Sec 3 Combined Science (Biology) Respiration in Humans notes

Air entering the mouth or nose passes the larynx, the voice box at the top of the airway, and continues down the trachea (windpipe), held open by rings of cartilage. In the chest the trachea splits into two bronchi — one for each lung — which branch into ever-narrower bronchioles, ending in clusters of microscopic air sacs: the alveoli. Each alveolus is wrapped in a net of capillaries, and that contact is the whole point: oxygen diffuses from alveolus to blood, and carbon dioxide diffuses from blood to alveolus, to be breathed out.

Order matters in labelling questions: larynx → trachea → bronchi → bronchioles → alveoli. Alveoli are the only exchange site — everything before them is plumbing.

Gaseous exchange runs on diffusion, so the alveolus is shaped to maximise it. First, numbers: hundreds of millions of alveoli give a total surface area of around 70 m² — roughly a tennis court packed inside the chest. Second, thin walls: the alveolus wall and the capillary wall are each only one cell thick, so the diffusion distance from air to blood is a couple of micrometres. Third, a moist lining lets oxygen dissolve before diffusing — gases only cross membranes in solution. Fourth, a dense capillary network constantly flows blood past, carrying oxygen away and delivering carbon dioxide, which keeps the concentration gradients steep on both sides.

Reading the table: expired air has lost about a quarter of its oxygen (taken up by the blood) and gained a hundredfold more carbon dioxide (released from the blood), while nitrogen passes through unchanged — it is not used by the body.

Nicotine is the addictive drug of smoke: it speeds the heart and narrows blood vessels, straining the circulation — and its addictiveness is what keeps smokers smoking. Tar is the sticky residue that coats the air passages and alveoli: it damages and kills the cells lining the airways, paralyses the cleaning mechanisms, and contains chemicals that cause cancer of the lung. Carbon monoxide is the silent thief: it binds to haemoglobin far more strongly than oxygen does, so smokers' blood carries less oxygen — the heart and tissues must work harder on a reduced supply.

Keep the trio distinct in answers: nicotine = addiction and vessel narrowing; tar = blockage, cell damage, cancer; carbon monoxide = oxygen starvation via haemoglobin. Mixing them — "tar is addictive" — is the standard mark-loser.

Aerobic respiration is the release of energy from glucose using oxygen, in the cells) mitochondria. Its word equation is the most important sentence in the topic: glucose + oxygen → carbon dioxide + water (+ energy). Notice it is combustion)s cousin — food "burning" slowly and controllably — which is exactly why the breathing system exists: to deliver the oxygen reactant and clear the carbon dioxide product.

Anaerobic respiration is the release of energy from glucose without oxygen. In human muscle its word equation is glucose → lactic acid (+ energy). During vigorous exercise the blood simply cannot deliver oxygen fast enough for the muscles) enormous demand, so muscle cells switch to anaerobic respiration to keep contracting. The price is lactic acid building up in the muscles — and an oxygen debt: the body now "owes" oxygen to break that lactic acid down.

Repayment is why you gasp after stopping: rapid, deep breathing continues after exercise, flooding the blood with the extra oxygen needed to dispose of the lactic acid. Only when the debt is cleared does breathing return to its resting pace.