Respiratory System Health Article

Larynx

Air passes from the pharynx to the larynx, which is approximately 5 cm (2 inches) long and situated near the middle of the neck. The larynx is comprised of several layers of cartilage, a tough and flexible tissue.

In addition to transporting air to the trachea, the larynx serves such other functions as:

• It prevents food and fluid from entering the air passage which would cause choking.
• Its mucous membranes and cilia-bearing cells help filter air.
• It plays a primary role in producing sound.
• The cilia in the larynx move airborne particles up toward the pharynx to be swallowed.

A thin, leaflike flap of tissue called the epiglottis prevents food and fluids from entering the larynx from the pharnyx. The epiglottis is held in a vertical position, like an open trap door when a person is breathing. When swallowing, a reflex forces the larynx and the epiglottis to move toward each other. This reflex diverts food and fluids to the esophagus. The swallowing reflex may not work if one eats or drinks too rapidly, or laughs while swallowing. Food or fluid enters the larynx and a coughing reflex is initiated to clear the obstruction. This situation may cause life-threatening choking if coughing does not clear the larynx of the obstruction.

Trachea, bronchi, and bronchioles

Air is passed from the larynx into the trachea, the largest airway in the respiratory system. The trachea is a tube located just below the larynx, approximately 12 to 15 cm (5 to 6 inches) long. Fifteen to twenty C-shaped rings of cartilage form the trachea. Air passes freely at all times because the trachea is held open by the rings of sturdy cartilage. The open part of the C-shaped cartilage rings is situated at the back of the trachea with the ends connected by muscle tissue. The trachea branches into two tubes at its base, located just below where the neck meets the trunk of the body. These two tubes are called the left and right bronchi and they deliver air to the left and right lungs, respectively. The bronchi branch into smaller tubes called bronchioles within the lungs. The trachea, bronchi, and the first few bronchioles are lined with mucous membranes and ciliated cells; thus they contribute to the cleansing action of the respiratory system by moving mucus upward to the pharynx.

Alveoli and lungs

The bronchioles divide many more times in the lungs into an upside-down tree-like structure with progressively smaller branches. Tiny air sacs called alveoli are at the end of the branches. Some of the bronchioles are no larger than 0.5 mm (0.02 inches) in diameter. The alveoli comprise most of the lung tissue, with about 150 million alveoli per lung, and resemble bunches of grapes. The alveoli send oxygen to the circulatory system while removing carbon dioxide. Alveoli have thin elastic walls, thus allowing air to flow into them when they expand; they collapse when the air is exhaled. Alveoli are arranged in clusters, and each cluster is surrounded by a dense network of capillaries. The walls of the capillaries are very thin; thus the air in the wall of the alveoli is very near to the blood in the capillaries (only about 0.1 to 0.2 microns). Carbon dioxide is a waste product that is dumped into the bloodstream from the cells. It flows throughout the body in the bloodstream to the heart, and then to the alveolar capillaries. The oxygen diffuses from the alveoli to the capillaries since the concentration of oxygen is much higher in the alveoli than in the capillaries. From the capillaries, the oxygen flows into larger vessels and is then carried to the heart where it is pumped to the rest of the body. The forces of exhalation cause the carbon dioxide to go back up through the respiratory passages and out of the body. Numerous macrophages are interspersed among the alveoli. Macrophages are large white blood cells that remove foreign substances from the alveoli that have not been previously filtered out. The presence of the macrophages ensures that the alveoli are protected from infection; they are the last line of defense of the respiratory system.

The lungs are the largest organ in the respiratory system and resemble large pink sponges. The left lung is slightly smaller than the right lung since it shares space with the heart, which is also located in the left side of the chest. Each lung is divided into lobes, with two in the left lung and three in the right. A slippery membrane called the pleura covers the lungs and lines the inside of the chest wall. It helps the lungs move smoothly during each breath. Normally, the two lubricated layers of the pleura have very little space between them. They glide smoothly over each other when the lungs expand and contract.

The diaphragm is the most important muscle involved in respiration. It lies just under the lungs and is a muscle shaped like a large dome. The sternum (or breastbone), ribs, and spine protect the lungs and the other organs in the chest. Twelve pairs of ribs curve around the chest and are joined to the vertebrae of the spine. The intercostal muscles are also important for respiration. They lie between the ribs and assist in breathing by helping to move the rib cage.