The most important function of the lungs is to take oxygen from the environment and transfer it to the bloodstream.
Taking more than 6 million breaths per year, the lungs affect every aspect of our bodies and health.
This article looks at the form and function of the lungs, diseases that affect the lungs, and how to maintain healthy lungs.
The lungs are located 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.
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.
The lungs are surrounded by two membranes, known as the pulmonary pleurae. The inner layer directly lines the outer surface of the lungs, and the outer layer is attached to the inner wall of the rib cage.
The space between the two membranes is filled with pleural fluid.
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.
Help is required from structures outside of the lungs 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 involved 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, and air is sucked in 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, air is sucked in 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 called 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 lung.
From there, like 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 little air sac endings.
Here, gas exchange occurs.
The alveoli are the end point of the oxygen’s journey from the outside world to the depths of the lungs.
Alveoli are minute sacs that are microscopic in size, each wrapped in a fine mesh of capillaries.
Each human has around 700 million individual alveoli. The total surface area of membrane that the alveoli provide is 70 meters squared. This is often said to be 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 the person has breathed in. 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, it is breathed back out into the atmosphere during exhalation.
Put simply, as the oxygen goes in, carbon dioxide comes out. This is gas exchange.
Surfactant has both hydrophilic and hydrophobic regions. Hydrophilic regions are attracted to water, and hydrophobic regions are repelled by water.
Pulmonary surfactant serves a number of vital functions.
- allowing for better breathing efficiency
- preventing the alveoli from collapsing on themselves
Each alveolus is like 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.
Before birth, surfactant production does not begin until the later weeks of gestation.
This is why babies who are born preterm have trouble breathing, referred to as infant respiratory distress syndrome (RDS).
Respiration is the best-known role of the lungs, but they carry out other important functions.
pH balance: Too much carbon dioxide can cause the body to become acidic. If the lungs detect a rise in acidity, they increase the rate of ventilation to expel more of the unwanted gas.
Filtering: The lungs filter small blood clots, and they can remove small air bubbles, known as air embolisms, if they occur.
Protective: The lungs can act as a shock absorber for the heart in certain types of collision.
Protection from infection: Certain membranes within the lungs secrete immunoglobulin A. This protects the lungs from some infections.
Mucociliary clearance: The mucus that lines the respiratory passages traps dust particles and bacteria. Tiny hair-like projections, known as cilia, move these particles upward to a position where they can be coughed out or swallowed and destroyed by the digestive system.
Blood reservoir: The lungs can vary how much blood they contain at any moment. This function can be useful, for example, during exercise. The amount of blood the lungs can contain can vary from 500 to 1,000 milliliters (ml). The lungs interact with the heart and can help the heart function more efficiently.
Speech: Without airflow, humanity would be without its favorite pastime.
Respiratory diseases can affect any part of the respiratory system, from the upper respiratory tract to the bronchi and down into the alveoli.
Diseases of the respiratory system are common. Every year, there are millions of cases of the common cold in the United States.
Inflammatory lung diseases
This group includes:
- 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.
- tobacco and wood smoke
- dust mites
- air pollution
- cockroach allergens
- some foods
No one knows exactly why asthma affects some people and not others.
Restrictive lung diseases
This means that the airway is restricted.
It can happen as a result of:
- the lungs becoming stiff
- a problem with the chest wall or breathing muscles, for example, as with idiopathic cystic fibrosis
- a curvature of the spine
The amount of air a person can take in is reduced, and breathing in becomes harder.
Respiratory tract infections
Infections can occur at any point in the respiratory tract. These may be described as:
Lower respiratory tract infection: The most common type is bacterial infection, and espeially bacterial pneumonia. Other causes of lower respiratory tract infection include viruses and fungi.
Complications can develop from these types of infections, including lung abscesses and the spread of infection to the pleural cavity.
Tumors of the respiratory system can be malignant or benign.
Malignant tumors: 14 percent of all new cancer diagnoses are primary lung cancer. Lung cancer is the second most common type of cancer and the leading cause of fatality from cancer in both men and women.
Most lung cancers are due to cigarette smoking. All the blood in the body travels from the heart through the lungs, so the cancer can easily spread to other parts of the body.
Benign tumors: Benign tumors are a less common cause of respiratory disease. One example is hamartoma. These can compress surrounding tissue, but they are usually asymptomatic.
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: Fluid builds up in the pleural cavity, often due to cancer in or near the chest cavity. It could also be related to congestive heart failure or liver cirrhosis. Other causes include inflammation of the pleura, which may occur with an infection.
Pneumothorax: This can result from trauma, for example, a bullet wound. Air inside the pleural cavity is called a pneumothorax. This compresses 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.
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 most often results from congestive heart failure. Fluid leaks from capillaries into the air spaces within the alveoli.
Pulmonary hemorrhage: Damaged and inflamed capillaries can leak blood into the alveoli. A symptom may be the coughing up of blood.
Ways of keeping the lungs healthy include:
Not smoking: Smoking tobacco, both first and second hand, can lead to lung cancer and COPD, including chronic bronchitis and emphysema. Smoking leads to a narrowing of the airways, it inflames the lungs and destroys the tissues over time. Make your home a smoke-free area.
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 impact 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: Chemicals used in the garden or home can damage the lungs. Wear a mask if using strong chemicals. Radon is a naturally occurring chemical that has been linked to 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. Keep moist surfaces clean and dry wherever possible. Keeping the home ventilated with external fresh air is a good idea.