Pulmonary alveolus

Your lungs have millions of tiny air sacs.

These sacs are shaped like small cups. They help you breathe every day. When you breathe in, the sacs take in good air.

This air moves into your blood. At the same time, the sacs let out old air. This old air leaves your body when you breathe out. These little sacs can stretch when they fill up. They also spring back when you breathe out.

Your body keeps making these sacs until you are eight years old.

Your lungs have millions of tiny air sacs. We call these alveoli.

They are shaped like small cups. They sit at the very end of your breathing tubes.

Alveoli have a very important job. They help you swap gases. This is a way to move air into your blood.

When you breathe in, oxygen moves through a thin wall. This wall is called the alveolar membrane. The oxygen enters tiny blood vessels called capillaries. At the same time, carbon dioxide moves out. This is the old air your body does not need.

Three main types of cells live in these sacs. Type I cells are very thin and flat. They make up most of the sac's wall. Type II cells are smaller. They make a fatty liquid called surfactant. This liquid keeps the sacs from collapsing. Finally, there are alveolar macrophages. These are tiny cells that act like scavengers. They move around to clean up dust or germs.

Your lungs grow as you get older. You keep making new alveoli until you are eight years old. A person has about 480 million of them!

Your lungs contain millions of tiny, hollow spaces called alveoli.

These small, cup-shaped cavities are part of the lung parenchyma. This term refers to the functional tissue that makes up 90 percent of your lung volume. Alveoli are found at the end of your breathing paths. They sit in the respiratory bronchioles and line the alveolar ducts. They also cluster together in groups called alveolar sacs.

These sacs are the basic units of breathing for mammals.

Alveoli work by swapping gases through a process called diffusion.

First, you breathe in oxygen through your airways. This oxygen moves across a very thin barrier called the alveolar membrane. This membrane is between 0.2 and 0.6 micrometers thick. The oxygen enters a network of tiny blood vessels called capillaries. At the same time, carbon dioxide moves from the blood into the alveoli. This carbon dioxide is then breathed out of your body.

This constant swap keeps your blood full of fresh oxygen.

Three main types of cells live inside these air sacs. Type I cells are very thin and flat. They cover more than 95 percent of the alveolar surface. Type II cells are smaller and more numerous. They release a fatty liquid called surfactant to keep the sacs open. Without surfactant, the alveoli would collapse.

The third type is the alveolar macrophage. These are mobile scavenger cells. They move around to eat dust, bacteria, and other tiny particles.

Human lungs are quite amazing in their scale and detail. A typical pair of human lungs contains about 480 million alveoli. Together, they create a massive surface area for gas exchange. This area is between 70 and 80 square meters. Each tiny alveolus is between 200 and 500 micrometers in diameter. About 70 percent of each sac is covered by a mesh of capillaries. This large surface area is necessary because oxygen does not dissolve easily in liquid.

Your lungs continue to grow and change as you age. New alveoli keep forming until you reach eight years of age. Most of this division happens within the first three years of life. Before birth, the lungs go through five stages of development. These stages include the embryonic and the alveolar stages. The alveolar stage begins around 36 weeks of development. As you grow, your lungs get bigger because you gain more alveoli. This growth helps you breathe better as you get older.

A pulmonary alveolus, often called an air sac, is a tiny, hollow, cup-shaped cavity in the lungs. These millions of structures are the primary sites for pulmonary gas exchange. This is the process where oxygen enters the blood and carbon dioxide leaves it. Alveoli make up the lung parenchyma, which is the functional tissue of the mammalian lungs. This tissue accounts for approximately 90 percent of the total lung volume.

The mechanism of gas exchange relies on a very thin barrier called the alveolar membrane. This membrane is also known as the respiratory membrane. It consists of several layers, including an alveolar lining fluid and the epithelial lining. The capillary endothelial basement membrane actually fuses with the basement membrane of the alveolar epithelium. This fusion creates the alveolar basement membrane. The entire membrane is incredibly thin, measuring between 0.2 $\mu$m at its thinnest and 0.6 $\mu$m at its thickest.

Gas exchange occurs through diffusion across this membrane. Oxygen moves from the alveolar air into the surrounding pulmonary capillaries. Simultaneously, carbon dioxide moves from the capillaries into the alveoli to be exhaled. This process is supported by a vast network of capillaries. In fact, a fine mesh of capillaries covers about 70 percent of each alveolus's area.

Alveoli are organized into specific respiratory units. They are first found in the respiratory bronchioles as scattered outpockets. These bronchioles have side branches called alveolar ducts, which number between two and eleven per bronchiole. These ducts open into five or six alveolar sacs. A cluster of alveoli within these sacs, along with the bronchioles and ducts, forms the acinus. The acinus is considered the basic unit of respiration.

Three distinct types of cells inhabit the alveolar walls. Type I pneumocytes are large, thin, and flat cells. They are squamous, meaning they are spread out, and they cover more than 95 percent of the alveolar surface. Because they are so thin, they allow for efficient diffusion. Type II pneumocytes are smaller, cuboidal, and more numerous. They serve a vital role by secreting pulmonary surfactant. This fatty substance, made of phospholipids, lowers surface tension so the alveoli do not collapse.

The third cell type is the alveolar macrophage. These are large, mobile phagocytic cells, sometimes called dust cells. They move through the lumens of the alveoli and the surrounding connective tissue. Their job is to act as scavengers by engulfing foreign particles like dust, bacteria, and carbon. They also play a role in the immune response by secreting cytokines to recruit other immune cells.

Human lung development is a complex, multi-stage process. It begins on day 22 and moves through five stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. The alveolar stage begins around 36 weeks of gestation. While most alveolar division happens in the first six months, it continues until a child is three years old. New alveoli continue to form until a person reaches eight years of age.

The scale of this system is massive. A typical pair of human lungs contains about 480 million alveoli. Each individual alveolus has a diameter between 200 and 500 $\mu$m. When you add them all up, they provide a total surface area for gas exchange between 70 and 80 square meters. This enormous surface area is necessary because oxygen has low solubility and does not diffuse easily.

Understanding alveoli is essential for medicine. For example, if there is not enough surfactant, a condition called atelectasis can occur, where the lungs collapse. Preterm infants may face infant respiratory distress syndrome (IRDS) if they lack sufficient surfactant. Additionally, inflammation from viruses or bacteria can cause pneumonia, which reduces the surface area available for breathing. These connections show how vital these tiny sacs are to the entire body's survival.