Definition
Air travels: nasal cavity pharynx larynx trachea bronchi bronchioles alveoli.
Fine hairs and mucus lining the nasal cavity filter out dust, pathogens, and other particles from incoming air, and also help warm and moisten it before it reaches the lungs.
The larynx (commonly called the voice box) contains the vocal cords, which vibrate as air passes over them to produce sound. Immediately after passing through the larynx, air enters the trachea.
The trachea splits into two bronchi (one to each lung), which repeatedly branch into progressively narrower bronchioles, finally ending in clusters of tiny air sacs called alveoli, where gas exchange actually happens.
Definition
The trachea and bronchi have C-shaped cartilage rings in their walls to keep these airways permanently open and prevent them from collapsing, even as air pressure changes during breathing — bronchioles don't have cartilage because they're narrow enough, and surrounded by enough other lung tissue and smooth muscle, that they don't need rigid structural support to stay open; their smooth muscle instead lets their diameter be actively adjusted.
Definition
The right lung has lobes; the left lung has lobes (slightly smaller, to make room for the heart).
The pulmonary artery is the blood vessel that carries deoxygenated blood from the heart to the lungs (unusual among arteries, which usually carry oxygenated blood, since here it's the direction — away from the heart — that defines it as an artery, not the oxygen content).
Definition
The pleura is a double membrane surrounding each lung, with pleural fluid between its two layers — this fluid reduces friction between the lungs and the chest wall as they move against each other during breathing, letting the lungs expand and contract smoothly.
Definition
Alveoli are surrounded by a dense network of capillaries so that oxygen and carbon dioxide can diffuse directly and efficiently between the air in the alveoli and the blood, over the shortest possible distance.
Features of alveoli that maximize gas exchange: extremely thin walls (just one cell thick, minimizing diffusion distance), a huge total surface area (millions of tiny alveoli combined), a moist inner lining (gases dissolve before diffusing, which requires moisture), and a dense capillary network (maintaining a steep concentration gradient by constantly carrying gases away/bringing them in).
Definition
Gaseous exchange is the physical diffusion of oxygen and carbon dioxide between the alveolar air and the blood (or, at the cellular level, between blood and body cells) — a passive process driven by concentration gradients. Breathing (ventilation) is the mechanical, muscular process of moving air into and out of the lungs, which maintains the concentration gradients that make gaseous exchange possible, but is a distinct, separate process from the actual gas exchange itself.
Breathing is also distinct from cellular respiration: breathing physically moves air, while cellular respiration is the biochemical process inside cells that uses oxygen to release energy from glucose (producing carbon dioxide as a by-product) — breathing supplies the oxygen and removes the waste that cellular respiration needs and produces, but the two are not the same process.
Definition
Inhalation: the diaphragm contracts and flattens (moving down), and the intercostal muscles contract, pulling the rib cage up and outward — together these increase the volume of the chest cavity, lowering pressure inside the lungs below atmospheric pressure, so air flows in.
Exhalation: the diaphragm relaxes and domes upward, and the intercostal muscles relax, letting the rib cage move down and inward — this decreases chest cavity volume, raising pressure inside the lungs above atmospheric pressure, so air flows out.
If the diaphragm stops working properly, breathing becomes seriously impaired or impossible unaided, since it's the primary muscle driving the pressure changes needed for normal, effortless breathing — a person could still breathe to some degree using intercostal and other accessory muscles, but far less efficiently.
Definition
Breathing rate is primarily controlled by the level of carbon dioxide in the blood, detected by the brain (specifically the medulla oblongata) — rising signals the brain to increase breathing rate and depth, while falling allows it to decrease. Average breathing rate at rest is roughly – breaths per minute, rising considerably (often to –+) during vigorous exercise, when the body produces much faster.
Definition
Composition of inspired vs expired air: oxygen falls from about (inspired) to about – (expired); carbon dioxide rises from about (inspired) to about (expired); nitrogen stays essentially unchanged at about in both, since the body neither absorbs nor produces nitrogen gas. Besides oxygen and carbon dioxide, expired air also contains noticeably more water vapour than inspired air, and is slightly warmer, both picked up from the moist, warm lining of the airways.
Definition
Bronchitis is inflammation of the bronchi's lining, usually from infection or irritants. Symptoms: persistent cough (often with mucus), wheezing, chest discomfort. Causes: viral/bacterial infection, smoking, air pollution. Treatment: rest, fluids, and in bacterial cases, antibiotics; avoiding irritants like smoke.
Emphysema is a chronic condition in which the walls of the alveoli are broken down and destroyed, merging many small alveoli into fewer, larger air spaces — this drastically reduces the total surface area available for gas exchange (contradicting the whole point of having millions of tiny alveoli), causing breathlessness. Symptoms: persistent breathlessness, chronic cough. Treatment: no cure (the alveolar damage is permanent), but symptoms are managed with bronchodilators, oxygen therapy, and stopping smoking to prevent further damage.
Pneumonia is caused by infection of the alveoli (typically bacterial or viral), which fill with fluid or pus — this interferes directly with gas exchange, since the alveoli's air spaces meant to hold air (and be moist for diffusion) instead get flooded with fluid. Symptoms: fever, cough, difficulty breathing, chest pain. Treatment: antibiotics for bacterial pneumonia, rest, and supportive care (fluids, sometimes oxygen).
During an asthma attack, the airways (particularly bronchioles) become inflamed and constricted, and patients find exhaling harder than inhaling because the narrowed airways trap air inside the lungs — inhaling can still pull some air in past the narrowing, but the same narrowed passages resist the outward flow of air even more during the relatively passive act of exhaling, since there's less pressure difference driving it out.
Definition
Carcinogens are substances that can cause cancer by damaging DNA and disrupting normal cell division control. Two carcinogens present in cigarette smoke: tar and various polycyclic aromatic hydrocarbons (benzopyrene is a commonly named example).
Nicotine is the chemical in cigarette smoke directly responsible for addiction, since it rapidly affects the brain's reward pathways, creating dependency.
Smoking leads to lung cancer mainly through the repeated exposure of lung tissue to carcinogens in tar and smoke, which progressively damage DNA in lung cells over years, eventually triggering uncontrolled cell division.
It's dangerous when smoke particles settle inside the lungs because they physically coat the delicate alveolar surfaces and irritate/damage the airway lining over time, impairing gas exchange, triggering chronic inflammation, and providing the ongoing carcinogen exposure that raises cancer risk.
Carbon monoxide in cigarette smoke harms the body by binding to haemoglobin far more strongly than oxygen does, reducing the blood's oxygen-carrying capacity and effectively starving tissues of oxygen even while breathing normally.
Beyond lung cancer, smoking's other harmful effects include chronic bronchitis and emphysema, increased risk of heart disease and stroke (from reduced oxygen delivery and blood vessel damage), and harm to nearly every organ system over time.
Remember
Vaping is inhaling a vapour produced by heating a liquid (often containing nicotine and other chemicals) using an electronic device. It's not actually a safer alternative to smoking because vaping liquids and their vapour can still contain nicotine (addictive) and various potentially harmful chemicals, and the long-term health effects of inhaling these substances are still not fully understood — 'different from cigarette smoke' doesn't mean 'harmless'.
Quick Recap
- ●Airway: nasal cavity (filters/warms) pharynx larynx (vocal cords) trachea/bronchi (cartilage rings, stay open) bronchioles (no cartilage) alveoli
- ●Alveoli: thin walls, huge surface area, moist, capillary network maximize gas exchange (diffusion, not breathing itself)
- ●Breathing gaseous exchange cellular respiration — three distinct but connected processes
- ●Inhalation: diaphragm down ribs up/out volume up, pressure down air in. Exhalation: reverse
- ●Breathing rate controlled by blood CO (medulla); expired air: less O (–), more CO (), more water vapour, same N
- ●Bronchitis (inflamed bronchi), emphysema (destroyed alveoli walls less surface area), pneumonia (alveoli fill with fluid), asthma (constricted airways, harder to exhale)
- ●Smoking: carcinogens (tar) DNA damage cancer; nicotine addiction; CO binds haemoglobin, reduces O delivery; vaping is not risk-free
