Lung

The lungs are the main organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs sit near the backbone on either side of the heart. Their job is to bring in oxygen from the air we breathe and put it into the blood. They also let out carbon dioxide from the blood back into the air. This process, called gas exchange, gives our bodies the energy they need.

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

The lungs start to form 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 when a baby takes their first breath after birth, the lungs begin to work. They keep working for the rest of a person’s life. Keeping the lungs healthy is very important because 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, and 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

Your lungs start to form very early when you are still a baby inside your mother. They begin as a tiny bud in your chest. This bud grows and splits into tubes. These tubes become the airways and your lungs. The tubes keep splitting into smaller tubes until they make tiny air sacs called alveoli. These sacs help your body take in oxygen and get rid of carbon dioxide.

After you are born, the fluid in your lungs goes away. When you take your first breath, your lungs open up and start working. Your lungs keep growing for many years, making new alveoli to help you 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. They also let out carbon dioxide, a waste gas, from our blood into the air. This happens in tiny air sacs in the lungs called alveoli. These sacs give a 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. This lets 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. 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, many genes help make proteins. Almost three out of four of these genes work in the lungs. Only a small number of genes are mainly found in 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. Other important lung proteins include dynein protein DNAH5 in cells with tiny hair-like structures, and SCGB1A1 in cells that make mucus, called goblet cells.

Clinical significance

Main articles: Respiratory disease and Pulmonology

Lungs can get sick in many ways. Pulmonology is a type of medicine that studies the lungs and the respiratory system. Some common problems with lungs are pneumonia, bronchitis, and asthma. These usually happen because of germs or swelling.

Other problems can make it harder for lungs to work well, like pulmonary embolism, when a clump of blood gets stuck in the lungs. Some diseases, such as asthma and chronic obstructive pulmonary disease (COPD), make breathing difficult. Lung cancer is a serious illness that can be caused by smoking. Doctors have many ways to help treat these problems and keep lungs healthy.

Culinary uses

Lung from animals is eaten in many places around the world. It is used with other animal parts like the heart and trachea. One example is a Scottish dish called haggis. In the United States, the Food and Drug Administration does not allow selling animal lungs for food. This is because of health concerns.

Other animals

Birds

Main article: Bird anatomy § Respiratory system

Birds have small lungs linked to eight or nine air sacs that run through their bodies. When a bird breathes in, air travels through its trachea into these air sacs. The air then moves through the lungs and into front air sacs before being breathed out. These lungs stay the same size and are called "circulatory lungs."

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

Reptiles

Main article: Reptile anatomy § Respiratory system

Most reptiles have lungs with one bronchus that splits into many pockets, making the lung look spongy. In tuataras, snakes, and some lizards, the lungs are simpler. Snakes and limbless lizards often have just one main lung, while some other reptiles have the opposite setup.

Both crocodilians and monitor lizards have lungs similar to birds, with a one-way airflow and air sacs. Reptiles usually breathe by moving their ribs, but crocodilians also use a special method with 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 push 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 well.

Fish

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

Invertebrates

Further information: Respiratory system of gastropods

Some invertebrates have structures that work 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. These sacs helped fish take in air when there was less oxygen in the water. They first appeared in bony fish. In most ray-finned fish, they became closed gas bladders. But in some fish like carp, trout, herring, catfish, and eels, they stay connected to the esophagus. In 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 animals. So, the lungs of animals today are similar in origin to the gas bladders of fish, but not to their gills.