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 in oxygen from the air we breathe and put it into the blood, while letting out carbon dioxide from the blood back into the air. This process, called gas exchange, is essential for giving our bodies the energy they need to work.

Humans have two lungs, a right lung and a left lung. The right lung is bigger, while the left lung shares space with the heart. Together, they weigh about as much as a small bag of sugar. Inside the lungs, air travels through a network of airways and ends up in tiny air sacs called alveoli, where the oxygen enters the blood. The lungs are protected by a slippery covering that lets them move easily as we breathe.

The lungs start developing very early in a baby’s life, even before they are born. At first, they are not used for breathing because the baby is surrounded by fluid. But shortly after birth, when a baby takes their first breath, the lungs begin to work, and they continue to do so for the rest of a person’s life. Keeping the lungs healthy is very important, as many diseases can affect them.

Structure

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

Each lung has sections called lobes. The right lung has three lobes, while the left lung has two. These lobes help the lungs expand and work properly. Inside the lungs, air travels through tiny tubes called bronchi and bronchioles, ending in small 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.

The lungs are surrounded by a thin, slippery layer called the pleura, which helps them move easily as we breathe.

Development

Further information: Sonic hedgehog § Lung development

The human lungs start to form very early in the development of a baby. They begin as a small bud in the chest area during the fourth week inside the mother. This bud then branches out into tubes that will become the airways and lungs. These tubes keep splitting into smaller and smaller branches, eventually forming the tiny air sacs called alveoli where oxygen is traded for carbon dioxide.

After a baby is born, the fluid that filled the lungs is quickly absorbed or breathed out. The baby's first breath triggers the lungs to expand and begin working properly. The lungs keep growing and developing well into young adulthood, with new alveoli forming even after birth to help the body get enough oxygen.

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, and they let out carbon dioxide, a waste gas, from our blood into the air. This process happens in tiny air sacs in the lungs called alveoli, which give a very large area for this exchange.

The lungs cannot inflate by themselves. When we breathe in, muscles around the lungs and the diaphragm pull the chest cavity open, letting the lungs expand. When we breathe out, these muscles relax, and the lungs shrink back to their resting size. The lungs also help protect the body from germs and particles in the air, and they play a role in many other body functions, like helping us speak.

Gene and protein expression

Further information: Bioinformatics § Gene and protein expression

In human cells, about 20,000 genes help make proteins, and almost 75% of these genes work in the lungs. Less than 200 of these genes are mainly found in the lungs, with fewer than 20 being very special to the lungs. Some important lung proteins include surfactant proteins like SFTPA1, SFTPB, and SFTPC, as well as napsin. These are made in special lung cells called type II pneumocytes. Other important lung proteins include dynein protein DNAH5 in cells with hair-like structures, and SCGB1A1 in cells that make mucus in the airways, called goblet cells.

Clinical significance

Main articles: Respiratory disease and Pulmonology

Lungs can be affected by many diseases and disorders. Pulmonology is the medical specialty that focuses on the lungs and respiratory system. Some common lung conditions include pneumonia, bronchitis, and asthma. These conditions often happen because of infections or inflammation.

Other conditions can affect how well the lungs work, such as pulmonary embolism, where a blood clot blocks blood flow in the lungs. There are also diseases that make it hard to breathe, like asthma and chronic obstructive pulmonary disease (COPD). Lung cancer is another serious condition, often linked to smoking. Doctors use different tests and treatments to help manage these diseases and improve lung health.

Culinary uses

Mammal lung is sometimes eaten as food in many parts of the world. It is grouped with other animal parts like the heart and trachea and is used in dishes such as Scottish haggis. However, in the United States, the Food and Drug Administration does not allow the sale of animal lungs for food because of certain 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 passes through the lungs and out to front air sacs before being exhaled. These lungs are fixed in size and are called "circulatory lungs."

Bird lungs contain many tiny passages called parabronchi, with small sacs called atria where gas exchange happens. The air sacs help the bird breathe but do not play a big role in exchanging gases.

Reptiles

Main article: Reptile anatomy § Respiratory system

Most reptiles have lungs with a single bronchus that branches into many pockets, giving the lung a sponge-like texture. In tuataras, snakes, and some lizards, the lungs are simpler. Snakes and limbless lizards often have only one main lung, while some other reptiles have the opposite arrangement.

Both crocodilians and monitor lizards have lungs similar to birds, with a unidirectional airflow and air sacs. Reptiles usually breathe by moving their ribs, but crocodilians also use a special method involving their liver to help move air.

Amphibians

Further information: Frog § Respiration and circulation

The lungs of frogs and other amphibians are simple and balloon-like, with gas exchange happening on their outer surface. Amphibians can also breathe through their skin in water and use a method called buccal pumping to force air into their lungs. This is different from most animals, who use a method that expands the rib cage.

Most salamanders do not have lungs and breathe through their skin. Their small size helps them do this effectively.

Fish

Lungs are found in some fish like the coelacanths, bichirs, and lungfish. The Australian lungfish has a single lung divided into two lobes, while other lungfish have two lungs. These fish lungs are located in the upper part of the body and have a network of walls that increase the area for gas exchange.

Invertebrates

Further information: Respiratory system of gastropods

Some invertebrates have structures that act like lungs. For example, spiders and scorpions have book lungs for breathing air. The coconut crab uses branchiostegal lungs to breathe on land. Pulmonates, which include many land snails and slugs, have a simple lung that develops from their mantle cavity.

Evolutionary origins

The lungs of land animals and the gas bladders of fish likely started as simple sacs in early fish, allowing them to take in air when oxygen was scarce in water. These sacs first appeared in bony fish. In most ray-finned fish, they became closed gas bladders, but in some like carp, trout, herring, catfish, and eels, they stay connected to the esophagus. In more basic bony fish such as gar, bichir, bowfin, and lobe-finned fish, these sacs developed into lungs. Lobe-finned fish are the ancestors of land-based tetrapods, meaning the lungs of vertebrates today are similar in origin to the gas bladders of fish, though not to their gills.