The Respiratory System
The respiratory system is a network of organs and tissues that help you breathe. It includes your airways, lungs, and blood vessels. It also contains the muscles that power your lungs and the organs that surround them.
The Lungs
The lungs are the largest part of your respiratory system. They bring oxygen into your body and remove waste gases, like carbon dioxide. The lungs are on each side of your heart, inside your chest cavity. They have a special job: exchanging oxygen and carbon dioxide with the atmosphere to give your body more energy, help fight diseases, and protect you from infections. They do this in tiny air sacs called alveoli, which are surrounded by capillaries and are filled with oxygen. They also release immune factors to prevent infection and secrete immunoglobulin A, which helps your body defend itself against bacteria. They also use mucociliary clearance to trap and cough out inhaled dust and germs so that you can breathe them out through your trachea.
Protection of the Lungs
The lungs are protected from outside threats by thin layers of tissue called pleura. These layers surround each lung lobe and separate the lungs from the chest wall.
Breathing Mechanism
In the lungs, your breathing is controlled automatically by muscles that contract and relax to control how much air comes in and goes out of your lungs. The diaphragm, the largest muscle of your lungs, is the primary muscle that controls this movement. The other main breathing muscle is the bronchial muscle, which controls the movement of air in and out of your lungs. The bronchial muscle has a small, circular layer of smooth muscle that can change its size and shape by relaxing or contracting.
Air Pathway
When you inhale, the air moves through your trachea (windpipe) to reach your mouth and nose, where mucous membranes warm and moisturize the air. This warms the air and traps any particles of foreign matter. Your trachea then branches into two main tubes called the bronchi. Each bronchi has many smaller airways that branch off of it called bronchioles, which are as small as one-half of a millimeter (or 2/100 of an inch) across. The smallest bronchioles are lined with cilia, which move in waves to filter out dust and other irritants from your airways.
Location and Structure of Lungs
You can see the lining of your lungs in a mirror or in an X-ray machine, but you can't feel them. Your lungs are also visible in ultrasound or CT scans, but these tests may not reveal the fine details of your lungs. The lungs are located in the upper parts of your chest and are on either side of your heart. They are divided into three lobes on the right side and two lobes on the left. Each lobe has several airways, which resemble an upside-down tree. The larger airways, or bronchi, are held open by semiflexible, fibrous connective tissue called cartilage. Bronchioles branch off of the bronchi and travel to the air sacs or alveoli in your lungs. The air sacs are surrounded by capillaries, which carry oxygen from the blood to your lungs.
