Pneumonia is acute INFLAMMATION of the lung PARENCHYMA (Parenchyma is medicalese for the substance of an organ; in the lung, this means the very smallest bronchi and the alveoli – of which, more later).

I’ve previously mentioned ‘acute’ and ‘chronic’ which to a doctor mean something slightly different to lay people. Inflammation that is ‘acute’ means it comes on quickly and involves mainly NEUTROPHILS (aka neutrophil polymorphonuclocytes), a type of white blood cell. Pus is dead neutrophils. Inflammation that is ‘chronic’ lasts a long time and involves mainly LYMPHOCYTES (another type of white blood cell), which become (ie change into) plasma cells, and MACROPHAGES (which occur in the blood but are also omni-present in tissues). Acute inflammation may go away, especially if humans intervene, but it may also become ‘acute-on-chronic’ (a mixture of the 2 types) and eventually ‘chronic’. If the body walls off the acute infections (by the formation of a fibrous tissue wall around it), then it may became an abscess (with pus inside).

The term’CHEST INFECTION’ is a generic term for (not surprisingly) infection in the chest, This can include BRONCHITIS (acute or chronic, referring to the type of white blood cell) which is inflammation of the bronchi (pleural of bronchus) the tubes that transfer air to the ALVEOLI (pleural of alveolus – it’s all Latin!), which are the end-sacs where air meets the blood and oxygen is transferred.

When pneumonia occurs, infection gets down to the alveoli, and these fill up with neutrophils (pus cells) and fluid (OEDEMA or, if you’re from the US, EDEMA). Thus air and blood do not interact properly and oxygen cannot transfer from air to blood. It can be LOBAR PNEUMONIA (meaning that an entire lung lobe is infected throughout – the right lung has 3 lobes, the left has 2 to make room for the heart) or BRONCHOPNEUMONIA (meaning that there is patchy involvement throughout the lobes, most commonly the lower lobes. DOUBLE PNEUMONIA means both right and left lung are involved. When looked at with the naked eye, the affected lung tissue is no longer like an empty sponge, it is like a sponge that that has been dipped in diluted blood and pus; it is heavy and solid, hence the term CONSOLIDATION.

Pneumonia is usually caused by bacteria; which bacteria cause it depends on who you are and what pre-existing conditions you have. Pneumonia de novo (‘from new’ – Latin again) tends to occur in the young and the old. If you have chronic bronchits, you are prone of pneumonia. The term PNEUMONITIS is reserved for infection of the lung parenchyma by viruses.

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