The lungs are the site of gas exchange between the blood and the air you breathe. The lungs are divided into lobes and smaller regions called segments based on how the supplying airways branch off. The right lung has three lobes, designated the upper lobe, middle lobe and lower lobe. The left lung, on the other hand, only has two lobes, the upper lobe and the lower lobe.
When you breathe, the air goes through your nose or mouth into a structure in your throat called the trachea. The airway is called the trachea until it splits into two bronchi. These bronchi then keep splitting into smaller and smaller bronchioles. The airways are sometimes called the bronchial tree due to their extensive branching pattern.
The terminal points of the airways are really small air sacs that are one cell thick, called alveoli. The alveoli are arranged into clusters, like grapes. The purpose of the alveoli is to make contact with capillaries to exchange gases between the blood and the atmosphere. Capillaries are really small blood vessels. Oxygen gets transferred from the air in the alveoli to a red blood cell in the capillaries, which travels back to the heart to get pumped out to the body. Carbon dioxide is also released from red blood cells into the alveoli so it can be breathed out and removed from the body.
The lungs have a rich blood supply. The capillaries converge to form larger blood vessels. The main large blood vessels going to the lungs are called the pulmonary arteries and the blood vessels returning to the heart from the lungs are called the pulmonary veins.
The lungs draw air in through negative pressure. There is a large, flat muscle called the diaphragm that lies directly underneath the lungs. When the diaphragm contracts, it makes the chest cavity larger and the lungs expand and fill with air. When the diaphragm starts to relax, it goes back up into its original position, making the chest cavity smaller and pushing the air back out of the lungs. There are other muscles in the body that can help with breathing, such as the muscles that attach to the ribs. These muscles, which are not as vital as the diaphragm to the process of breathing, are known as accessory breathing muscles.