The most important function of the lungs is to extract oxygen from the environment and transfer it to the bloodstream.
Taking more than 6 million breaths per year, these integral parts of the human anatomy deserve our attention and respect.1
In this article, we will look at the form and function of the lungs; we will also learn about diseases that affect the lungs and how to maintain healthy lungs.
Contents of this article:
Here are some key points about the lungs. More detail and supporting information is in the main article.
- The left and right lungs are different sizes
- The lungs fill the majority of the chest cavity
- Each person takes more than 6 million breaths per year
- At the most basic level, lungs work like a set of bellows
- Without the production of surfactant, the lungs would collapse
- The lungs also play a part in regulating the acidity of the body
- Smoking tobacco is the biggest cause of lung-related complaints
- Lung tumors make up 15% of all diagnosed cancers and 30% of all cancer deaths
- There are a number of simple ways to help keep your lungs healthy.
Structure of the lungs
The left lung is smaller than the right lung.
The lungs are located in the chest, beneath the rib cage on either side of the heart. They are roughly conical in shape with a rounded point at their apex and a flat base where they meet the diaphragm.
Despite being a pair, the lungs are not equal in size and shape.
The left lung has an indentation where the heart resides (the cardiac notch), and the right lung is shorter to make space for the liver below.
Overall, the left lung has a slightly smaller weight and capacity than the right.
The lungs are contained within two membranes - the pulmonary pleurae. The inner layer directly lines 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.
Function of the lungs
The lungs' primary role is to bring in air from the atmosphere and pass life-giving oxygen into the bloodstream; from here, it can be circulated to the rest of the body.Lungs have no musculature of their own, and so the mechanics of breathing are reliant upon the muscles of the diaphragm (to which it is attached), the intercostal muscles (between the ribs) and the muscles of the abdomen and neck.
The diaphragm - a conical muscle that sits below the lungs - powers the majority of the work involved in breathing. As it contracts, it moves down, stretching the chest cavity and consequently increasing the lungs' capacity. This increase in volume decreases the pressure inside and sucks in air through the nose or mouth.
As the diaphragm relaxes and returns to its resting position, the volume of the lungs decreases, causing a pressure increase that expels the air.
The lungs are like bellows: as they expand, air is sucked in and, as they compress, carbon dioxide waste is pushed back out.
When air enters the nose or mouth, it travels down the trachea and beyond until it reaches a section called the carina. At the carina, the tube splits into two, creating two bronchi that lead to the left and right lungs.
From there, the pipe-like bronchi further split into smaller and smaller bronchioles. This ever-decreasing pipework eventually terminates in the alveoli. It is within the alveoli that gas exchange occurs.
The role of alveoli
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 (around 20 µm in diameter), each wrapped in a fine mesh of capillaries.
Each human has around 700 million individual alveoli. The total area of membrane that the alveoli provide is a staggering 75 m2, roughly equivalent to the area of half a tennis court.2
Blood from the rest of the body that has been sapped of its oxygen passes through the heart and travels to the alveoli's capillaries. The oxygen within the alveoli passes across a thin membrane and enters the blood; at the same time, the accumulated carbon dioxide in the blood enters the alveoli and is breathed back out into the atmosphere.
Blood travels away from the alveoli and the lungs fully saturated with oxygen.
Surfactant production in the lungs
Without surfactant, the lungs would collapse on expiration.
A compound known as pulmonary surfactant is produced by specific cells of the alveoli.
Constructed of lipid and protein building blocks, pulmonary surfactant has hydrophilic and hydrophobic regions (attracted to and repelled by water respectively).
This ingenious chemical serves a number of vital functions, one of which is keeping the lungs supple. However, surfactant's most vital role is preventing the lungs from collapsing in on themselves during expiration.
Each alveolus can be likened to a plastic bag that is wet inside. Under normal conditions, the bag would collapse in on itself and the internal sides would stick together. Surfactant prevents this from occurring.3
Pulmonary surfactant carries out its role by reducing the amount of surface tension and, by doing so, reducing the effort necessary to inflate the alveoli, minimizing their propensity to close up.
The production of surfactant does not begin until towards the end of the gestation period; it is for this reason that premature babies have trouble breathing, referred to as infant respiratory distress syndrome (RDS).
Other functions of the lungs
Although respiration is the most well-known role of the lungs, they carry out other important functions:
- Regulation of blood pressure: the lungs play a role in the renin-angiotensin system. Angiotensin I is converted to angiotensin II (a potent vasoconstrictor) within the lungs4
- pH balance: carbon dioxide can cause the body to become acidic. If a rise in acidity is detected, the lungs increase the rate of ventilation to expel more of the unwanted gas
- Filtering: small blood clots are filtered by the lung; they can also remove small air embolisms (bubbles) 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 which protects the lungs from some infections
- Mucociliary clearance: the mucus that lines the respiratory passages traps dust particles and bacteria. Cilia (tiny hair-like projections) move the offending items upwards to a position where they can be swallowed and destroyed by the digestive system
- Blood reservoir: the lungs can vary how much blood they contain at any moment in time. On average, around 9% of the total blood is within the lungs, and this level can be modified up to around 18% if necessary. This function can be life-saving if hemorrhage should occur; blood can then be moved from the lungs to the rest of the body
- Speech: without airflow, humanity would be without its favorite pastime.
Respiratory diseases can affect any part of the organs responsible for gaseous exchange, from the upper respiratory tract to the bronchi and down into the alveoli.
Diseases of the respiratory system are particularly common. For instance, there are approximately 1 billion cases of the common cold per year in the US.5
In 2010, there were around 6.8 million emergency department visits for respiratory disorders for patients under 18.6
Inflammatory lung diseases
Smoking is the most common cause of lung disease.
This group includes asthma (one of the deadliest lung diseases), cystic fibrosis, emphysema, chronic obstructive pulmonary disorder (normally caused by the damage smoking causes to the lungs7) and acute respiratory distress syndrome.
Asthma is characterized by a narrowing and swelling of the airways and the production of excess mucus. This triggers shortness of breath and wheezing.
No one knows why asthma affects some people and not others. It can be triggered by airborne allergens, infections, physical activity, cold air, some medications and stress.8
Restrictive lung diseases
This category is characterized by a loss of lung compliance; in other words, the lungs lose their elasticity and become stiffer. As a result, the maximum volume of air that they can hold is reduced, and inhaling is harder work.9
Respiratory tract infections
Infections can occur at any point in the respiratory tract and are often split into the following two categories:
- Upper respiratory tract infection: the most frequently contracted upper respiratory tract infection is the common cold. Also classed within this group are laryngitis, pharyngitis, ear infection, sinusitis and tonsillitis
- Lower respiratory tract infection: the most common infection in this category is pneumonia that is bacterial in origin. Other causes of pneumonia include tuberculosis, viruses and fungi. Complications can develop from these types of infections, including lung abscesses and the spread of infection to the pleural cavity.
Lung cancers make up 15% of all diagnosed cancers.
Tumors of the respiratory system can be divided into malignant and benign:
- Malignant tumors: 15% of all diagnosed cancers and 30% of all cancer deaths are due to malignant tumors. The majority of these tumors are caused by smoking. Because the entire output of blood travels from the heart through the lung,s it is common for the tumors to be swept up and develop in other regions of the body (metastasize)
- Benign tumors: benign tumors are a less common cause of respiratory disease. One example is hamartoma; they can compress surrounding tissue, but generally they are asymptomatic.
Pleural cavity diseases
The pleural cavity is the gap between the inner and outer pleural membranes that encase the lungs.
- Pleural effusion: a build-up of fluid within the pleural cavity. This can result from congestive heart failure or cirrhosis. Pleural effusion may also be due to inflammation of the pleura in conjunction with an infection
- Pneumothorax: a hole in the pleural membrane allowing air to escape from the lungs is called a pneumothorax. The affected lung can collapse like a balloon.
Pulmonary vascular disease
Pulmonary vascular diseases affect the vessels of the pulmonary circulation. Examples of pulmonary vascular diseases include:
- Pulmonary embolism: a blood clot can form elsewhere in the body and travel to the heart, lodging in the lungs. This can be fatal and cause a sudden death. More rarely, an embolism can consist of fat, amniotic fluid or air
- Pulmonary arterial hypertension: increased pressure in the pulmonary arteries. Often this is idiopathic (of unknown cause)
- Pulmonary edema: most often caused by congestive heart failure, fluid can leak from capillaries into the air spaces within alveoli
- Pulmonary hemorrhage: damaged and inflamed capillaries can leak blood into the alveoli. A symptom of pulmonary hemorrhage may be the coughing up of blood.
Maintaining lung health
Avoiding exercise outdoors on bad air days can help maintain lung health.
A set of organs as important as the lungs are well worth keeping in good working order.
Below are some words of advice from the American Lung Association10 and the Public Health Association of Canada.11
- Do not smoke: smoking tobacco is the leading cause of lung cancer, COPD, bronchitis and emphysema. Smoke narrows the airways, inflames the lung and destroys the tissues over time. It is never too late to stop smoking
- Avoid pollutant exposure: second-hand smoke and chemicals used in the garden or home can damage the lungs. Make your home a smoke-free area and wear a mask if using strong chemicals. Get your home checked for radon, a naturally occurring chemical thought to cause 21,000 lung cancer deaths annually in the US12
- Prevent infection: there are number of ways to help defend against respiratory infections, including washing hands, avoiding crowds in flu season, good oral hygiene and influenza and pneumonia vaccinations
- Check-ups: regular health checks, even when feeling well, can catch issues early on
- Control moisture: keep indoor humidity down (30-50%) 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
- Exercise: aerobic exercise improves lung capacity and staying fit can ward off other diseases that might impact the lungs.
A drug approved to treat melanoma has shown promise for the treatment of advanced nonsmall cell lung cancer, according to researchers from the University of California-Los Angeles.
Chronic obstructive pulmonary disease is a global public health concern, and the third leading cause of death worldwide. New research published in The Lancet Respiratory Medicine suggests that genetic factors, as well as smoking, may trigger lung disease.