American Medical Association Family Medical Guide - American Medical Association [382]
The pleura
Each lung is surrounded by a thin membrane called the pleura. The pleura serves as a cover for the lungs and the inner surface of the rib cage and diaphragm (the sheet of muscle that arches into your chest cavity and separates the cavity from your abdomen). A tiny space called the pleural space between the two layers of the pleura contains a small amount of fluid that allows the two layers to slide easily against each other when you breathe.
Deep in the chest, the trachea divides into two main tubes called bronchi. Each bronchus leads to a lung, where the bronchus divides into smaller air passages called bronchioles. At the tip of each bronchiole are clusters of balloonlike structures called alveoli; the lungs contain about 300 million alveoli. The exchange of oxygen for carbon dioxide occurs through tiny blood vessels in the thin walls of the alveoli.
The body uses several muscles to draw air into the lungs. The diaphragm is a dome-shaped muscle attached to the lower ribs that separates the chest cavity from the abdomen. When the diaphragm contracts (along with other muscles between the ribs), the contraction creates a mild vacuum inside your lungs, causing the lungs to expand and draw air into the respiratory tract. When the muscles that help draw air into the lungs relax, the lungs spring back, forcing the air back out of the lungs.
The Nose
The nose is the uppermost part of the respiratory tract. Protective hairs just inside the nose trap large dust particles and other debris. A mucous membrane that contains many tiny blood vessels lines the nose. This lining cleans, moistens, and warms the air you breathe as it goes through the nasal passage toward your throat and lungs. The nasal passage is winding rather than straight, running along the top of the palate (the roof of the mouth) and turning down toward the throat. In several places, the nasal passage opens into sinuses (air-filled cavities in bones of the skull).
Respiration
How you breathe
When you breathe in, your diaphragm is pulled down and flattened (top left). The muscles between your ribs may also contract and pull your rib cage upward and outward (top right), expanding and drawing air into your lungs. The stronger this muscle action is, the more air that enters your lungs. Your breathing rate is determined by the amount of carbon dioxide expelled from your bloodstream. When you breathe out, your chest muscles and diaphragm relax (bottom left), making your rib cage sink (bottom right) and your lungs contract and squeeze out air.
Cross section of the nose
The nasal passage is linked to sinuses, which are air-filled cavities in the skull. The sensitive hairlike endings of the olfactory nerves project into the nasal passage. The olfactory nerves detect odors in the air and pass the information to the olfactory bulb, which transmits the information to the brain (which recognizes or perceives the odors).
Nasal Polyps
Nasal polyps are benign (noncancerous) growths of the mucous membrane that lines the nose. They protrude into the nasal cavity. Polyps result from an immune response that triggers fluid buildup and swelling in the lining of the nose. Nasal polyps develop most frequently in people who have chronic conditions such as asthma (see page 640) that cause recurring inflammation in the nasal cavity or in people who are sensitive to aspirin.
Symptoms
Nasal polyps don’t always cause symptoms. If they are large or numerous, they can obstruct the nasal passages, making breathing through the nose difficult and impairing your sense of smell and taste. You may have a headache or pain in your face if they cause an infection and block the opening between the nasal cavity