Your lungs are made of a spongy tissue divided into sections or lobes: The right lung has three lobes, and the left has two. A thin lining called the pleura encases the lungs.
Each day, between 2,500 and 5,000 gallons of air pass through your lungs. In your lungs, oxygen (the fuel all your cells need) passes into your blood, and carbon dioxide (the waste product of cell metabolism) is removed from it. The everyday act of respiration makes up approximately a quarter of your daily energy expenditure.
Air flows down your trachea, or windpipe, which divides into two branches called bronchi, one to each lung. The bronchi then divide into a series of smaller and smaller branching airways. These eventually reach millions of tiny, elastic, balloon-like sacs called alveoli. The alveoli are arranged in clusters. In the alveoli, oxygen passes from air in the lungs into the blood and carbon dioxide passes from the blood into the lungs. All this takes less than a second.
Although your lungs are internal organs, they are always exposed to the world outside you. The air that enters the lungs can contain pollens, dust, viruses, bacteria, tobacco smoke, animal dander and many other substances, some harmless and others not so benign. Mucus-secreting cells, cells with tiny hairs called cilia, and cells from the immune system line the airways and protect the lungs by trapping pollen, bacteria, viruses and dust to prevent them from entering the lungs. The airways are surrounded by muscle cells. If the airways become inflamed from an allergy or infection, for example, the mucus cells increase production and the muscles around the airways can tighten, narrowing the airway. These defenses lead to coughing and wheezing, some of the common symptoms of respiratory infections and asthma.
Once you reach adulthood, physical training seems to have little effect on making your lungs more efficient. But regular exercise does help your heart and muscles use oxygen more quickly and efficiently. That's why athletes don't breathe hard during moderate exertion.
After strenuous exercise, you feel "winded" and breathe deeper and faster because your blood becomes overloaded with carbon dioxide. When muscles burn sugar and oxygen to make energy, they produce carbon dioxide. When you exercise, these carbon dioxide levels increase. Sensors in major arteries detect the imbalance between carbon dioxide and oxygen. The brain sends out powerful signals to the muscles that control breathing, telling them to exhale as quickly as possible in order to expel the excess carbon dioxide.
From the very first drag, cigarette smoke begins to disable the lungs' defense mechanisms. Cigarette smoke slows -- and eventually kills -- the tiny hair-like cilia that line the air passages and push tiny particles up and out of the lungs. Deep in the lungs, cigarette smoke causes the alveoli to lose elasticity and destroys their walls. These changes lead to emphysema and chronic bronchitis, together called chronic obstructive pulmonary disease (COPD). Breathing chemical fumes, vapors and dust over time can also cause COPD. Smoking damages the cells that attack microscopic intruders such as viruses and bacteria. That's why people who smoke are more susceptible to colds and other infections in the upper respiratory tract, as well as pneumonia, which is an infection in the lung. Cigarette smoking is a cause of the most common type of lung cancer, bronchogenic carcinoma.
As you age, your lungs, like every organ in the body, also age. Because the airways lose elasticity, the lungs lose some of their ability to get air in and out. Most people lose about 40 percent of breathing capacity between the ages of 20 and 70, according to the National Institute on Aging. That's one reason strenuous exercise becomes more difficult for older people.
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