Respiratory System

General Points about the respiratory system

The respiratory system is divided into upper and lower zones. Above the larynx, the respiratory system contains normal flora. Part of the normal flora may be disease-causing organisms, but they are often carried as normal flora (i.e.: if some organisms from here are taken and put somewhere else in the body, then they might cause disease).

The respiratory system is exposed to air; hence it has many defence mechanisms against infective organisms. The lower respiratory tract has macrophages which can be mobilised in case of infection. The upper respiratory tract has functional cilia, which act to remove mucus from the tract. If there is any impairment in these defences, then there is  incidence of infections (i.e.: smoking impairs ciliary function – therefore mucus is stagnant and provides a favourable environment to infection).

Nomenclature

Pneumonia: infections involving the lung parenchyma and air spaces, often showing consolidation. Cause: typically bacterial  Strep Pneumoniae.

Bronchitis: infections of the bronchial mucosa WITHOUT involvement of lung parenchyma (if involved, then we called it Bronchopneumonia)

Pneumonitis: interstitial inflammation caused by viral infections or toxic compounds.

Lobar pneumonia: infection of lung parenchyma within anatomic boundaries of a lobe. Cause: Strep Pneumoniae, Klebsiella Pneumonia (not common).

Aetiological agents lung infections

Broadly speaking pneumonia can be caused by: bacteria, virus, fungi, parasites. Some organisms can cause disease in healthy individuals (i.e.: Mycoplasma pneumoniae, Strep. Pneumoniae), some organisms cause disease in immunocompromised individuals (i.e.: Pneumocystis carinii), other organisms cause disease in young children where immune system is imcompletely developed (i.e.: respiratory syncytial virus).

Category: Microbiology Notes