Lung

The lungs are the primary organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the backbone on either side of the heart. Their job is to take oxygen from the air we breathe and put it into our blood, while letting out carbon dioxide from the blood back into the air. This process helps our bodies get the air they need to stay alive and work properly.

Humans have two lungs, a right lung and a left lung. The right lung is bigger, while the left shares space with the heart. Together, they weigh about as much as a small bag of sugar. Inside the lungs, air travels through many tiny paths until it reaches very small air sacs called alveoli, where the oxygen and carbon dioxide are exchanged. The lungs are protected by thin layers that let them move easily as we breathe.

Lungs can be affected by several illnesses, such as pneumonia or lung cancer. Long-term problems like chronic obstructive pulmonary disease can happen from smoking or being around harmful fumes. Even before we are born, lungs start forming inside us, getting ready to help us breathe when we take our first breath at birth.

Structure

The lungs are the main organs that help animals, including humans, breathe. In humans, the lungs sit in the chest on either side of the heart, inside the rib cage. They have a cone shape, with a rounded top and a wide, flat base.

The left lung has a notch to make space for the heart. Both lungs have a central area where blood vessels and airways enter. The lungs are covered by thin layers of tissue called pleurae that keep them moist and help them move easily while breathing.

Each lung is divided into sections called lobes. The right lung has three lobes, and the left lung has two. These lobes help the lungs expand and contract smoothly as we breathe. Inside the lungs, air travels through a network of airways that end in tiny air sacs called alveoli. It is in these alveoli where oxygen from the air enters the bloodstream, and carbon dioxide from the bloodstream is released to be breathed out.

Main article: Respiratory tract

Development

Further information: Sonic hedgehog § Lung development

The human lungs start forming very early in the development before birth. They begin as a small bud near the top part of the digestive system and grow into a complex branching structure. This structure includes the tubes that carry air in and out of the body, and tiny air sacs where oxygen and carbon dioxide are exchanged.

After a baby is born, their lungs go through many more changes. At first, the lungs are filled with fluid and are not ready to work yet. But very soon after birth, the baby's body makes the first breath, and the fluid is removed. The lungs then start to work, getting bigger and more complex over many years as the child grows.

Function

Main articles: Respiratory system, Breathing, and Gas exchange

The lungs are important organs that help animals, including humans, breathe. They take in oxygen from the air we breathe and put it into our blood. At the same time, they take carbon dioxide from our blood and release it into the air when we breathe out. This process happens in tiny air sacs called alveoli, which give the lungs a very large surface area for this exchange.

The lungs cannot breathe on their own; they need help from muscles in our chest and abdomen. When we breathe in, muscles like the diaphragm and intercostal muscles work to expand the chest cavity, allowing the lungs to fill with air. When we breathe out, these muscles relax, and the lungs return to their normal size. Even when we breathe heavily during exercise, the lungs always keep some air inside. The lungs also help protect our bodies by trapping dust and germs in mucus, which is moved up and out by tiny hair-like structures called cilia. They also help regulate blood pressure and play a role in speaking by providing the air needed for our voices.

Gene and protein expression

Further information: Bioinformatics § Gene and protein expression

Our bodies have around 20,000 genes that help make important working parts called proteins. Almost three out of every four of these genes work in our lungs. A small group of about 200 genes work especially in the lungs, and only about 20 of these are found mostly in the lungs.

Some of the most important lung proteins help keep our airways open. These include proteins called SFTPA1, SFTPB, and SFTPC, as well as napsin. Other proteins help cells that have tiny hairs move things along, like the dynein protein DNAH5. There are also proteins such as SCGB1A1 that help make mucus in special cells called goblet cells.

Clinical significance

Main articles: Respiratory disease and Pulmonology

Lungs can be affected by many different health problems. Doctors who specialize in caring for lungs are called pulmonologists, and they study diseases that affect breathing. Some of these diseases include infections that make it hard to breathe, changes in the blood flow to the lungs, and conditions that make it difficult to move air in and out of the lungs.

Lungs can become inflamed or infected, which can happen when germs like bacteria or viruses make us sick. Other times, the lungs can be injured by things like blows to the chest or by breathing in harmful substances. Some people are born with lung problems, and others develop lung issues later in life due to things like smoking or long-term exposure to dust and other pollutants. There are also diseases that cause the lungs to become stiff or scarred, making it hard to breathe properly.

Culinary uses

Some people around the world eat the lungs from animals, along with other parts like the heart. This is known as offal. However, in the United States, rules prevent selling animal lungs as food because of health concerns.

Other animals

Birds

Main article: Bird anatomy § Respiratory system

Birds have small lungs connected to eight or nine air sacs that stretch through their bodies. When a bird inhales, air moves through its trachea into these air sacs. The air then flows through the lungs and out through front air sacs when the bird exhales. Bird lungs have many tiny passages called parabronchi where blood flows in a way that helps exchange gases efficiently.

Reptiles

Main article: Reptile anatomy § Respiratory system

Most reptiles have lungs with a single bronchus that branches into many pockets, giving the lungs a sponge-like texture. Some reptiles, like snakes, often have only one large lung. Crocodilians and monitor lizards have lungs similar to birds, with air moving in one direction and even having air sacs. Reptiles usually breathe by moving their ribs, while crocodiles also use a special muscle to help pull air into their lungs.

Amphibians

Further information: Frog § Respiration and circulation

Frogs and other amphibians have simple, balloon-like lungs. They breathe by using a method called buccal pumping, where they force air into their lungs by pushing their throat against their skull. Amphibians can also absorb gases through their skin when in water, which helps them get enough oxygen. Most salamanders do not have lungs and breathe through their skin.

Fish

Lungs are found in some fish groups like coelacanths, bichirs, and lungfish. Bichirs have simple lung sacs, while lungfish have more complex lungs with many chambers. In lungfish, the lungs are located in the upper part of the body, with a duct that curves around the esophagus.

Invertebrates

Further information: Respiratory system of gastropods

Some invertebrates have structures that work like lungs. Spiders and scorpions have book lungs for breathing air. The coconut crab uses special lungs to breathe on land. Land snails and slugs have developed simple lungs from their mantle cavity, with an opening called the pneumostome to take in air.

Evolutionary origins

The lungs of land animals and the gas bladders of fish today are thought to have started from simple sacs in early fish. These sacs were outpocketings of the oesophagus, allowing fish to gulp air when oxygen levels in water were low. In most ray-finned fish, these sacs became closed gas bladders, but in some fish like carp, trout, herring, catfish, and eels, the sac stays open to the oesophagus. In more basic bony fish such as the gar, bichir, bowfin, and lobe-finned fish, these sacs developed into lungs. The lobe-finned fish are the ancestors of land animals, called tetrapods. So, the lungs of vertebrates are similar in origin to the gas bladders of fish, though not to their gills.