passes from the nasal passages, through the pharynx and into the trachea. This is a tube which lies in front of the esophagus 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 esophagus. The cartilage prevents collapse of the tube during inspiration (breathing in). The cartilage can be seen in a section of the trachea. At its lower end the trachea splits into two bronchi. Within the lungs each bronchus subdivides many times into 

much smaller tubes called bronchi oles. 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 alveolar. A summary of these structures and their main features is provided in. The walls of most of the respiratory passage are lined with cilia ted 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 buccaneer cavity where the mucus is swallowed. Note that it is not the cilia which trap the particles don't confuse them with the hairs in the nose. Mucus also moistens the incoming air.