The respiratory system is a network of organs and tissues that moves air into and out of your body, delivering oxygen to your blood and removing carbon dioxide. It includes everything from your nose down to the tiny air sacs deep inside your lungs. At rest, a healthy adult breathes 12 to 20 times per minute, and the average lung capacity is about 6 liters of air.
Beyond breathing, the respiratory system also regulates the acid-base balance of your blood, powers your voice, and enables your sense of smell. It even runs its own defense system to keep harmful particles and germs from reaching your lungs.
Upper and Lower Respiratory Tract
The respiratory system is divided into two sections. The upper respiratory tract consists of your nose, nasal cavity, and pharynx (throat). These structures warm, moisten, and filter the air you breathe before it travels deeper. The nasal cavity also houses a small patch of specialized tissue called the olfactory epithelium, located high in the superior nasal cavity, where receptor neurons detect airborne chemicals and give you your sense of smell.
The lower respiratory tract picks up where the upper tract leaves off. It includes the larynx (voice box), the trachea (windpipe), the bronchi (the two main airways that branch into each lung), and the lungs themselves. The larynx contains the vocal folds, sometimes called the true vocal cords. These are membranous folds that vibrate as air passes over them, producing sound. Their size varies from person to person. Males generally have larger vocal folds, which is why their voices tend to be deeper.
How Air Moves In and Out
Breathing depends on pressure changes inside your chest, and the key muscle driving those changes is the diaphragm. This dome-shaped sheet of muscle sits below the lungs, separating the chest cavity from the abdomen. When you inhale, the diaphragm contracts and flattens downward, expanding the chest cavity. That expansion lowers the pressure inside your lungs relative to the air outside, and air rushes in to equalize the difference. Muscles between your ribs, called the intercostal muscles, along with certain neck muscles, assist by lifting and spreading the rib cage.
Exhaling at rest is mostly passive. The diaphragm relaxes and moves back up, and the natural elasticity of the lungs and chest wall pushes air out. During exercise or heavy breathing, your abdominal and intercostal muscles contract more forcefully to speed up the process.
Newborns breathe much faster than adults. The median respiratory rate at birth is about 44 breaths per minute, dropping to around 26 breaths per minute by age two as the lungs grow and become more efficient.
Gas Exchange in the Lungs
The real purpose of moving air in and out is gas exchange, which happens in the alveoli. These are microscopic, balloon-like sacs clustered at the ends of the smallest airways in the lungs. Together, the alveoli create an enormous surface area for gas exchange, roughly 118 square meters in a healthy adult. That is about the size of half a tennis court, all folded inside your chest.
Oxygen crosses from the alveoli into the blood, and carbon dioxide moves in the opposite direction, entirely through diffusion. Gases pass through several thin layers: a film of fluid lining the alveoli, the alveolar wall itself, a shared basement membrane, and then the wall of the surrounding capillary. Despite those layers, the barrier is so thin that gas exchange reaches equilibrium about one-third of the way along each capillary. Blood arriving at the lungs carries oxygen at a partial pressure of about 40 mmHg. By the time it leaves, that number has risen to 100 mmHg. Carbon dioxide, meanwhile, drops from 46 mmHg to 40 mmHg as it exits into the alveolar air to be exhaled.
How Breathing Controls Blood pH
Your blood needs to stay within a very narrow pH range to keep your cells functioning properly. The respiratory system is one of the body’s main tools for maintaining that balance, and it does so through carbon dioxide. When CO2 dissolves in blood, it combines with water to form carbonic acid, which lowers pH. Exhaling removes CO2 from the body, reducing acid levels and raising pH.
This works as a responsive system. If your blood becomes too acidic, perhaps because of a metabolic problem where the kidneys aren’t clearing enough acid, your respiratory system compensates by increasing your breathing rate. More breaths per minute means more CO2 exhaled, which pulls the equation back toward balance. The reverse is also true: if you hyperventilate and exhale too much CO2, your blood becomes too alkaline. Slow, shallow breathing can cause the opposite problem, allowing CO2 to build up and making the blood more acidic. This is why breathing rate matters for more than just oxygen supply.
Built-In Defense Systems
Every breath you take pulls in more than just air. Dust, pollen, bacteria, and other particles ride along with it. The respiratory system has a layered defense to deal with these threats, and the primary one is called mucociliary clearance.
The airways are lined with cells that have tiny, hair-like structures called cilia on their surface. These cells are coated with a two-part liquid layer: a sticky mucus layer on top that traps inhaled particles and pathogens, and a thinner, more watery layer underneath that lubricates the surface and allows the cilia to beat freely. The cilia move in coordinated waves, sweeping the mucus and everything trapped in it upward toward the throat, where it can be swallowed or coughed out.
Coughing serves as a backup mechanism when the normal mucus-clearing process is overwhelmed or when larger particles need to be expelled quickly. The system also relies on anatomical barriers (the twists and turns of the nasal passages force large particles to impact the walls before reaching deeper airways) and immune cells stationed in the lungs that can engulf anything that slips through.
How the Respiratory and Circulatory Systems Work Together
The respiratory system cannot deliver oxygen to your tissues on its own. It depends entirely on the circulatory system to carry that oxygen from the lungs to the rest of the body and to bring carbon dioxide back for disposal. Deoxygenated blood flows from the heart into the pulmonary arteries, which carry it to the capillary networks surrounding the alveoli. After gas exchange, oxygen-rich blood returns through the pulmonary veins to the heart, which then pumps it out to every organ and tissue.
This partnership means that problems in one system often show up in the other. Conditions that reduce blood flow to the lungs impair gas exchange just as much as conditions that damage the airways or alveoli directly. The surface area available for gas exchange, the thickness of the membrane between air and blood, and the volume of blood flowing through the capillaries all determine how efficiently your body gets the oxygen it needs.