What is the function and structure of the lungs, and how to do a lung function test

Function

The lungs’ main role is to bring in air from the atmosphere and pass oxygen into the bloodstream. From there, it circulates to the rest of the body.

The organs require help from surrounding structures in the body in order to breathe properly. To breathe, we use the muscle of the diaphragm, the intercostal muscles between the ribs, the muscles of the abdomen, and sometimes even muscles in the neck.

The diaphragm is a muscle that is domed at the top and sits below the lungs. It powers most of the work necessary in breathing. As it contracts, it moves down, allowing more space in the chest cavity and increasing the lungs’ capacity to expand. As the chest cavity volume increases, the pressure inside goes down, sucking in air through the nose or mouth and down into the lungs.

As the diaphragm relaxes and returns to its resting position, the lung volume decreases because the pressure inside the chest cavity goes up, and the lungs expel the air. The lungs are like bellows — as they expand, they suck in air for oxygen. As they compress, the exchanged carbon dioxide waste is pushed back out during exhalation.

When air enters the nose or mouth, it travels down the trachea, also known as the windpipe. After this, it reaches a section called the carina. At the carina, the windpipe splits into two, creating two mainstem bronchi. One leads to the left lung and the other to the right.

From there, similar to branches on a tree, the pipe-like bronchi split again into smaller bronchi and then even smaller bronchioles. This ever-decreasing pipework eventually terminates in the alveoli, which are tiny air sacs. Here, gas exchange occurs.

Learn more about how the respiratory system works.

Structure

The lungs reside in the chest, behind the rib cage on either side of the heart. They are roughly conical in shape with a rounded point at their apex and a flatter base where they meet the diaphragm.

Although they are a pair, the lungs are not equal in size and shape. While the left lung has an indentation bordering where the heart resides, called the cardiac notch, the right lung is shorter to allow space for the liver below. Overall, the left lung has a slightly smaller weight and capacity than the right.

Two membranes, known as the pulmonary pleurae, surround the lungs. The inner layer directly lines the outer surface of the lungs, and the outer layer attaches to the inner wall of the rib cage. The space between the two membranes contains pleural fluid, which helps the pleura stay moist and reduces friction between the membranes when breathing.

Anatomy

Click on the BodyMap above to interact with a 3D model of the lungs.

Lung function tests

Lung function tests, also known as pulmonary function tests, check how well the lungs work and can help investigate breathing problems. For example, tests can help screen and diagnose lung conditions such as asthma and COPD. They can also measure levels of function, monitor lung conditions, and see if treatments are working.

Some examples of lung function tests include:

• Spirometry: This test measures how much and how quickly people can move air in and out of their lungs.
• Plethysmography: Also known as a lung volume test, this measures the amount of air individuals can hold in the lungs and how much air remains after breathing out.
• Gas diffusion test: This test can measure gases, such as oxygen, moving from the lungs to the bloodstream.
• Exercise stress test: This test typically involves a bike or treadmill to investigate how exercise affects lung function.

A doctor may also suggest some imaging scans to help diagnose, screen, and monitor lung function.

The alveoli

The alveoli are the endpoint of the oxygen’s journey from the outside atmosphere to the depths of the lungs. Alveoli are minute sacs that are microscopic in size, each wrapped in a fine mesh of capillaries. The total surface area of membrane that the alveoli provide is roughly 70 meters squared, or around the size of half a tennis court.

After the lungs, the body takes oxygen out of the bloodstream to its other tissues as it travels around the circulatory system. The blood that has given up its oxygen in exchange for carbon dioxide from the tissues then passes through the heart and travels to the lungs to reach the capillaries surrounding the alveoli.

The alveoli now contain a new supply of oxygen that a person has inhaled. This oxygen passes across a membrane, called the alveolar-capillary membrane, into the bloodstream. At the same time, the carbon dioxide that has collected in the bloodstream during its travels around the body enters the alveoli. From there, a person’s body breathes it back out into the atmosphere during exhalation.

Put simply, as the oxygen goes in, carbon dioxide comes out — an example of a gas exchange process.

Surfactant in the lungs

Special cells in the alveoli produce a compound known as pulmonary surfactant. It comprises lipids, proteins, and carbohydrates. Surfactant features both hydrophilic and hydrophobic regions — water attracts hydrophilic regions but repels hydrophobic regions.

Pulmonary surfactant has several vital functions, including allowing for better breathing efficiency and preventing the alveoli from collapsing on themselves. Each alveolus is similar to a plastic bag that is wet inside. If there were no surfactant, the bag would collapse in on itself and the internal sides would stick together. Surfactant prevents this from happening to the alveoli.

Pulmonary surfactant carries out its role by reducing the amount of surface tension. By doing this, it reduces the effort necessary to inflate the alveoli.

Other functions of the lungs

Respiration is the best-known role of the lungs, but they carry out other important functions, including:

• Balancing pH: Too much carbon dioxide can cause the body to become acidic. If the lungs detect a rise in acidity, they increase the ventilation rate to expel more of the unwanted gas, which helps balance pH.
• Protecting the body: The lungs help protect the body from harmful substances. For example, they can secrete immunoglobulin A, which can prevent infection. Additionally, the lungs can use mucociliary clearance. This involves mucus trapping pathogens and dust particles, after which small hair-like projections, known as cilia, move these particles upward so the body can either cough them out or swallow and destroy them using the digestive system.
• Speech: Without airflow, a person would be unable to speak.

Respiratory disease

Respiratory diseases can affect any part of the respiratory system, from the upper respiratory tract to the bronchi and down into the alveoli. Some examples of conditions that affect the lungs include the below.

Inflammatory lung diseases

This group includes:

• asthma
• cystic fibrosis
• acute respiratory distress syndrome
• chronic obstructive pulmonary disease (COPD), including emphysema and chronic bronchitis

COPD usually results from the damage that tobacco smoking causes to the lungs.

Asthma involves an obstructive narrowing and swelling of the airways and the production of excess mucus. This triggers shortness of breath and wheezing. Triggers can include:

• tobacco and wood smoke
• dust mites
• air pollution
• cockroach allergens
• mold
• stress
• infections
• some foods

Restrictive lung diseases

This means that the airway is restricted, so the amount of air a person can take in is reduced, and breathing in becomes harder. It can occur due to:

• the lungs becoming stiff
• airways filling with phlegm
• a problem with the chest wall or breathing muscles, for example, as with idiopathic pulmonary fibrosis
• a curvature of the spine
• obesity

Respiratory tract infections

Infections can occur at any point in the respiratory tract. Some examples include:

• Upper respiratory tract infection (URTI): The most frequent URTI is the common cold. Others include laryngitis, pharyngitis, and tonsillitis.
• Lower respiratory tract infection (LRTI): The most common type is bacterial infection, especially bacterial pneumonia. Other causes of LRTI include viruses and fungi.

Complications can develop from these types of infections, including lung abscesses and the spread of infection to the pleural cavity.

Lung cancer

Lung cancer is when cells in the lungs divide uncontrollably. This can cause tumors to grow, which can reduce a person’s ability to breathe. Evidence suggests lung cancer is the third most common cancer and the main cause of cancer-related death in the U.S.

Smoking is the most common cause of lung cancer, but other risk factors can include a family history of the disease and exposure to radiation or certain chemicals.

Pleural cavity diseases

The pleural cavity is the gap between the inner and outer pleural membranes that encase the outside of the lungs.

Pleural effusion describes a build-up of fluid in the pleural cavity. It always results from other conditions, such as cancer, congestive heart failure, or liver cirrhosis. A collapsed lung, also known as pneumothorax, occurs when air gets into the space between the chest wall and the lung, called the pleural space. This can compress the lungs, and when severe, it causes them to collapse like a balloon.

Pulmonary vascular disease

Pulmonary vascular diseases affect the vessels that carry blood through the lungs. Examples can include:

• Pulmonary artery embolism: A blood clot forms elsewhere in the body and travels in the bloodstream to the heart and then to the lungs, where it becomes lodged. This can result in sudden death. More rarely, an embolism can consist of fat, amniotic fluid, or air.
• Pulmonary arterial hypertension: Increased pressure can build up in the pulmonary arteries. Sometimes, the reasons for this are unclear.
• Pulmonary edema: This occurs when fluid leaks from capillaries into the air spaces within the alveoli. It most often happens due to congestive heart failure.

Tips for good lung health

Some ways of keeping the lungs healthy include:

• Not smoking: Smoking tobacco, including secondhand smoke, can lead to lung cancer and COPD, including chronic bronchitis and emphysema. Over time, it causes a narrowing of the airways, lung inflammation, and tissue damage.
• Prevent infection: Ways to prevent the spread of respiratory infections include washing hands, avoiding crowds in flu season, and asking your health provider about influenza and pneumonia vaccinations.
• Exercise: Aerobic exercise improves lung capacity, and staying fit can ward off other diseases that might affect the lungs.
• Check-ups: Regular health checks, even when feeling well, can detect problems in the early stages, when it is easier to treat them.
• Avoiding pollutant exposure: Garden or home chemicals may damage the lungs. With this in mind, wear a mask if using strong chemicals. Additionally, radon is a naturally occurring chemical that has associations with 21,000 lung cancer deaths a year in the U.S. Around 2,900 of these people have never smoked.
• Control moisture: Keep indoor humidity down to an acceptable level by using exhaust fans and vents. A person should also aim to keep moist surfaces clean and dry wherever possible and keep the home ventilated with external fresh air.