The Respiratory System

   Breathing describes the process of inhaling and exhaling air. The exchange of gases (oxygen
   and carbon dioxide) between living cells and the environment is a process known as
   respiration. The respiratory system, which controls breathing and respiration, consists of the
   respiratory tract and the lungs.

   The respiratory tract cleans, warms, and moistens air on its way to the lungs. The tract can
   be divided into an upper and a lower part. The upper part consists of the nose, nasal cavity,
   pharynx (throat), larynx, and upper part of the trachea (windpipe). The lower part consists of
   the lower part of the trachea, bronchi, and lungs (which contain bronchioles and alveoli).

  The nose and nasal cavity

   The nose is the only external part of the respiratory system. It is made of bone and cartilage
   (tough connective tissue) and is covered with skin. The two openings to the outside, called
   nostrils, allow air to enter or leave the body during breathing. The nostrils are lined with
   coarse hairs that prevent large particles such as dust, insects, and sand from entering.

   The nostrils open into a large cavity, the nasal cavity. This cavity is divided into right and
   left cavities by a thin plate of bone and cartilage called the nasal septum. The hard portion
   of the palate forms the floor of the entire nasal cavity, separating it from the mouth or oral
   cavity below.

  The pharynx

   The pharynx or throat is a short, muscular tube extending about 5 inches (12.7 centimeters)
   from the nasal cavity and mouth to the esophagus and trachea. It serves two separate systems:
   the digestive system (by allowing the passage of solid food and liquids) and the respiratory
   system (by allowing the passage of air).

  The larynx

   The larynx, commonly called the voice box, forms the upper part of the trachea. The larynx is
   made of nine pieces of cartilage connected by ligaments. The largest of these cartilages is
   the shield-shaped thyroid cartilage, which may protrude at the front of the neck, forming the
   so-called Adam's apple. The upper cartilage is the epiglottis, a flaplike piece of tissue.
   During swallowing, the larynx rises up and the epiglottis folds down to cover the glottis, or
   the larynx's opening. This prevents food or liquids from passing into the lower respiratory
   tract.

   Mucous membrane lines the larynx. A pair of elastic folds in that lining form the vocal cords.
   During silent breathing, the vocal cords lie against the walls of the larynx. During speech,
   the cords are stretched across the opening of the larynx and air that passes through causes
   them to vibrate, generating sound waves. Various muscles produce tension on the cords, making
   them tighter (shorter) or looser (longer). The tighter the tension, the higher the pitch of
   the sound produced. Since men's larynges tend to be larger than women's, their vocal cords
   tend to be thicker and longer. The male voice thus tends to be lower in pitch.

   

  The trachea

   The trachea is a tough, flexible tube about 1 inch (2.5 centimeters) in diameter and 4.5
   inches (11.4 centimeters) in length. Located in front of the esophagus, it is the principal
   tube that carries air to and from the lungs. The walls of the trachea are supported by 16 to
   20 C-shaped cartilage rings. Elastic fibers in the tracheal walls allow the trachea to expand
   and contract during breathing, while the cartilage rings prevent it from collapsing. Mucous
   membrane containing cilia lines the trachea. The mucus produced by the membrane traps dust
   particles and other debris. The cilia move continuously in a direction opposite that of the
   incoming air, helping propel the mucus away from the lungs to the throat where it can be
   swallowed or spat out.

  The bronchi

   The trachea divides behind the sternum (breastbone) to form a right and left branch called
   primary bronchi (singular: bronchus). Each bronchus passes into a lung—the right bronchus into
   the right lung and the left bronchus into the left lung. The right bronchus is wider, shorter,
   and straighter than the left. As a result, accidentally inhaled objects (such as pieces of
   food) most often enter the right primary bronchus.

   By the time incoming air reaches the primary bronchi, it is warm, moistened, and cleansed of
   most particles or other impurities.

  The lungs

   The lungs are two broad, cone-shaped organs located on either side of the heart in the
   thoracic or chest cavity. They extend from the collarbones to the diaphragm, a membrane of
   muscle separating the thoracic cavity from the abdominal cavity. The base of each lung rests
   directly on the diaphragm. The rib cage forms a wall around the lungs, protecting them.

   At birth, the lungs are pale pink in color. As people age, their lungs grow darker. Each lung
   is divided into lobes separated by deep grooves or fissures. The right lung, which is larger,
   is divided into three lobes. The left lung is divided into only two lobes. Combined, the two
   soft and spongy lungs weigh about 2.5 pounds (1.1 kilograms).

   A membrane sac, called the pleura, surrounds and protects each lung. One layer of the pleura
   attaches to the wall of the thoracic cavity; the other layer encloses the lung. A fluid
   (pleural fluid) between the two membrane layers reduces friction and allows smooth movement of
   a lung during breathing.

   After the bronchi enter the lungs, they subdivide repeatedly into smaller and smaller bronchi
   or branches. Eventually they form thousands of tiny branches called bronchioles, which have a
   diameter of about 0.02 inch (0.5 millimeter). This branching network of bronchial tubes within
   the lungs is called the bronchial tree.

   The bronchioles branch to form even smaller passageways that open into clusters of cup-shaped
   air sacs called alveoli (singular: alveolus). The average person has a total of about 700
   million alveoli (which resemble clusters of grapes) in his or her lungs. These provide an
   enormous surface area-roughly the size of a tennis court—for gas exchange. A network of
   capillaries surrounds each alveolus. As blood passes through these vessels and air fills the
   alveoli, the exchange of gases takes place: oxygen passes from the alveoli into the
   capillaries while carbon dioxide passes from the capillaries into the alveoli.

   The membranes of the alveoli are extremely delicate and thin to allow the gases to pass easily
   through them. The inner lining of those membranes is coated with a thin layer of tissue fluid
   (a gas must be dissolved in a liquid in order to enter or leave a cell). To prevent the walls
   of the alveoli from sticking together (like the inside walls of a wet plastic bag), cells in
   the alveoli also produce an oily secretion, called pulmonary surfactant, that mixes with the
   tissue fluid (pulmonary refers to anything relating to or affecting the lungs).

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