Lungs

The lungs are spongy organs that surround the heart in the midline of the thoracic cavity. Air from the oral and nasal passages enters the lungs via the trachea which branches into two bronchi as it enters the lungs. The bronchi branch further inside the lungs becoming bronchioles which terminate at alveoli, clusters of air sacs where oxygen and carbon dioxide are exchanged with tiny blood vessels. The action of breathing is a muscular operation, the muscles involved are: the diaphragm which separates the thoracic from the abdominal cavity, and the intercostal muscles found between the ribs (bone). These muscles, when contracted, increase the thoracic cavity size during inhalation to allow for the air to rush in.