Each lung is between 10 to 12 inches long. The lung on the left is a bit smaller than the lung on the right because it has to make room for the heart to fit in the chest. Each lung weighs 1.1 kilograms, therefore making the entire organ about 2.3 kilograms.

Since the lungs process air, they are the only internal organs that are constantly exposed to the external environment. The air that we breathe contains several gases, including oxygen that is important for keeping us alive, because body cells need oxygen for energy and growth. With each breath, our lungs add fresh oxygen to our blood, which then carries it to our cells. The lungs also enable our body to get rid of carbon dioxide when the air is breathed out.

Respiration is the set of events that results in the exchange from the atmosphere 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. Lungs breathe in between 2100 and 2400 gallons of air, each day.

Occupying most of the space within the thoracic cavity, the lungs extend laterally from the heart to the ribs on both sides of the chest and continue posteriorly towards the spine. Each soft, spongy lung is roughly cone-shaped. The base of the lungs is concave to follow the contour of the diaphragm. The left lung is slightly smaller than the right lung because 2/3 of the heart is located on the left side of the body. Each lung consists of several distinct lobes. The right lung (the larger of two) has 3 lobes while the smaller left lung has only two lobes.

At the bottom of the trachea, or windpipe, there are two large tubes called the main stem bronchi. One of the heads into the left lung, while the other heads into the right lung. Inside each lung, the bronchus divides into smaller and still smaller tubes, like branches on a big tree. The tiniest tubes, called bronchioles, eventually end in clusters of microscopic air sacs called alveoli. Each alveolus has a mesh-like covering of very small blood vessels called capillaries.

The air that we inhale through our mouth and nose goes down our windpipe. On the way, tiny hairs called cilia filter out the mucus and dirt. Passing through the series of bronchi and bronchioles, the inhaled air finally ends up in the alveoli where oxygen from the air is absorbed into the blood in the tiny capillaries. From here, the oxygen hitches a ride on red blood cells and travels through layers of blood vessels to the heart. The heart then sends the oxygenated blood out to all the cells in the body. Carbon dioxide, a waste product of metabolism, travels in the reverse order - through the bronchioles, out the bronchi, out the trachea, and finally out through our mouth and nose.