Respiratory System

The human respiratory system consists of the respiratory tract and the lungs. The respiratory tract can again be divided into an upper and a lower part. The upper part consists of the nose, nasal cavity, pharynx (throat) and larynx (voicebox). The lower part consists of the trachea (windpipe), bronchi, and bronchial tree. The respiratory tract cleans, warms, and moistens air during its trip to the lungs. The nose has openings to the outside that allow air to enter. Hairs inside the nose trap dirt and keep it out of the respiratory tract. The external nose leads to a large cavity within the skull. This cavity and the space inside the nose make up the nasal cavity. A nasal septum, supported by cartilage and bone, divides the nasal cavity into a right and left side. Epithelium, a layer of cells that secrete mucus and cells equipped with cilia, lines the nasal passage. Mucus moistens the incoming air and traps dust. The cilia move pieces of the mucus with its trapped particles to the throat, where it is spit out or swallowed. Stomach acids destroy bacteria in swallowed mucus. Sinuses, epithelium-lined cavities in bone, surround the nasal cavity. Blood vessels in the nose and nasal cavity release heat and warm the entering air.

Air leaves the nasal cavity and enters the throat or pharynx. From there it passes into the larynx, which is located between the pharynx and the trachea or windpipe. A framework of cartilage pieces supports the larynx, which is covered by the epiglottis, a flap of elastic cartilage that moves up and down like a trap door. When we breathe, the epiglottis stays open, but when we swallow, it closes. This valve mechanism keeps solid particles and liquids out of the trachea. If we breathe in something other than air, we automatically cough and expel it. Should these protective mechanisms fail, allowing solid food to lodge in and block the trachea, the victim is in imminent danger of asphyxiation.

Air enters the trachea in the neck. Epithelium lines the trachea as well as all the other parts of the respiratory tract. C-shaped cartilage rings reinforce the wall of the trachea and all the passageways in the lower respiratory tract. Elastic fibers in the trachea walls allow the airways to expand and contract when we inhale and exhale, while the cartilage rings prevent them from collapsing. The trachea divides behind the sternum to form a left and right bronchus, each entering a lung. Inside the lungs, the bronchi subdivide repeatedly into smaller airways. Eventually they form tiny branches called terminal bronchioles. Terminal bronchioles have a diameter of about 0.02 in (0.5 mm). The branching air-conducting network within the lungs is called the bronchial tree.

The respiratory tract is not dedicated to respiration alone but plays a major role in many other bodily functions as well. The pharynx in particular is a multipurpose organ. It is a passageway for food as well as air, since the mouth cavity also leads to it. The back of the pharynx leads into the esophagus (food tube) of the digestive system. The front leads into the larynx and the rest of the respiratory system. Small amounts of air pass between holes in the pharynx and the Eustachian tubes of the ear to equalize the gas pressure inside the ears, nose, and throat. The pharynx also contains lymph glands called tonsils and adenoids, which play a role in the immune system. Finally, the pharynx, which doubles as a resonating chamber, also plays a role in the production of sound, to which many other parts of the respiratory tract also contribute.

The vocal cords, a pair of horizontal folds inside the larynx, vibrate to produce sound from exhaled air. When we speak, muscles change the size of the vocal cords and the space between them, known as the glottis. The shape and size of the vocal cords determine the pitch of the sound produced. The glottis widens for deep tones and narrows for high-pitched ones. Longer, thicker vocal cords, which vibrate more slowly, produce a deeper sound. The force with which air is expelled through the larynx determines the volume of the sound produced. Voice quality also depends on several other factors, including the shape of the nasal cavities, sinuses, pharynx, and mouth, which all function as resonating chambers.