Respiratory system is the network of organs and passages by which air is taken into the lungs and carbon dioxide and oxygen are exchanged within the body.
Air enters the respiratory system through the mouth and nose, where it is warmed and moistened. Then air is filtered by the coarse hairs that line the nostrils and trap large dust particles. Smaller particles are trapped in a sticky fluid (mucus) produced by the cells lining the passage between nose and the mouth. This mucus is moved away by the beating of minute hair-like projections (cilia).
Then air travels through the throat (pharynx), voice box (larynx), and windpipe (trachea). At the entry to the windpipe is a flap, the epiglottis, which prevents choking when food is swallowed. The windpipe divides into two tubes or bronchi, and one bronchus enters each lung. The windpipe and bronchi are stiffened by rings of cartilage.
Within each lung, the bronchi split into smaller bronchi and then into many bronchioles. The bronchioles branch through the lungs and lead into millions of air sacs (alveoli) of the lung tissue. Gases are exchanged in the air sacs. Each air sac is meshed with small blood vessels (capillaries) carrying carbon dioxide and water from the heart. Oxygen breathed in passes into the blood and carbon dioxide and water vapor are released into the air sacs to be breathed out. The blood, now rich in oxygen, flows into the pulmonary vein and back to the heart for redistribution.
The lungs are housed in bony cage made up of the ribs, breastbone, and backbone. The floor of the cage is formed by a sheet of muscle called the diaphragm. When a person breathes in the diaphragm is pulled downward, the ribcage is pulled up and out by contraction of the muscles between the ribs, and air rushes in. When a person breathes out, the diaphragm and rib muscles relax and the chest subside.
Respiration takes place 10 to 15 times a minute and is controlled by respiratory center, a collection of cells in the brain. The cells in the respiratory center are extremely sensitive to carbon dioxide concentrations. When the carbon dioxide reaches a certain level, messages are sent from the respiratory center to the diaphragm and rib muscles that trigger contraction. As the lungs expand during inhalation, stretch receptors send signals back to the respiratory center. The center instructs the muscles of ribs and diaphragm to relax so that exhaling takes place.
Respiration is not always a quiet process. The presence of many dust particles in the nose can trigger sneezing. Irritants or too much mucus in the windpipe and bronchi cause coughing.
Speech is also a special sort of “noisy” breathing. The sounds of speech are produced in the voice box (larynx) and molded into word in the mouth.
The larynx consists of a box of cartilage. Across the inside of the box are flap-like structures, the vocal cords. The cords move by the action of muscles attached to the cartilages of the larynx. During normal breathing, the vocal cords are held apart. For speech, the cords are pulled together after a breath in; during breathing out, the air is, thus, forced between the cords, making them vibrate. The tighter the cords are pulled together, the higher is the pitch of the sound produced. For loud sounds, the air is forced through the cords faster then it is for soft sounds. The movements of the lips and of the tongue against the teeth and roof of the mouth achieve articulation. The resonance of spoken sounds and the characteristics of every individual voice are created by the cavities of the sinuses, nose, throat, and chest.
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