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Anatomy and Function of the Normal Lung

To understand your lung condition, you should be familiar with how the lungs normally work.

How do the lungs normally work?

The chest contains two lungs, one lung on the right side of the chest, the other on the left side of the chest. Each lung is made up of sections called lobes. The lung is soft and protected by the ribcage. The purposes of the lungs are to bring oxygen (abbreviated O2), into the body and to remove carbon dioxide (abbreviated CO2). Oxygen is a gas that provides us energy while carbon dioxide is a waste product or "exhaust" of the body.

How do the lungs protect themselves?

The lungs have several ways of protecting themselves from irritants. First, the nose acts as a filter when breathing in, preventing large particles of pollutants from entering the lungs. If an irritant does enter the lung, it will get stuck in a thin layer of mucus (also called sputum or phlegm) that lines the inside of the breathing tubes. An average of 3 ounces of mucus are secreted onto the lining of these breathing tubes every day. This mucus is "swept up" toward the mouth by little hairs called cilia that line the breathing tubes. Cilia move mucus from the lungs upward toward the throat to the epiglottis. The epiglottis is the gate, which opens allowing the mucus to be swallowed. This occurs without us even thinking about it. Spitting up sputum is not "normal" and does not occur unless the individual has chronic bronchitis or there is an infection, such as a chest cold, pneumonia or an exacerbation of chronic obstructive pulmonary disease (COPD).

Another protective mechanism for the lungs is the cough. A cough, while a common event, is also not a normal event and is the result of irritation to the bronchial tubes. A cough can expel mucus from the lungs faster than cilia.

The last of the common methods used by the lungs to protect themselves can also create problems. The airways in the lungs are surrounded by bands of muscle. When the lungs are irritated, these muscle bands can tighten, making the breathing tube narrower as the lungs try to keep the irritant out. The rapid tightening of these muscles is called bronchospasm. Some lungs are very sensitive to irritants. Bronchospams may cause serious problems for people with COPD and they are often a major problem for those with asthma, because it is more difficult to breathe through narrowed airways.

How does air get into the body?

Photo 5To deliver oxygen to the body, air is breathed in through the nose, mouth or both. The nose is the preferred route since it is a better filter than the mouth. The nose decreases the amount of irritants delivered to the lung, whilst also heating and adding moisture (humidity) into the air we breathe. When large amounts of air are needed, the nose is not the most efficient way of getting air into the lungs and therefore mouth breathing may be used. Mouth breathing is commonly needed when exercising.

After entering the nose or mouth, air travels down the trachea or "windpipe". The trachea is the tube lying closest to the neck. Behind the trachea is the esophagus or "food tube". When we inhale air moves down the trachea and when we eat food moves down the esophagus. The path air and food take is controlled by the epiglottis, a gate that prevents food from entering the trachea. Occasionally, food or liquid may enter the trachea resulting in choking and coughing spasms.

The trachea divides into one left and one right breathing tube, and these are termed bronchi. The left bronchus leads to the left lung and the right bronchus leads to the right lung. These breathing tubes continue to divide into smaller and smaller tubes called bronchioles. The bronchioles end in tiny air sacs called alveoli. Alveoli, which means "bunch of grapes" in Italian, look like clusters of grapes attached to tiny breathing tubes. There are over 300 million alveoli in normal lungs. If the alveoli were opened and laid out flat, they would cover the area of a doubles tennis court. Not all alveoli are in use at one time, so that the lung has many to spare in the event of damage from disease, infection or surgery.

Which muscles help in the breathing process?

Many different muscles are used in breathing. The largest and most efficient muscle is the diaphragm. The diaphragm is a large muscle that lies under the lungs and separates them from the organs below, such as the stomach, intestines, liver, etc. As the diaphragm moves down or flattens, the ribs flare outward, the lungs expand and air is drawn in. This process is called inhalation or inspiration. As the diaphragm relaxes, air leaves the lungs and they spring back to their original position. This is called exhalation or expiration. The lungs, like balloons, require energy to blow up but no energy is needed to get air out.



Photo 7

The other muscles used in breathing are located between the ribs and certain muscles extending from the neck to the upper ribs. The diaphragm, muscles between the ribs and one of the muscles in the neck called the scalene muscle are involved in almost every breath we take. If we need more help expanding our lungs, we "recruit" other muscles in the neck and shoulders. In some conditions, such as emphysema, the diaphragm is pushed down so that it no longer works properly. This means that the other muscles must work extra hard because they aren’t as efficient as the diaphragm. When this happens, patients may experience breathlessness or shortness of breath.

Last Reviewed: February 2015