How The Lungs Work
Your lungs are the pair of spongy, pinkish-gray organs in your chest.
When you inhale (breathe in), air enters your lungs, and oxygen from that air moves to your blood. At the same time, carbon dioxide, a waste gas, moves from your blood to the lungs and is exhaled (breathed out). This process, called gas exchange, is essential to life.
The lungs are the centerpiece of your respiratory system. Your respiratory system also includes the trachea (windpipe), muscles of the chest wall and diaphragm, blood vessels, and other tissues. All of these parts make breathing and gas exchange possible. Your brain controls your breathing rate (how fast or slow you breathe), by sensing your body’s need to get oxygen and also get rid of carbon dioxide.
The Respiratory System
Your lungs are on each side of your heart, inside your chest cavity. They are the main organs of the respiratory system. The right lung is divided into three lobes (sections), and the left lung is divided into two lobes. Your left lung is slightly smaller than your right lung, since your heart takes up some space on the left side. When you breathe in, air enters your airways and travels down into the air sacs, or alveoli, in your lungs. This is where gas exchange takes place.
The circulatory system, which is made up of the heart and blood vessels, supports the respiratory system by bringing blood to and from the lungs. The circulatory system helps deliver nutrients and oxygen from the lungs to tissues and organs throughout the body. It also helps remove carbon dioxide and waste products. Other body systems that work with the respiratory system include the nervous system, lymphatic system, and immune system.
The airways are pipes that carry oxygen-rich air to the alveoli in your lungs. They also carry the waste gas carbon dioxide out of your lungs. The airways include these body parts:
- Nose and linked air passages called the nasal cavity and sinuses
- Larynx (voice box)
- Trachea (windpipe)
- Tubes called bronchial tubes, or bronchi, and their branches
- Smaller tubes called bronchioles that branch off of the bronchial tubes
Air comes into your body
Air first enters your body through your nose or mouth, which moistens and warms the air since cold, dry air can irritate your lungs. The air then travels past your voice box and down your windpipe. Rings of tough tissue, called cartilage, acts as a support to keep the bronchial tubes open.
Inside your lungs, the bronchial tubes branch into thousands of thinner tubes called bronchioles. The bronchioles end in clusters of tiny air sacs called alveoli.
Air fills your lung’s air sacs
Your lungs have about 150 million alveoli. Normally, your alveoli are elastic, meaning that their size and shape can change easily. Alveoli are able to easily expand and contract because their insides are coated with a substance called surfactant. Surfactant reduces the work it takes to breathe by helping the lungs inflate more easily when you breathe in. It also prevents the lungs from collapsing when you breathe out.
Each of these alveoli is made up of a mesh of tiny blood vessels called capillaries. The capillaries connect to a network of arteries and veins that move blood through your body.
Blood low in oxygen flows through the lungs
The pulmonary artery and its branches deliver blood to the capillaries that surround the alveoli. This blood is rich in carbon dioxide and low in oxygen.
Oxygen flows into your blood
Carbon dioxide moves from the blood into the air inside the alveoli. At the same time, oxygen moves from the air into the blood in the capillaries.
The lungs are surrounded by the pleura, a membrane with two layers. The space between these two layers is called the pleural cavity. A slippery liquid called pleural fluid acts as a lubricant to reduce friction during breathing.
The muscles used for breathing
The lungs are like sponges; they cannot get bigger on their own. Muscles in your chest and abdomen tighten or contract to create a slight vacuum around the lungs. This causes air to flow in. When you exhale, the muscles relax and the lungs deflate on their own, much like an elastic balloon will deflate if left open to the air.
Your breathing muscles include:
- The diaphragm: This dome-shaped muscle below your lungs separates the chest cavity from the abdominal cavity. The diaphragm is the main muscle used for breathing.
- The muscles between your ribs: Called intercostal muscles, these muscles play a role in breathing during physical activity.
- Abdominal muscles: You use these muscles to help you breathe out when you are breathing fast, such as during physical activity.
- The muscles of the face, mouth, and pharynx: These control the lips, tongue, soft palate, and other structures to help with breathing. The pharynx is the part of the throat right behind the mouth. Problems with any of these muscles can narrow the airway, make it more difficult to breathe, and contribute to sleep apnea.
- Muscles in the neck and collarbone area: You use these muscles to help you breathe in.
Damage to the nerves in the upper spinal cord can interfere with the movement of your diaphragm and other muscles in your chest, neck, and abdomen. This can happen due to a spinal cord injury, a stroke, or a degenerative disease that affects the muscles, such as muscular dystrophy that causes muscle weakness or muscle loss. The damage can cause respiratory failure. Ventilator support or oxygen therapy may be necessary to maintain oxygen levels in the body and protect the organs from damage.
The nervous system
Your breathing usually does not require any thought, because it is controlled by the autonomic nervous system, also called the involuntary nervous system.
- The parasympathetic system slows your breathing rate. It causes your bronchial tubes to narrow and the pulmonary blood vessels to widen.
- The sympathetic system increases your breathing rate. It makes your bronchial tubes widen and the pulmonary blood vessels narrow.
Your breathing changes depending on how active you are and the condition of the air around you. For example, you need to breathe more often when you do physical activity. At times, you can control your breathing pattern, such as when you hold your breath or sing.
To help adjust your breathing to changing needs, your body has sensors that send signals to the breathing centers in the brain.
- Sensors in the airways detect lung irritants. The sensors can trigger sneezing or coughing. In people who have asthma, the sensors may cause the muscles around the airways in the lungs to contract. This makes the airways smaller.
- Sensors in the brain and near blood vessels detect carbon dioxide and oxygen levels in your blood.
- Sensors in your joints and muscles detect the movement of your arms or legs. These sensors may play a role in increasing your breathing rate when you are physically active.
When you breathe in, or inhale, your diaphragm contracts and moves downward. This increases the space in your chest cavity, and your lungs expand into it. The muscles between your ribs also help enlarge the chest cavity. They contract to pull your rib cage both upward and outward when you inhale.
As your lungs expand, air is sucked in through your nose or mouth. The air travels down your trachea, or windpipe, and into your lungs. After passing through your bronchial tubes, the air travels to the alveoli, or air sacs.
Every time you breathe in, oxygen from the air you inhale passes through the thin walls of the alveoli into the surrounding capillaries, where red blood cells pick it up using a protein called hemoglobin. At the same time, carbon dioxide, the waste gas carried back to the lungs from the cells of the body, trades places with the oxygen, moving from the blood in the capillaries back into the alveoli.
Blood loaded up with oxygen-rich red blood cells travels to the left side of the heart through the pulmonary veins. The heart then pumps the oxygenated blood to the rest of the body, where it moves from your blood vessels to your cells. The cells need this oxygen to make the energy your body needs to work. When cells make that energy, they create the waste product carbon dioxide. That carbon dioxide has to be removed from the blood and the body, which is why it is pushed from the cells back to the blood.
The carbon dioxide, once in the bloodstream, travels back to the heart, where it enters the right side. From there, it travels through the pulmonary artery to the lungs, where it flows from the capillaries back into the alveoli in exchange for the incoming oxygen. From the alveoli, the carbon dioxide is breathed back out.
When you breathe out, or exhale, your diaphragm and rib muscles relax, reducing the space in the chest cavity. As the chest cavity gets smaller, your lungs deflate, similar to how air releases from a balloon. At the same time, carbon dioxide-rich air flows out of your lungs through the windpipe and then out of your nose or mouth.
Breathing out requires no effort from your body unless you have a lung disease or are doing physical activity. When you are physically active, your abdominal muscles contract and push your diaphragm against your lungs even more than usual. This rapidly pushes air out of your lungs.
Conditions that affect the respiratory system
Damage, infection, or inflammation in the lungs or airways or both, can lead to the following conditions:
- Acute respiratory distress syndrome
- Hypersensitivity pneumonitis
- Idiopathic pulmonary fibrosis
- Pleural disorders
- Primary ciliary dyskinesia
- Sleep apnea
Exposure to cigarette smoke, air pollutants, or other substances can damage the airways, and can make a condition you already have more serious.