Respiratory Medicine for Nurses and Paramedics T Balasubramanian
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Anatomy of Respiratory System1

The organs of the respiratory system extend from the nose to the lungs and are divided into the upper and lower respiratory tracts. The upper respiratory tract consists of the nose and the pharynx, or throat. The lower respiratory tract includes the larynx, or voice box; the trachea, or windpipe, which splits into two main branches called bronchi; which further subdivides into smaller bronchioles; and the lungs, a pair of saclike, spongy organs. The nose, pharynx, larynx, trachea, bronchi, and bronchioles conduct air to and from the lungs. The lungs interact with the circulatory system to deliver oxygen and remove carbon dioxide.
 
NOSE
The flow of air from outside of the body to the lungs begins with the nose, which is divided into the left and right nasal passages. The nasal passages are lined by a membrane known as nasal mucosa, composed primarily of one layer of flat, closely packed cells called epithelial cells. Each epithelial cell is densely covered with thousands of microscopic cilia, fingerlike extensions of the cells. Interspersed among the epithelial cells are goblet cells, specialized cells that produce mucus, a sticky, thick, moist fluid that coats the epithelial cells and the cilia. Numerous tiny blood vessels called capillaries lie just under the mucous membrane, near the surface of the nasal passages. While transporting air to the pharynx, the nasal passages play two critical roles: they filter the air to remove potentially disease-causing particles; and they moisten and warm the air to protect the structures in the respiratory system.
Filtering prevents airborne bacteria, viruses, other potentially disease-causing substances from entering the lungs, where they may cause infection. Filtering also eliminates smog and dust particles, 2which may clog the narrow air passages in the smallest bronchioles. Coarse hairs found just inside the nostrils of the nose trap airborne particles as they are inhaled. The particles drop down onto the mucous membrane lining the nasal passages. The cilia embedded in the mucous membrane wave constantly, creating a current of mucus that propels the particles out of the nose or downward to the pharynx. In the pharynx, the mucus is swallowed and passed to the stomach, where the particles are destroyed by stomach acid. If more particles are in the nasal passages than the cilia can handle, the particles build up on the mucus and irritate the membrane beneath it. This irritation triggers a reflex that produces a sneeze to get rid of the polluted air.
The nasal passages also moisten and warm air to prevent it from damaging the delicate membranes of the lung. The mucous membranes of the nasal passages release water vapor, which moistens the air as it passes over the membranes. As air moves over the extensive capillaries in the nasal passages, it is warmed by the blood in the capillaries. If the nose is blocked or “stuffy” due to a cold or allergies, a person is forced to breathe through the mouth. This can be potentially harmful to the respiratory system membranes, since the mouth does not filter, warm, or moisten air.
In addition to their role in the respiratory system, the nasal passages house cells called olfactory receptors, which are involved in the sense of smell. When chemicals enter the nasal passages, they contact the olfactory receptors. This triggers the receptors to send a signal to the brain, which creates the perception of smell.
 
PHARYNX
Air leaves the nasal passages and flows to the pharynx, a short, and funnel-shaped tube about 13 cm (5 in) long that transports air to the larynx. Like the nasal passages, the pharynx is lined with a protective mucous membrane and ciliated cells that remove impurities from the air. In addition to serving as an air passage, the pharynx houses the tonsils, lymphatic tissues that contain white blood cells. The white blood cells attack any disease-causing organisms that escape the hairs, cilia, and mucus of the nasal passages and pharynx. The tonsils are strategically located to prevent these organisms from moving further into the body. One tonsil, called the adenoids, is found high in the rear wall of the pharynx.3
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Fig 1.1: Anatomy of respiratory system
A pair of tonsils, the palatine tonsils, is located at the back of the pharynx on either side of the tongue. Another pair, the lingual tonsils, is found deep in the pharynx at the base of the tongue. In their battles with disease-causing organisms, the tonsils sometimes become swollen with infection. When the adenoids are swollen, they block the flow of air from the nasal passages to the pharynx, and a person must breathe through the mouth (Fig. 1.1).
 
LARYNX
Air moves from the pharynx to the larynx, a structure about 5 cm (2 in) long located approximately in the middle of the neck. Several layers of cartilage, a tough and flexible tissue, comprise most of the larynx. A protrusion in the cartilage called the Adam's apple sometimes enlarges in males during puberty, creating a prominent bulge visible on the neck.4
While the primary role of the larynx is to transport air to the trachea, it also serves other functions. It plays a primary role in producing sound; it prevents food and fluid from entering the air passage to cause choking; and its mucous membranes and cilia-bearing cells help filter air. The cilia in the larynx waft airborne particles up toward the pharynx to be swallowed.
Food and fluids from the pharynx usually are prevented from entering the larynx by the epiglottis, a thin, leaf like tissue. The “stem” of the leaf attaches to the front and top of the larynx. When a person is breathing, the epiglottis is held in a vertical position, like an open trap door. When a person swallows, however, a reflex causes the larynx and the epiglottis to move toward each other, forming a protective seal, and food and fluids are routed to the esophagus. If a person is eating or drinking too rapidly, or laughs while swallowing, the swallowing reflex may not work, and food or fluid can enter the larynx. Food, fluid, or other substances in the larynx initiate a cough reflex as the body attempts to clear the larynx of the obstruction. A surgical procedure called a tracheotomy is used to bypass the larynx and get air to the trachea in extreme cases of choking.
 
TRACHEA
Air passes from the larynx into the trachea, a tube about 12 to 15 cm (about 5 to 6 in) long located just below the larynx. The trachea is formed of 15 to 20 C-shaped rings of cartilage. The strong cartilage rings hold the trachea open, enabling air to pass freely at all times. The open part of the C-shaped cartilage lies at the back of the trachea, and the ends of the “C” are connected by muscle tissue.
The base of the trachea is located a little below where the neck meets the trunk of the body. Here the trachea branches into two tubes, the left and right bronchi, which deliver air to the left and right lungs, respectively. The junction where trachea divides into right and left bronchi is known as the carina. Within the lungs, the bronchi branch into smaller tubes called bronchioles. The trachea, bronchi, and the first few bronchioles contribute to the cleansing function of 5the respiratory system, and they, too, are lined with mucous membranes and ciliated cells that move mucus upward to the pharynx.
 
LUNGS
The lungs are the most important organ of respiration. This is where blood receives its oxygen supply. They are two in number, placed inside the chest cavity (thorax). Lungs float in water when immersed because of the presence of air inside it. The two lungs are separated from each other by the heart and the big blood vessels like the aorta. The lungs are approximately shaped like a cone with the apex pointing up. The inner surface (medial surface) of the lung has a depressed area known as the hilum through which structures like bronchi and blood vessels enter and leave the lungs. The whole lung is covered by a membrane known as pleura. This membrane is double layered and there is a small space between these layers known as the pleural cavity. Secretions known as pleural fluid fills this space lubricating the lungs making it free to inflate and deflate. The lungs receive its blood supply through the pulmonary artery which carries impure blood for oxygenation, while the pure / oxygenated blood is carried out of the lungs through pulmonary veins. This is the only place in the body where vein carries oxygenated blood and artery carries deoxygenated blood. The pulmonary artery arises directly from the right ventricle of the heart, while the pulmonary vein coming out of the lungs reaches the left atrium delivering pure blood to the heart. This enables the left side of the heart to pump pure blood through the aorta.
Each of these lungs is divided into lobes. The left lung has two lobes and the right has three lobes. The main bronchi on entering the lung through its hilum divides into two main branches on the right side and three main branches on the left to supply the lobes.
The bronchioles divide many more times in the lungs to create an impressive tree with smaller and smaller branches, some no larger than 0.5 mm (0.02 in) in diameter. These branches dead-end into tiny air sacs called alveoli. The alveoli deliver oxygen to the circulatory system and remove carbon dioxide. Interspersed among the alveoli are numerous macrophages, large white blood cells that patrol the alveoli and remove foreign substances that have not been filtered out 6earlier. The macrophages are the last line of defense of the respiratory system; their presence helps ensure that the alveoli are protected from infection so that they can carry out their vital role.
The alveoli number about 150 million per lung and comprise most of the lung tissue. Alveoli resemble tiny, collapsed balloons with thin elastic walls that expand as air flows into them and collapse when the air is exhaled. Alveoli are arranged in grapelike clusters, and each cluster is surrounded by a dense hairnet of tiny, thin-walled capillaries. The alveoli and capillaries are arranged in such a way that air in the wall of the alveoli is only about 0.1 to 0.2 microns from the blood in the capillary. Since the concentration of oxygen is much higher in the alveoli than in the capillaries, the oxygen diffuses from the alveoli to the capillaries. The oxygen flows through the capillaries to larger vessels, which carry the oxygenated blood to the heart, where it is pumped to the rest of the body.
Carbon dioxide that has been dumped into the bloodstream as a waste product from cells throughout the body flows through the bloodstream to the heart, and then to the alveolar capillaries. The concentration of carbon dioxide in the capillaries is much higher than in the alveoli, causing carbon dioxide to diffuse into the alveoli. Exhalation forces the carbon dioxide back through the respiratory passages and then to the outside of the body.
 
SUMMARY
  1. Respiratory system extends from the nose to the lungs.
  2. Respiratory system can be divided into upper and lower respiratory tracts. The upper respiratory tract extends from the nose to pharynx, and the lower respiratory tract begins from the larynx and extends up to the terminal bronchioles.
  3. Nose protects the lower airway from particulate matter and micobes.
  4. The nasal mucosa moistens the inspired air. This process is also known as air conditioning.
  5. Strategically located lymphoid tissue known as the tonsils protects the lower airway from microbes.
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  6. Larynx is made up of layers of cartilage and it protects the lower airway from food particles and saliva.
  7. Lungs are two air filled sacs which inflate and deflate depending on whether patient is inspiring or expiring.
  8. Alveoli are small air sacs in which the small terminal bronchioles end. Gas exchange occurs here.
  9. Lungs are supplied by blood directly from the heart. It is the only organ which receives impure blood via the pulmonary artery and sends out pure blood through the pulmonary vein.