Oxygen is the correct answer! The air we breathe is about 21% oxygen, 78% nitrogen, 1% argon, and trace gases (including 0.04% carbon dioxide). We exhale about 16% oxygen and 4% carbon dioxide. The respiratory system works closely with the circulatory system: oxygen is picked up by red blood cells in the lungs and transported to tissues; carbon dioxide is transported back to the lungs to be exhaled. A person breathes about 12-20 times per minute at rest (about 17,000-30,000 breaths per day). The total surface area of the lungs (including alveoli) is about 70-100 m² – roughly half the size of a tennis court. Without oxygen, brain cells begin to die within 4-6 minutes. Breathing is controlled by the brainstem (medulla oblongata and pons), which monitors carbon dioxide levels (not oxygen). That is why you feel the need to breathe when CO₂ builds up (after holding your breath).

The trachea is the windpipe! It is lined with cilia (tiny hairs) and mucus to trap particles and pathogens. The cough reflex helps clear the trachea of foreign objects. The larynx (voice box) contains the vocal cords, which vibrate to produce sound. The epiglottis is a flap of tissue that covers the trachea during swallowing to prevent food from entering the lungs (choking). The bronchi (singular: bronchus) are two tubes that branch into the left and right lungs. The bronchioles are smaller branches that lead to the alveoli. The right lung has three lobes (superior, middle, inferior); the left lung has two lobes (superior, inferior) to make room for the heart. The lungs are not hollow; they are spongy because they are filled with millions of tiny air sacs (alveoli). The alveoli are surrounded by capillaries where gas exchange occurs. If laid flat, the alveoli would cover about 100 square meters (1,076 square feet).

Gas exchange occurs in the alveoli! The walls of the alveoli and capillaries are only one cell thick, allowing oxygen and carbon dioxide to diffuse quickly. Red blood cells pass through the capillaries single file, maximizing exchange. Surfactant is a fluid in the alveoli that reduces surface tension, preventing the alveoli from collapsing. Premature babies often lack surfactant, causing respiratory distress syndrome (treated with artificial surfactant). Alveoli are highly elastic, expanding during inhalation and recoiling during exhalation. The total surface area of the alveoli is about the size of a tennis court (70-100 m²). Alveolar macrophages are immune cells that engulf dust, bacteria, and other particles. Smoking destroys alveoli and reduces lung function (emphysema). Once destroyed, alveoli do not regenerate. The number of alveoli increases from birth to about age 8 (then remains stable). Each lung contains about 150-250 million alveoli. The total area of the alveolar-capillary membrane is about 50-100 m².

The diaphragm is the main breathing muscle! The intercostal muscles (between the ribs) also help by lifting the ribs during deep breathing. The phrenic nerve (from the cervical spine, C3-C5) controls the diaphragm. A spinal cord injury above C3 (neck) can paralyze the diaphragm, requiring a ventilator. Hiccups are involuntary spasms of the diaphragm followed by sudden closure of the vocal cords (producing the "hic" sound). Yawning is thought to increase oxygen intake and cool the brain (though the exact purpose is still debated). Sneezing is a forced exhalation that clears the nasal passages (speeds of up to 100 mph / 160 km/h). Coughing clears the lower airways. The average tidal volume (air moved in a normal breath) is about 500 mL (0.5 L) at rest. Vital capacity (maximum air exhaled after maximum inhalation) is about 4-5 L in adults. Residual volume (air remaining in lungs after forceful exhalation) is about 1.2 L (prevents lung collapse). Total lung capacity is about 6 L in adults. Spirometry measures lung function (important for diagnosing asthma, COPD).

Diffusion is the correct answer! Oxygen concentration is higher in the alveoli (about 100 mmHg) than in the capillaries (about 40 mmHg), so oxygen diffuses into the blood. Carbon dioxide concentration is higher in the blood (about 45 mmHg) than in the alveoli (about 40 mmHg), so CO₂ diffuses into the alveoli. The diffusion distance is very short (about 0.5 micrometers) because the alveolar and capillary walls are each one cell thick. Hemoglobin in red blood cells carries oxygen (four oxygen molecules per hemoglobin). The oxyhemoglobin dissociation curve shows how hemoglobin releases oxygen in tissues (lower pH, higher temperature, higher CO₂ increase oxygen release – the Bohr effect). Fetal hemoglobin has a higher affinity for oxygen, allowing it to pull oxygen from maternal blood. Carbon monoxide (CO) binds to hemoglobin 200-250 times more strongly than oxygen, causing carbon monoxide poisoning (cherry-red skin, headache, nausea, death). Hyperbaric oxygen therapy is used to treat CO poisoning. A pulse oximeter measures blood oxygen saturation (normal: 95-100%). Low oxygen (hypoxia) causes shortness of breath, confusion, and blue skin (cyanosis).

Asthma is the correct answer! About 300 million people worldwide have asthma. Asthma can be controlled with medications but cannot be cured. An asthma attack (status asthmaticus) is a medical emergency. COPD (chronic obstructive pulmonary disease) includes emphysema (destruction of alveoli) and chronic bronchitis (inflammation of the bronchi). COPD is the third leading cause of death worldwide (after heart disease and cancer). Smoking causes about 85-90% of COPD cases. Pneumonia is an infection that fills the alveoli with fluid, causing cough, fever, and difficulty breathing. Pneumonia is a leading cause of death in children under 5 (about 800,000 deaths per year). Tuberculosis (TB) is caused by Mycobacterium tuberculosis; it spreads through coughs and sneezes. TB was called "consumption" historically (wasting away). Multidrug-resistant TB (MDR-TB) is a growing problem. COVID-19 (SARS-CoV-2) caused a global pandemic (2020-2023), infecting millions. Lung cancer is the leading cause of cancer death worldwide. Smoking causes about 85% of lung cancers. Early symptoms include persistent cough, chest pain, coughing up blood. Asbestos exposure also causes lung cancer (and mesothelioma). Cystic fibrosis is a genetic disorder causing thick mucus that clogs the lungs and pancreas. Respiratory syncytial virus (RSV) is a common cause of bronchiolitis in infants. Preventive measures: handwashing, masks, vaccines (flu, pneumonia, COVID-19), and not smoking.

The larynx is the voice box! The vocal cords vibrate when air from the lungs passes between them, producing sound. Whispering involves forcing air through the cords without vibration (just friction). The pitch of the voice can be controlled by tightening or relaxing the vocal cords. The vocal cords are covered by a mucous membrane; infections (laryngitis) cause swelling, hoarseness, or loss of voice. The epiglottis covers the larynx during swallowing to prevent food from entering the lungs. The cricoid cartilage (below the thyroid cartilage) is used for emergency airways (cricothyrotomy). The hyoid bone (U-shaped, above the larynx) is the only bone that does not articulate with any other bone; it supports the tongue and larynx. The size of the larynx is influenced by puberty: testosterone causes the larynx to grow, dropping the voice (voice breaking in boys). The larynx also plays a role in the cough reflex. Professional singers train to control their vocal cords and resonators to produce a wide range of sounds. Vocal fold nodules (singers' nodes) can develop from overuse, causing hoarseness. Laryngectomy is the surgical removal of the larynx (usually due to cancer), after which speech is restored using an electrolarynx or esophageal speech.

Hemoglobin (iron) makes blood red! The myth that deoxygenated blood is blue comes from the appearance of veins through skin (which scatter blue light). Blood is never blue; it is always red (shades from bright to dark). Blood appears bright red when oxygenated; dark red when deoxygenated. The iron in hemoglobin forms a complex with oxygen (oxyhemoglobin). Carbon monoxide (CO) binds to hemoglobin 200-250 times more strongly, forming carboxyhemoglobin (cherry-red). That is why CO poisoning causes skin to appear pink (not cyanotic). Some deep-sea fish have antifreeze proteins in their blood to survive in freezing waters. The Greenland shark has a special compound (trimethylamine oxide) to prevent protein denaturation under pressure. Blood doping (increasing red blood cells artificially) is banned in sports because it increases oxygen delivery (but also blood viscosity, risking stroke). Myoglobin in muscle tissue stores oxygen (gives meat its red color). Myoglobin has a higher affinity for oxygen than hemoglobin, allowing it to pull oxygen from blood. Hemoglobin levels are measured to diagnose anemia (low red blood cell count). Anemia can be caused by iron deficiency, vitamin B12 deficiency, blood loss, or bone marrow disorders. Sickle cell anemia is a genetic disorder causing abnormal hemoglobin (sickle cell shape) that clogs blood vessels.

Emphysema is the correct answer! Emphysema is a type of COPD (chronic obstructive pulmonary disease). The alveolar walls break down, creating fewer, larger air sacs instead of many small ones. This reduces surface area for gas exchange. The lungs lose elasticity, making it hard to exhale fully. People with emphysema often have a "barrel chest" from overinflation of the lungs. Emphysema is irreversible; treatment includes bronchodilators, oxygen therapy, and lung volume reduction surgery (in severe cases). Chronic bronchitis involves inflammation and excess mucus production in the bronchi, causing chronic cough ("smoker's cough"). Smoking is responsible for about 85% of lung cancer cases. Lung cancer is the leading cause of cancer death worldwide (about 1.8 million deaths per year). Cilia in the respiratory tract are paralyzed and eventually destroyed by smoking; without cilia, mucus accumulates, leading to smoker's cough. Quitting smoking can partially reverse damage: cilia regrow within weeks to months, lung function improves, and the risk of lung cancer decreases over time (but may never return to that of a non-smoker). Smoking also harms the cardiovascular system (increases risk of heart attack and stroke) and causes other cancers (bladder, pancreas, kidney, mouth, esophagus, larynx). The WHO estimates that tobacco kills up to half of its users (about 8 million people per year). Electronic cigarettes (vapes) are not safe; they contain nicotine (addictive), heavy metals, and flavoring chemicals that can cause lung injury (EVALI).

Nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli is the correct order! The pharynx (throat) is a passageway for both air and food. The epiglottis covers the larynx during swallowing to prevent choking. The trachea divides into the left and right bronchi (singular: bronchus). The bronchioles are smaller branches without cartilage. The alveoli are the terminal air sacs where gas exchange occurs. The total number of alveoli is about 300-500 million. The lungs are not hollow; they are spongy due to the millions of air sacs. The right lung is larger and has three lobes; the left lung has two lobes (to accommodate the heart). The pleural membranes (visceral and parietal) surround the lungs, with a thin layer of pleural fluid that reduces friction during breathing. Pleurisy is inflammation of the pleural membranes, causing sharp chest pain. You have learned about the anatomy of the respiratory system, gas exchange, breathing mechanics, diseases, and the effects of smoking. You are now a respiratory system expert!