The lungs are a pair of spongy, air-filled organs located on either side of the chest (thorax). The trachea (windpipe) conducts inhaled air into the lungs through its tubular branches, called bronchi. The bronchi then divide into smaller and smaller branches (bronchioles), finally becoming microscopic.
The bronchioles eventually end in clusters of microscopic air sacs called alveoli. In the alveoli, oxygen from the air is absorbed into the blood. Carbon dioxide, a waste product of metabolism, travels from the blood to the alveoli, where it can be exhaled. Between the alveoli is a thin layer of cells called the interstitium, which contains blood vessels and cells that help support the alveoli.
The lungs are covered by a thin tissue layer called the pleura. The same kind of thin tissue lines the inside of the chest cavity -- also called pleura. A thin layer of fluid acts as a lubricant allowing the lungs to slip smoothly as they expand and contract with each breath.
Chronic obstructive pulmonary disease (COPD): Damage to the lungs results in difficulty blowing air out, causing shortness of breath. Smoking is by far the most common cause of COPD.
Emphysema: A form of COPD usually caused by smoking. The fragile walls between the lungs' air sacs (alveoli) are damaged, trapping air in the lungs and making breathing difficult.
Chronic bronchitis: Repeated, frequent episodes of productive cough, usually caused by smoking. Breathing also becomes difficult in this form of COPD.
Pneumonia: Infection in one or both lungs. Bacteria, especially Streptococcus pneumoniae, are the most common cause.
Asthma: The lungs' airways (bronchi) become inflamed and can spasm, causing shortness of breath and wheezing. Allergies, viral infections, or air pollution often trigger asthma symptoms.
Acute bronchitis: An infection of the lungs' large airways (bronchi), usually caused by a virus. Cough is the main symptom of acute bronchitis.
Pulmonary fibrosis: A form of interstitial lung disease. The interstitium (walls between air sacs) become scarred, making the lungs stiff and causing shortness of breath.
Source: Web MD