22.1 Anatomy and normal microbiota of the respiratory tract

Learning objectives

• Describe the major anatomical features of the upper and lower respiratory tract
• Describe the normal microbiota of the upper and lower respiratory tracts
• Explain how microorganisms overcome defenses of upper and lower respiratory-tract membranes to cause infection
• Explain how microbes and the respiratory system interact and modify each other in healthy individuals and during an infection

The primary function of the respiratory tract is to exchange gases (oxygen and carbon dioxide) for metabolism. However, inhalation and exhalation (particularly when forceful) can also serve as a vehicle of transmission for pathogens between individuals.

Anatomy of the upper respiratory system

The respiratory system can be conceptually divided into upper and lower regions at the point of the epiglottis , the structure that seals off the lower respiratory system from the pharynx during swallowing ( [link] ). The upper respiratory system is in direct contact with the external environment. The nares (or nostrils) are the external openings of the nose that lead back into the nasal cavity , a large air-filled space behind the nares. These anatomical sites constitute the primary opening and first section of the respiratory tract, respectively. The nasal cavity is lined with hairs that trap large particles, like dust and pollen, and prevent their access to deeper tissues. The nasal cavity is also lined with a mucous membrane and Bowman’s glands that produce mucus to help trap particles and microorganisms for removal. The nasal cavity is connected to several other air-filled spaces. The sinuses, a set of four, paired small cavities in the skull, communicate with the nasal cavity through a series of small openings. The nasopharynx is part of the upper throat extending from the posterior nasal cavity. The nasopharynx carries air inhaled through the nose. The middle ear is connected to the nasopharynx through the eustachian tube . The middle ear is separated from the outer ear by the tympanic membrane , or ear drum. And finally, the lacrimal glands drain to the nasal cavity through the nasolacrimal ducts (tear ducts). The open connections between these sites allow microorganisms to move from the nasal cavity to the sinuses, middle ears (and back), and down into the lower respiratory tract from the nasopharynx.