Lungs and Respiratory System

About the Lungs and Respiratory System

Breathing is so vital to life that it happens automatically. Each day, you breathe about 20,000 times, and by the time you're 70 years old, you'll have taken at least 600 million breaths.

All of this breathing couldn't happen without the respiratory system, which includes the nose, throat, voice box, windpipe, and lungs.

At the top of the respiratory system, the nostrils (also called nares) act as the air intake, bringing air into the nose, where it's warmed and humidified. Tiny hairs called cilia protect the nasal passageways and other parts of the respiratory tract, filtering out dust and other particles that enter the nose through the breathed air.

Air can also be taken in through the mouth. These two openings of the airway (the nasal cavity and the mouth) meet at the pharynx, or throat, at the back of the nose and mouth. The pharynx is part of the digestive system as well as the respiratory system because it carries both food and air. At the bottom of the pharynx, this pathway divides in two, one for food (the esophagus, which leads to the stomach) and the other for air. The epiglottis, a small flap of tissue, covers the air-only passage when we swallow, keeping food and liquid from going into the lungs.

The larynx, or voice box, is the uppermost part of the air-only pipe. This short tube contains a pair of vocal cords, which vibrate to make sounds.

The trachea, or windpipe, extends downward from the base of the larynx. It lies partly in the neck and partly in the chest cavity. The walls of the trachea are strengthened by stiff rings of cartilage to keep it open. The trachea is also lined with cilia, which sweep fluids and foreign particles out of the airway so that they stay out of the lungs.

Trachea and Bronchi

At its bottom end, the trachea divides into left and right air tubes called bronchi, which connect to the lungs. Within the lungs, the bronchi branch into smaller bronchi and even smaller tubes called bronchioles. Bronchioles end in tiny air sacs called alveoli, where the exchange of oxygen and carbon dioxide actually takes place. Each lung houses about 300-400 million alveoli.

The lungs also contain elastic tissues that allow them to inflate and deflate without losing shape and are encased by a thin lining called the pleura. This network of alveoli, bronchioles, and bronchi is known as the bronchial tree.

The chest cavity, or thorax, is the airtight box that houses the bronchial tree, lungs, heart, and other structures. The top and sides of the thorax are formed by the ribs and attached muscles, and the bottom is formed by a large muscle called the diaphragm. The chest walls form a protective cage around the lungs and other contents of the chest cavity.

Separating the chest from the abdomen, the diaphragm plays a lead role in breathing. It moves downward when we breathe in, enlarging the chest cavity and pulling air in through the nose or mouth. When we breathe out, the diaphragm moves upward, forcing the chest cavity to get smaller and pushing the gases in the lungs up and out of the nose and mouth.

What the Lungs and Respiratory System Do

The air we breathe is made up of several gases. Oxygen is the most important for keeping us alive because body cells need it for energy and growth. Without oxygen, the body's cells would die.

Carbon dioxide is the waste gas produced when carbon is combined with oxygen as part of the energy-making processes of the body. The lungs and respiratory system allow oxygen in the air to be taken into the body, while also enabling the body to get rid of carbon dioxide in the air breathed out.

Respiration

Respiration is the set of events that results in the exchange of oxygen from the environment and carbon dioxide from the body's cells. The process of taking air into the lungs is inspiration, or inhalation, and the process of breathing it out is expiration, or exhalation.

Air is inhaled through the mouth or through the nose. Cilia lining the nose and other parts of the upper respiratory tract move back and forth, pushing foreign matter that comes in with air (like dust) either toward the nostrils to be expelled or toward the pharynx. The pharynx passes the foreign matter along to the stomach to eventually be eliminated by the body. As air is inhaled, the mucous membranes of the nose and mouth warm and humidify the air before it enters the lungs.

When you breathe in, the diaphragm moves downward toward the abdomen, and the rib muscles pull the ribs upward and outward. In this way, the volume of the chest cavity is increased. Air pressure in the chest cavity and lungs is reduced, and because gas flows from high pressure to low, air from the environment flows through the nose or mouth into the lungs.

In exhalation, the diaphragm moves upward and the chest wall muscles relax, causing the chest cavity to contract. Air pressure in the lungs rises, so air flows from the lungs and up and out of respiratory system through the nose or mouth.

Every few seconds, with each inhalation, air fills a large portion of the millions of alveoli. In a process called diffusion, oxygen moves from the alveoli to the blood through the capillaries (tiny blood vessels) lining the alveolar walls. Once in the bloodstream, oxygen gets picked up by the hemoglobin in red blood cells. This oxygen-rich blood then flows back to the heart, which pumps it through the arteries to oxygen-hungry tissues throughout the body.

In the tiny capillaries of the body tissues, oxygen is freed from the hemoglobin and moves into the cells. Carbon dioxide, which is produced during the process of diffusion, moves out of these cells into the capillaries, where most of it is dissolved in the plasma of the blood. Blood rich in carbon dioxide then returns to the heart via the veins. From the heart, this blood is pumped to the lungs, where carbon dioxide passes into the alveoli to be exhaled.

Problems of the Lungs and Respiratory System

The respiratory system is susceptible to a number of diseases, and the lungs are prone to a wide range of disorders caused by pollutants in the air.

The most common problems of the respiratory system are:

Asthma. More than 20 million people in the United States have asthma, and it's the #1 reason that kids chronically miss school. Asthma is a chronic inflammatory lung disease that causes airways to tighten and narrow. Often triggered by irritants in the air such as cigarette smoke, asthma flares involve contraction of the muscles and swelling of the lining of the tiny airways. The resulting narrowing of the airways prevents air from flowing properly, causing wheezing and difficulty breathing, sometimes to the point of being life-threatening. Management of asthma starts with an asthma management plan, which usually involves avoiding asthma triggers and sometimes taking medications.

Bronchiolitis. Not to be confused with bronchitis, bronchiolitis is an inflammation of the bronchioles, the smallest branches of the bronchial tree. Bronchiolitis affects mostly infants and young children, and can cause wheezing and serious difficulty breathing. It's usually caused by specific viruses in the wintertime, including respiratory syncytial virus (RSV).

Chronic obstructive pulmonary disease (COPD). COPD is a term that describes two lung diseases — emphysema and chronic bronchitis:

• Long-term smoking often causes emphysema, and although it seldom affects kids and teens, it can have its roots in the teen and childhood years. Talking to your kids about smoking is a key part of preventing smoking-related diseases. In emphysema, the lungs produce an excessive amount of mucus and the alveoli become damaged. It becomes difficult to breathe and get enough oxygen into the blood.
• In bronchitis, a common disease of adults and teens, the membranes lining the larger bronchial tubes become inflamed and an excessive amount of mucus is produced. The person develops a bad cough to get rid of the mucus. Cigarette smoking is a major cause of chronic bronchitis in teens.

Other Conditions

Common cold. Caused by over 200 different viruses that cause inflammation in the upper respiratory tract, the common cold is the most common respiratory infection. Symptoms may include a mild fever, cough, headache, runny nose, sneezing, and sore throat.

Cough. A cough is a symptom of an illness, not an illness itself. There are many different types of cough and many different causes, ranging from not-so-serious to life-threatening. Some of the more common causes affecting kids are the common cold, asthma, sinusitis, seasonal allergies, croup, and pneumonia. Among the most serious causes of cough are tuberculosis (TB) and whooping cough (pertussis).

Cystic fibrosis (CF). Affecting more than 30,000 kids and young adults in the United States, cystic fibrosis is the most common inherited disease affecting the lungs. Affecting primarily the respiratory and digestive systems, CF causes mucus in the body to be abnormally thick and sticky. The mucus can clog the airways in the lungs and make a person more vulnerable to bacterial infections.

Lung cancer. Caused by an abnormal growth of cells in the lungs, lung cancer is a leading cause of death in the United States and is usually caused by smoking cigarettes. It starts in the lining of the bronchi and takes a long time to develop. Symptoms include a persistent cough that may bring up blood, chest pain, hoarseness, and shortness of breath. Radon gas exposure (radon is a gas that occurs in soil and rocks) may also be a cause of lung cancer. Radon is more likely to occur in certain parts of the United States. You can check your home's radon level by purchasing a radon kit at your local home supply or hardware store.

Pneumonia. This inflammation of the lungs usually occurs because of bacterial or viral infection. Pneumonia causes fever and inflammation of lung tissue, and makes breathing difficult because the lungs have to work harder to transfer oxygen into the bloodstream and remove carbon dioxide from the blood. Common causes of pneumonia are influenza and infection with the bacterium Streptococcus pneumoniae.

Pulmonary Hypertension

This condition occurs when the blood pressure in the arteries of the lungs is abnormally high, which means the heart has to work harder to pump blood against the high pressure. Pulmonary hypertension may occur in children because of a heart defect present at birth or because of a health condition such as HIV infection.

Respiratory Diseases of Newborns

Several respiratory conditions can affect a newborn baby just starting to breathe for the first time.  Premature babies are at increased risk for conditions such as:

• Respiratory distress syndrome of the newborn. Babies born prematurely may not have enough surfactant in the lungs. Surfactant helps to keep the baby's alveoli open; without surfactant, the lungs collapse and the baby is unable to breathe.
• Apnea of prematurity (AOP). Apnea is a medical term that means someone has stopped breathing. Apnea of prematurity (AOP) is a condition in which premature infants stop breathing for 15 to 20 seconds during sleep. Apnea of prematurity generally occurs after 2 days of life and up to a week of life. The lower the infant's weight and level of prematurity at birth, the more likely the baby is to have AOP spells.
• Bronchopulmonary dysplasia (BPD). BDP involves abnormal development of lung tissue. Sometimes called chronic lung disease or CLD, it's a disease in infants characterized by inflammation and scarring in the lungs. It develops most often in premature babies who are born with underdeveloped lungs.
• Meconium aspiration. Meconium aspiration occurs when a newborn inhales (aspirates) a mixture of meconium (baby's first feces, ordinarily passed after birth) and amniotic fluid during labor and delivery. The inhaled meconium can cause a partial or complete blockage of the baby's airways.

Additional Conditions

• Persistent pulmonary hypertension of the newborn (PPHN). In the uterus, a baby's circulation bypasses the lungs. Normally, when a baby is born and begins to breathe air, his or her body quickly adapts and begins the process of respiration. PPHN occurs when a baby's body doesn't make the normal transition from fetal circulation to newborn circulation. This condition can cause symptoms such as rapid breathing, rapid heart rate, respiratory distress, and cyanosis (blue-tinged skin).
• Transient tachypnea of the newborn (TTN). Rapid breathing in a full-term newborn (more than 60 breaths a minute) is called transient tachypnea.

Although some respiratory diseases can't be prevented, many chronic lung and respiratory illnesses can be prevented by avoiding smoking, staying away from pollutants and irritants, washing hands often to avoid infection, and getting regular medical checkups.

Reviewed by: Steven Dowshen, MD
Date reviewed: November 2009

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