Human Respiratory System

The human respiratory system is a single, continuous, branching passage running from the two external nostrils down to the microscopic alveoli deep within a pair of lungs, and every organ along this passage plays a specific part in conducting, cleaning, warming and moistening the air a person breathes before it ever reaches the site where gases are actually exchanged with the blood.

Air first enters through the external nostrils, a pair of openings guarded by fine hairs that trap larger dust particles, and passes into the nasal cavity, a chamber lined internally by a mucus-secreting, ciliated epithelium richly supplied with blood vessels. This lining performs three jobs at once as air passes over it: it warms the incoming air toward body temperature, moistens it with water vapour, and filters out fine dust and many microorganisms, which are trapped in the mucus and swept by the beating cilia back toward the throat to be swallowed or expelled, so that the air reaching the lower respiratory passage is considerably cleaner, warmer and more humid than the air that first entered the nostrils.

From the nasal cavity, air passes into the pharynx, a common passage at the back of the throat that is shared with the digestive tract -- both the path taken by inhaled air and the path taken by swallowed food and drink cross here, and a flap of elastic cartilage called the epiglottis normally folds down over the opening of the airway during swallowing, to prevent food or liquid from being misdirected into the respiratory passage.

The pharynx narrows below into the larynx, popularly called the voice box because it houses the vocal cords, a pair of elastic membranous folds that vibrate as air passes over them to produce sound; the larynx also continues the work of warming, moistening and filtering the air, and its cartilaginous wall keeps this short passage permanently open. Below the larynx, the larynx continues as the trachea, a fairly wide, straight tube running down through the neck and into the thorax; the trachea's wall is reinforced by a series of C-shaped rings of cartilage, incomplete at the back where the tube lies against the food pipe (oesophagus), and it is this cartilage that keeps the trachea permanently open and prevents it from collapsing even as pressure inside the thorax changes during breathing.

Within the thorax, the trachea divides into a right and a left primary bronchus, one entering each lung; because the two lungs must serve very different volumes on the two sides of the thorax (the right lung is somewhat larger than the left, since the heart occupies space on the left side), the right bronchus is typically slightly wider and more vertically oriented than the left. Once inside a lung, each primary bronchus branches repeatedly, first into several secondary and then tertiary bronchi, and then into a very large number of progressively finer bronchioles, in a highly ramified, tree-like pattern often described as the bronchial tree; this progressive branching greatly multiplies the total number of small airways carrying air deeper and deeper into the lung tissue.

Each of the finest bronchioles finally opens into a cluster of thin-walled, balloon-like sacs called alveoli (singular: alveolus) -- and it is here, and only here, that actual gas exchange with the blood takes place, described in a later section. Each alveolus is surrounded by an extremely dense network of pulmonary blood capillaries, and its wall, together with the wall of the adjoining capillary, is only a single cell thick on each side, keeping the diffusion distance for oxygen and carbon dioxide as short as physically possible. …