Human Respiratory Anatomy: Airways and Alveoli

The human respiratory system brings air into the body and exchanges gases with the blood, using a pathway of airways, the nose, trachea, bronchi and bronchioles, that ends in millions of tiny air sacs called alveoli inside the lungs. In brief: air travels nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. The alveoli are where oxygen enters the blood and carbon dioxide leaves it, and the diaphragm is the main muscle that powers breathing.

Every cell needs oxygen to release energy by respiration, and the respiratory system supplies it. This guide follows air along the airways, describes the structure of the lungs and alveoli, explains how the structure suits gas exchange, compares the parts in a table, works through a surface area example and corrects typical mistakes.

What is the respiratory system?

The respiratory system consists of the airways, the lungs and the muscles that move air. Its main functions are to supply oxygen to the blood, remove carbon dioxide and, to a lesser extent, to help regulate blood pH. The lungs also warm and humidify air, filter out particles and allow speech through the larynx.

Ventilation (breathing) is the movement of air in and out of the lungs. Gas exchange is the diffusion of oxygen and carbon dioxide between the air in the alveoli and the blood. Cellular respiration is the separate chemical process in cells that uses oxygen.

What happens in the nose and upper airway?

Air normally enters through the nose and the nasal cavity. The nasal cavity is lined with mucous membrane and fine hairs. Mucus traps dust and microbes, and the rich blood supply warms the air. Moisture added to the air prevents the delicate lung surfaces from drying out. People can also breathe through the mouth, but the air is then not filtered or warmed as well.

The air passes through the pharynx (throat), which is shared with the food route, and then the larynx (voice box). The larynx contains the vocal cords, which vibrate to make sound. The epiglottis, a flap of cartilage, closes over the larynx when swallowing so that food goes down the esophagus.

What are the trachea, bronchi and bronchioles?

The trachea (windpipe) is a tube about 10 to 12 centimeters long and about 2 centimeters wide. Its wall contains C-shaped rings of cartilage that keep it open. Its lining has goblet cells that produce mucus and ciliated cells whose cilia beat to move the mucus, with trapped particles, upward to the throat, where it is swallowed. This is known as the mucociliary escalator. Smoking damages the cilia and impairs this cleaning system.

At its lower end, the trachea divides into two bronchi (singular: bronchus), one for each lung. Each bronchus branches repeatedly into smaller tubes, the bronchioles. The branches are often compared to an upside-down tree. The larger airways have cartilage, but the smallest bronchioles have none and are surrounded by smooth muscle that can narrow them, as in an asthma attack. After about 16 divisions, the airways reach the gas-exchange region, and after roughly 23 divisions, they end in clusters of alveoli.

What are the lungs and the diaphragm?

The two lungs are spongy organs in the chest cavity (thorax), protected by the rib cage. The right lung has three lobes, and the left lung has two, leaving room for the heart. Each lung is covered by two thin membranes called pleura, with a small amount of fluid between them that lets the lungs slide smoothly during breathing.

The diaphragm is a dome-shaped sheet of muscle below the lungs that separates the chest from the abdomen. The intercostal muscles between the ribs also help in breathing. Their action is explained in the article on the mechanism of breathing.

How are alveoli adapted for gas exchange?

Alveoli (singular: alveolus) are tiny air sacs, about 0.2 to 0.3 millimeters in diameter, grouped like bunches of grapes at the end of the bronchioles. Each lung is thought to contain several hundred million of them. A commonly quoted total is about 300 to 500 million for both lungs together.

• Large surface area: the huge number of alveoli gives a total gas exchange area of roughly 50 to 100 square meters, depending on the method of estimate and the person.
• Thin walls: the wall of an alveolus and the wall of the adjacent capillary are each one cell thick, so the diffusion distance is less than one micrometer.
• Rich blood supply: a dense network of capillaries takes away oxygen-rich blood and brings oxygen-poor blood, which maintains steep concentration gradients.
• Moist lining: gases dissolve in a thin film of water before diffusing.
• Ventilation: breathing keeps fresh air in the alveoli, with more oxygen and less carbon dioxide than in the blood.
• Surfactant: a soapy fluid made by some alveolar cells reduces surface tension and prevents the alveoli from collapsing.

Respiratory structures compared

Worked example: the surface area of the alveoli

Estimate the gas exchange area using a simple model. Assume each alveolus is a sphere with a diameter of 0.25 mm and that both lungs contain 480 million alveoli.

1. Radius: 0.125 mm = 0.000125 m.
2. Surface area of one sphere: 4πr² = 4 × 3.14 × (0.000125)² = 4 × 3.14 × 1.5625 × 10⁻⁸ ≈ 1.96 × 10⁻⁷ m².
3. Total: 480 × 10⁶ × 1.96 × 10⁻⁷ ≈ 94 m².

The result of about 94 m² fits the commonly quoted range of 50 to 100 m², although part of each alveolus is shared with its neighbors, so the effective area is somewhat smaller. For comparison, a singles tennis court is about 195 m², so the area of the lung is roughly half of this. The calculation assumes perfect spheres and an exact number, and real values vary between people.

How was lung anatomy discovered?

The anatomist Andreas Vesalius described the airways in his 1543 book on human anatomy. Marcello Malpighi observed the alveoli and the capillaries around them in frog lungs in the 1660s. In the following centuries, the work of Antoine Lavoisier, Joseph Priestley and others showed that air is used and altered in respiration, and the detailed process of gas exchange was clarified in the early twentieth century.

Which mistakes are common?

Mistake 1: Saying that the lungs contain muscle that inflates them

Lungs have no muscles of their own. The diaphragm and intercostal muscles change the volume of the chest.

Mistake 2: Mixing up the trachea and the esophagus

The trachea carries air and has cartilage rings. The esophagus carries food.

Mistake 3: Describing alveoli as the site of respiration

Alveoli are the site of gas exchange. Respiration takes place in cells.

Mistake 4: Forgetting thin walls in the adaptations

Short diffusion distance is important. Mention a one-cell-thick wall, a large surface and a good blood supply.