The Respiratory System
Our lungs are the main organs of the respiratory system. The lungs are located inside the upper part of our chest on either side of the heart, and they are protected by the ribcage. The breastbone (sternum) is at the center front of the chest, and the spine is at the center of the back of the chest.
The inside of the chest cavity and the outside of the lungs are covered by the pleura, a slippery membrane that allows the lungs to move smoothly as they fill up with and empty out air when we inhale and exhale. Normally, there is a small amount of lubricating fluid between the two layers of the pleura. This helps the lungs glide inside the chest as they change size and shape during breathing.
With each breath, our lungs are filled with air that comes into our body through the nose or mouth. It flows down the throat (pharynx) and through the voice box (larynx). A small flap of tissue (epiglottis) covers the entrance to the larynx, and it automatically closes when we swallow to prevent food or liquids from getting into our airways.
Our largest airway is the windpipe (trachea), which is between three-and-a-half and six inches long and a little over half an inch in diameter. It brings air to the chest, where it branches into two smaller airways: the left and right bronchi, which lead to the left and right lungs.
The bronchi themselves divide many times into smaller and smaller airways (bronchioles). Because the pattern of these increasingly smaller passages looks like an upside-down tree, this part of the system is sometimes called the bronchial tree. The airways are held open by flexible, fibrous connective tissue called cartilage. Circular airway muscles can make the airways wider or narrower. The smallest bronchiole is only half a millimeter across.
At the end of each bronchiole are clusters of air sacs called alveoli. Each air sac is surrounded by a dense network of tiny blood vessels (capillaries). The extremely thin barrier between the air and the blood allows the blood to pick up oxygen and release carbon dioxide into the alveoli.