RESPIRATORY SYSTEM
The Respiratory system is a network of organs and passages through 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 the air is filtered by 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 the nose and the mouth. This mucus is moved away by the beating of hair-like projections (cilia).
Then air travels through the throat (pharynx), voicebox (larynx) and the windpipe (trachea). At the entry to the windpipe is the flap, the epiglottis, which prevents choking when food is swallowed. The windpipe divides into two tubes (bronchi) and one bronchus enters each lung. Windpipe and bronchi are stiffened by ring 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 air sacs (alveoli) of the lung tissue. Gases are exchanged in the air sacs. Each air sac is meshed with small blood vessels (capillaries) which carry 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. Then the blood, rich in oxygen, flows into the pulmonary vein and back to the heart for distribution. The lungs are housed in a 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 subsides.
Respiration takes place 10 to 15 times a minute and is controlled by the 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, massages 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 and exhaling takes place.
Respiration is not a quiet process. The presence of many dust particles in the nose can trigger sneezing. Irritation or too much mucus in the windpipe and bronchi cause coughing.