The respiratory system brings in air through the nose and windpipe to the lungs, where oxygen enters the blood and carbon dioxide leaves, driven by the diaphragm.
Path of air
- Nose or mouth.
- Pharynx and larynx.
- Trachea.
- Bronchi and bronchioles.
- Alveoli, about 480 million tiny air sacs in the lungs.
Gas exchange
Oxygen passes into the blood while carbon dioxide passes out.
Breathing
The diaphragm contracts and moves down. The chest expands, drawing air in. When it relaxes, air leaves.
Rate
Adults breathe 12 to 20 times a minute at rest.
Parts of the airway
- Nose and mouth: warm, moisten and filter air.
- Pharynx and larynx: pass air, with the larynx housing the vocal cords.
- Trachea: a tube about 10 to 12 centimeters long with C shaped rings of cartilage.
- Bronchi: two main branches, one to each lung, dividing into smaller bronchioles.
- Alveoli: around 300 to 500 million tiny air sacs in the lungs, where gas exchange happens.
Surface area
The total surface area of the alveoli is about 70 square meters, roughly the size of half a tennis court, according to physiology textbooks.
Diaphragm
The diaphragm is a dome shaped muscle under the lungs. When it contracts, it flattens, enlarging the chest cavity and lowering pressure, so air flows in. When it relaxes, air flows out.
Gas exchange
Oxygen moves across thin alveolar walls into capillaries and binds to hemoglobin in red blood cells. Carbon dioxide moves in the opposite direction and is breathed out.
Air composition
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Inhaled air has about 21% oxygen and 0.04% carbon dioxide. Exhaled air has around 16% oxygen and 4% carbon dioxide.
Breathing rate
Normal adults breathe 12 to 20 times a minute, moving about 6 liters of air per minute at rest. Total lung capacity is about 6 liters in men and 4 to 5 liters in women.
Defense
Cilia and mucus trap particles and move them upward to be swallowed or coughed out. Smoking damages cilia.
Control by the brain
The brainstem controls breathing automatically, responding mostly to carbon dioxide levels in the blood.
Common conditions
Asthma, COPD, pneumonia and lung cancer. According to the WHO, COPD is the third leading cause of death worldwide.
Airway anatomy and numbers
Air flows through the nose or mouth, pharynx, larynx, trachea, bronchi and bronchioles to reach the alveoli. An adult has about 300 to 500 million alveoli, giving a gas exchange surface area of roughly 70 square meters. At rest, adults breathe 12 to 20 times per minute, moving about 500 ml of air each breath. The diaphragm contracts and flattens to expand the chest, pulling air in, and relaxes to push air out. Cilia and mucus trap particles and move them upward. The brainstem controls breathing, mainly in response to rising carbon dioxide levels in the blood.
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Gas exchange and common problems
Inhaled air is about 21 percent oxygen and 0.04 percent carbon dioxide, and exhaled air contains about 16 percent oxygen and 4 percent carbon dioxide. Oxygen moves across thin alveolar walls into capillaries and binds to hemoglobin, while carbon dioxide moves the other way. Common problems include asthma, which affects around 25 million Americans, COPD, pneumonia and lung cancer. Smoking is the leading cause of preventable lung disease. Call a doctor for breathlessness that is sudden, worsening or comes with chest pain, blue lips or confusion.
Based on data from the American Lung Association and the CDC
