Respiratory System: How Your Lungs and Airways Work

What Is the Respiratory System?

The respiratory system is the network of organs and tissues responsible for breathing, gas exchange, and supplying oxygen to the body while removing carbon dioxide. It includes the airways (nose, throat, larynx, trachea, bronchi) and the lungs where actual gas exchange occurs.

Your body needs oxygen to survive. Every cell in your body requires a constant supply of oxygen to produce energy through a process called cellular respiration. The respiratory system is the remarkable biological machinery that makes this possible, working tirelessly every second of your life to bring fresh oxygen into your body and remove the waste product carbon dioxide.

The respiratory system can be divided into two main sections: the upper respiratory tract and the lower respiratory tract. The upper respiratory tract includes the nose, nasal cavity, sinuses, pharynx (throat), and larynx (voice box). The lower respiratory tract consists of the trachea (windpipe), bronchi, bronchioles, and the lungs themselves. Each component plays a specific role in the complex process of breathing and gas exchange.

When you take a breath, air travels a carefully designed pathway. It enters through your nose or mouth, passes through the pharynx and larynx, travels down the trachea, branches into the bronchi, continues through smaller bronchioles, and finally reaches the alveoli - tiny air sacs where the magic of gas exchange occurs. This entire journey takes just a fraction of a second, yet it involves numerous protective mechanisms that filter, warm, and humidify the air to protect your delicate lung tissue.

The Six Parts of the Airways

The airways can be divided into six distinct anatomical regions, each with specific functions:

• Nasal cavity: Filters, warms, and humidifies incoming air
• Sinuses: Air-filled spaces that lighten the skull and affect voice resonance
• Pharynx (throat): Shared passage for air and food
• Larynx (voice box): Contains vocal cords and protects the lower airways
• Trachea (windpipe): Main airway leading to the lungs
• Bronchi and bronchioles: Branching airways within the lungs

How Does the Nose Filter and Warm Air?

The nose filters incoming air using nasal hairs and mucus, warms it to body temperature through a rich network of blood vessels, and adds humidity through moist mucous membranes. This protects the delicate lung tissue from cold, dry, or contaminated air.

The nose is far more than just a passageway for air - it's a sophisticated filtration and conditioning system. Inside the nose is a hollow space called the nasal cavity, which is divided into two halves by the nasal septum. Each nostril leads into its respective half of the nasal cavity. Within each nasal cavity are three bony projections called nasal conchae (or turbinates), which increase the surface area for air conditioning.

The nose is composed of both cartilage and bone. Cartilage is a tissue that's softer than bone but both strong and flexible. This combination gives the nose its characteristic shape while allowing some movement and flexibility. The external nose you see is primarily cartilage, while the nasal cavity is surrounded by various skull bones.

One of the nose's most important functions is protecting the lungs from harmful particles. Just inside the nostrils, coarse hairs called vibrissae trap larger particles like dust and debris. Beyond these hairs, the entire nasal cavity is lined with mucous membrane covered in tiny hair-like structures called cilia. The mucous membrane produces sticky mucus that traps smaller particles, bacteria, and viruses. The cilia beat in coordinated waves, moving this contaminated mucus toward the throat where it can be swallowed and neutralized by stomach acid.

The Mucous Membrane and Cilia

The inside of the nose is covered with a specialized lining called the respiratory mucosa. This mucous membrane has two critical functions: producing mucus to trap particles and housing millions of cilia that constantly sweep debris away from the lungs. Beneath the mucous membrane lies a dense network of small blood vessels called the nasal plexus.

This rich blood supply serves an important purpose: it warms the incoming air to near body temperature. Cold air can damage delicate lung tissue and impair gas exchange, so warming the air before it reaches the lungs is essential. The blood vessels can expand or contract to regulate airflow and temperature - this is why your nose can feel stuffy when you have a cold, as the blood vessels swell in response to infection.

The moisture from the mucous membrane also humidifies the air. Dry air can irritate and damage the airways and interfere with proper gas exchange in the alveoli. By the time air reaches your lungs, it has been warmed to body temperature and is nearly 100% saturated with water vapor, regardless of the temperature or humidity of the air you breathed in.

What Is the Function of the Sinuses?

The paranasal sinuses are air-filled cavities in the skull bones that lighten the head's weight, affect voice resonance, and produce mucus that drains into the nasal cavity. They include the maxillary, frontal, ethmoid, and sphenoid sinuses.

The sinuses (paranasal sinuses) are located near the nose and connect to the nasal cavity through narrow passages. These are hollow spaces within the bones of the skull. On the inside, the sinuses are lined with the same type of mucous membrane found in the nasal cavity, allowing them to participate in mucus production and air conditioning.

You have sinuses in several locations within your facial bones: the maxillary sinuses in your cheekbones (the largest sinuses), the frontal sinuses in your forehead above your eyebrows, the ethmoid sinuses between your eyes, and the sphenoid sinuses deep behind your nose. Each pair of sinuses has openings (ostia) that drain into the nasal cavity.

The primary functions of the sinuses are to make the skull lighter (hollow bones weigh less than solid ones) and to affect voice quality. The sinuses act as resonating chambers that amplify and modify sound, giving your voice its unique character. This is why your voice sounds different when you have a sinus infection or severe nasal congestion - the resonating chambers are blocked.

The sinuses also produce mucus that helps moisten the nasal passages and trap particles. However, if the narrow drainage passages become blocked - often due to swelling from allergies or infection - mucus can accumulate in the sinuses, leading to sinusitis (sinus infection) with symptoms like facial pain, pressure, and headache.

How Does the Throat (Pharynx) Work?

The pharynx (throat) is a muscular tube that serves as a shared passage for both air and food. It connects the nasal and oral cavities to the larynx and esophagus, and contains the tonsils which help protect against infection.

The pharynx is a muscular tube located behind the nasal cavity and mouth. It serves as a crossroads where both the respiratory and digestive systems share a common pathway. The pharynx continues downward, eventually dividing into two separate passages: the esophagus (leading to the stomach) and the larynx (leading to the lungs).

The pharynx can be divided into three regions based on location. The nasopharynx is the uppermost section, located behind the nasal cavity. It is lined with the same type of respiratory mucosa found in the nose and sinuses. The oropharynx is the middle section, located behind the mouth - this is the part of your throat visible when you open your mouth wide. The laryngopharynx (or hypopharynx) is the lowest section, which leads to both the larynx and esophagus.

The Mucous Membrane Lining

The uppermost part of the pharynx is covered with the same mucous membrane found in the nasal cavity and sinuses. This respiratory epithelium contains cilia that help move mucus and trapped particles toward the throat. The lower part of the pharynx has a thicker, more durable mucous membrane designed to withstand the passage of food and drink. This stratified squamous epithelium is similar to the lining of the mouth and can handle mechanical stress.

In the upper part of the pharynx, passages called the Eustachian tubes connect to the middle ear. These tubes help equalize pressure between the middle ear and the atmosphere - this is why yawning or swallowing can help "pop" your ears during altitude changes. At the back of the pharynx hangs a small flap of tissue called the uvula (commonly called the "dangly thing").

The Tonsils and Immune Defense

The tonsils are two masses of lymphoid tissue located on either side of the pharynx, roughly at the level of the back of the tongue. The medical term for tonsils is palatine tonsils, and they get their common name from their shape and size in adults, which resembles an almond.

Tonsils are made of lymphoid tissue, which is part of the body's immune system. They serve as a first line of defense against pathogens (bacteria and viruses) that enter through the mouth and nose. The tonsils contain immune cells that can identify and respond to infectious agents, helping protect both the respiratory tract and the digestive system from infection.

Interestingly, tonsils are proportionally larger in children and typically shrink during adolescence. In adults, they are usually much smaller than in children. This is because young children are still developing their immune systems and benefit more from the additional immune surveillance that large tonsils provide.

What Does the Larynx (Voice Box) Do?

The larynx (voice box) protects the lower airways by closing during swallowing, keeps the airway open for breathing, and produces sound through the vibration of the vocal cords. The epiglottis is a flap that covers the larynx entrance during swallowing to prevent food from entering the windpipe.

The larynx is a complex structure made of several cartilages connected by muscles and ligaments. These cartilage components work together to form the entrance to the trachea (windpipe). You can feel your larynx on the front of your neck - it's commonly known as the "Adam's apple," though everyone has one (it's typically more prominent in males after puberty).

The larynx has two critical functions that might seem contradictory: it must keep the airway open so you can breathe, and it must close the airway completely when you swallow to prevent food and liquid from entering your lungs. These functions are accomplished through a sophisticated system of muscles and cartilages that can rapidly switch between open and closed configurations.

The Epiglottis Protects the Airway

A leaf-shaped flap called the epiglottis partially covers the entrance to the larynx. During swallowing, the epiglottis folds down like a lid to cover the laryngeal opening, directing food and liquid into the esophagus instead of the trachea. This reflex action prevents food from "going down the wrong pipe" - when it fails, you experience choking or aspiration.

The inside of the larynx is lined with the same type of mucous membrane found in the nose, sinuses, and upper pharynx. This respiratory epithelium with its cilia and mucus helps keep the lower airways clean and protected from particles that might have escaped the upper filtering mechanisms.

The Vocal Cords Produce Sound

Inside the larynx, two bands of tissue called the vocal cords (or vocal folds) stretch across the airway. These elastic structures are attached to cartilages that can move them together or apart. When the cartilages move, the vocal cords are tensioned or relaxed, and the gap between them (called the glottis) opens or closes.

When the glottis is open, air can flow freely through the trachea for breathing. When you speak or sing, the vocal cords come close together and air from the lungs is forced between them, causing them to vibrate. These vibrations create sound waves, which are then modified and amplified by the resonating chambers of the chest, throat, mouth, nose, and sinuses. The sounds are further shaped by the movements of the cheeks, teeth, lips, and tongue to form words.

How Does the Trachea (Windpipe) Function?

The trachea (windpipe) is a tube about 10 cm long that carries air from the larynx to the lungs. It is reinforced with C-shaped cartilage rings that keep it open, and is lined with mucous membrane and cilia that continue to filter and protect the airways.

From the larynx, the trachea continues downward through the chest. The trachea lies directly in front of the esophagus (the tube that carries food to the stomach). This anatomical relationship is why the larynx and epiglottis must work so precisely - the two tubes are neighbors, and food must be directed to the correct one.

The trachea is constructed of rings of cartilage stacked on top of each other, connected by muscles and connective tissue. These cartilage rings are not complete circles but are C-shaped, with the open part of the "C" facing backward toward the esophagus. This design allows the esophagus to expand when you swallow large bites of food without being restricted by the trachea.

Structure and Protection

The cartilage rings serve a critical purpose: they keep the trachea open at all times. Without this rigid support, the trachea could collapse during breathing, especially during vigorous inhalation when negative pressure is created in the chest. The cartilage ensures that air can always flow freely to and from the lungs.

The inside of the trachea is covered with the same type of respiratory mucosa found throughout the upper airways. This mucous membrane produces mucus that traps any remaining particles that escaped the filtering in the nose and throat. The cilia lining the trachea beat upward, moving mucus and trapped debris up toward the throat where it can be swallowed or coughed out.

The trachea is approximately 10 centimeters (4 inches) long in adults. At its lower end, it divides into two main branches called the primary bronchi - one leading to each lung.

What Are the Bronchi and Bronchioles?

The bronchi are the large airways that branch from the trachea into each lung. Inside the lungs, they continue to divide into smaller and smaller bronchioles until they end in tiny air sacs called alveoli. The larger airways contain cartilage for support, while the smallest bronchioles have thin walls without cartilage.

The bronchi are the continuation of the airway system from the trachea into the lungs. When the trachea divides, it forms two primary (main) bronchi - the right main bronchus leads to the right lung, and the left main bronchus leads to the left lung. Inside the lungs, each primary bronchus divides repeatedly into progressively smaller airways.

The branching pattern of the bronchi resembles an upside-down tree, which is why this system is sometimes called the "bronchial tree." The larger bronchi contain cartilage to keep them open, similar to the trachea. As the airways get smaller, the amount of cartilage decreases. The smallest airways, called bronchioles, have very thin walls with no cartilage at all - instead, they are surrounded by smooth muscle that can contract or relax to regulate airflow.

At the very end of the bronchial tree, the smallest bronchioles (terminal bronchioles) lead into even smaller respiratory bronchioles, which finally open into clusters of tiny air sacs called alveoli. It is in these alveoli that the actual exchange of oxygen and carbon dioxide takes place.

How Do the Lungs Work?

The lungs are the primary organs of gas exchange, containing approximately 300-400 million alveoli that provide about 70 square meters of surface area for oxygen and carbon dioxide exchange. The right lung has 3 lobes while the left lung has 2 lobes. Each lung is surrounded by a protective membrane called the pleura.

The lungs are located in the chest cavity, sitting on either side of the heart. Between the lungs you'll find the trachea, esophagus, heart, and major blood vessels. The lungs' primary function is to take up oxygen from the air you breathe and deliver it to the bloodstream, while simultaneously removing carbon dioxide from the blood and releasing it into the air you exhale.

The body's cells require a constant supply of oxygen to function. The brain is particularly sensitive to oxygen deprivation and can suffer damage within just a few minutes without oxygen. Carbon dioxide, on the other hand, is a waste product of cellular metabolism that must be removed from the body. If carbon dioxide accumulates in the cells and tissues, it becomes toxic and disrupts normal cellular function.

The Lobes of the Lungs

The right lung is divided into three sections called lobes (upper, middle, and lower), while the left lung has only two lobes (upper and lower). The left lung is slightly smaller because it must accommodate the heart, which sits slightly to the left of center in the chest. Each lobe is further divided into smaller segments, and each segment receives its own branch of the bronchial tree and its own blood vessels.

Each lung is surrounded by a double-layered membrane called the pleura. The inner layer (visceral pleura) adheres to the lung surface, while the outer layer (parietal pleura) lines the chest wall. Between these two layers is a thin space containing a small amount of lubricating fluid. This pleural fluid allows the lungs to slide smoothly against the chest wall during breathing.

The Structure of Lung Tissue

The lungs are made of a specialized tissue containing millions of tiny air sacs called alveoli. This gives lung tissue a spongy, lightweight consistency - healthy lung tissue actually floats in water. Each lung contains approximately 300-400 million alveoli. The walls of these alveoli are extremely thin to allow gases to pass through easily - they are only one cell thick.

The thin walls of the alveoli are surrounded by a network of tiny blood vessels called capillaries. The total surface area of all the alveoli in both lungs is approximately 70 square meters - about the size of a tennis court! This enormous surface area is crucial for efficient gas exchange between the air and the blood.

How Does the Breathing Process Work?

Breathing occurs when the diaphragm contracts and the ribs lift, expanding the chest cavity and creating negative pressure that draws air into the lungs. During exhalation, the diaphragm relaxes and the chest cavity shrinks, pushing air out. At rest, we breathe about 12-20 times per minute, inhaling approximately 500 ml of air with each breath.

Under normal circumstances, we breathe through the nose. The process of breathing involves the coordinated action of several muscles, most importantly the diaphragm - a dome-shaped muscle that separates the chest cavity from the abdominal cavity. When you inhale, the diaphragm contracts and flattens, and the intercostal muscles between your ribs contract to lift and expand the rib cage.

These actions expand the chest cavity, which causes the pressure inside the pleural space (around the lungs) to drop below atmospheric pressure. This negative pressure causes the lungs to expand, and air rushes in through the airways to fill the expanding lung tissue. The air eventually reaches the alveoli, where gas exchange occurs.

When you exhale, the process reverses. The diaphragm relaxes and returns to its dome shape, pushing upward into the chest cavity. The intercostal muscles relax, allowing the rib cage to fall back down. The chest cavity shrinks, compressing the lungs and forcing air out through the airways. At rest, exhaling is largely passive - the elastic recoil of the lung tissue and chest wall is enough to push air out.

With each breath at rest, approximately 500 milliliters (half a liter) of air flows into and out of the lungs - this is called the tidal volume. When you're resting, you typically breathe about 12 times per minute. During exercise or exertion, both your breathing rate increases and you take deeper breaths, dramatically increasing the amount of air (and thus oxygen) that reaches your lungs.

The Brain Controls Breathing

Breathing is controlled by signals from a region called the respiratory center, located in the brainstem - specifically in the part called the medulla oblongata. Various reflexes also help regulate breathing. If the carbon dioxide level in your blood increases, you automatically breathe more frequently and deeply to expel the excess carbon dioxide. In this way, breathing automatically adjusts to your body's needs.

Although breathing is primarily automatic (you don't have to think about it to breathe), you can also consciously control your breathing to some extent. This voluntary control comes from the cerebral cortex and allows you to hold your breath, take deep breaths, or modify your breathing for activities like speaking, singing, or playing a wind instrument.

How Does Gas Exchange Work in the Lungs?

Gas exchange occurs in the alveoli, where oxygen diffuses from the air into the blood capillaries while carbon dioxide diffuses from the blood into the alveoli to be exhaled. This exchange happens across extremely thin membranes - the alveolar walls and capillary walls together are only about 0.5 micrometers thick.

The alveoli are tiny air sacs located at the ends of the smallest bronchioles. It is in the alveoli that the exchange between oxygen and carbon dioxide takes place. Surrounding the alveoli are thin blood vessels called capillaries. Both oxygen and carbon dioxide can pass through the thin walls of the capillaries and the alveoli.

The process of gas exchange relies on a principle called diffusion - gases naturally move from areas of higher concentration to areas of lower concentration. When you inhale, the air in your alveoli contains a high concentration of oxygen and a low concentration of carbon dioxide. The blood arriving at the alveoli from the body has used up much of its oxygen and accumulated carbon dioxide from cellular metabolism.

Oxygen Transport to Body Tissues

The air you breathe in contains oxygen that your body needs. During inhalation, oxygen travels through the airways, to the lungs, and all the way to the alveoli. The oxygen then passes from the alveoli into the capillaries surrounding them. From there, it is transported through vessels called the pulmonary veins to the heart and then out to the rest of the body.

In the blood, most oxygen is bound to a protein called hemoglobin, which is found in red blood cells. Hemoglobin has a remarkable ability to bind oxygen when oxygen levels are high (as in the lungs) and release oxygen when oxygen levels are low (as in body tissues that are actively using oxygen). This allows efficient delivery of oxygen to the cells that need it most.

Carbon Dioxide Removal from the Body

The capillaries surrounding the alveoli also contain carbon dioxide. This carbon dioxide has been transported to the lungs from the body's various tissues. The blood vessels that carry carbon dioxide to the lungs are called the pulmonary arteries. Inside the lungs, the carbon dioxide passes from the capillaries through their thin walls and into the alveoli. The air you breathe out contains this carbon dioxide.

Carbon dioxide is transported in the blood in several ways: some is dissolved directly in the plasma, some is bound to hemoglobin (at a different site than oxygen), and most is carried as bicarbonate ions. When the blood reaches the lungs, these processes reverse, releasing carbon dioxide into the alveoli to be exhaled.

How Do the Airways Protect the Lungs?

The airways protect the lungs through multiple mechanisms: nasal hairs and mucus trap particles, cilia sweep debris toward the throat, the cough reflex expels irritants, and immune cells in the airways neutralize pathogens. The warming and humidification of air also protects delicate lung tissue.

The respiratory system has evolved sophisticated defense mechanisms to protect the delicate alveoli from damage and infection. The lungs are essentially exposed to the outside environment with every breath, making these defenses essential for health.

The first line of defense begins in the nose, where coarse hairs trap large particles. The mucous membrane throughout the airways produces sticky mucus that captures smaller particles, bacteria, and viruses. The cilia - tiny hair-like projections - beat in coordinated waves to move this contaminated mucus up toward the throat, where it can be swallowed or expelled. This "mucociliary escalator" operates continuously, clearing about a tablespoon of mucus from your airways every day.

When irritants penetrate deeper into the airways, the cough reflex provides a powerful defense. A deep breath followed by forceful expulsion of air can generate wind speeds exceeding 100 miles per hour, effectively ejecting irritants and excess mucus from the airways. Sneezing serves a similar function for the nasal passages.

The airways also contain various immune cells, including macrophages that engulf and destroy pathogens, and lymphocytes that mount targeted immune responses. The tonsils and other lymphoid tissue in the throat provide additional immune surveillance at the entrance to the respiratory tract.