PRINCIPLES OF TOXICOLOGY - Biology East Borneo

170 PULMONOTOXICITY: TOXIC EFFECTS IN THE LUNGInhaled air is highly “ conditioned” before it leaves the upper airway system. Relatively cold air,for instance, will be warmed to body temperature (37 °C) before it reaches the lung. In like manner,air that is at an elevated temperature will be cooled to body temperature within the nasopharyngealsystem. The lung and the portion of the respiratory system below the upper airway is a very moistphysiological system and is quite sensitive to humidity. The inhaled air is therefore highly humidifiedduring its passage from the nares to the lung, and the air that enters the nares is cleared of the largerparticles. The nose hairs function to some extent in this process, and the turbulent nature of the airpassages in the nares also contributes to the deposition of the larger particles, preventing them frombeing inhaled into the lower passages of the respiratory system.The lining of the nasal wall is known as the mucosa and is highly inundated with blood capillaries.Therefore, air that is inhaled through the nose comes immediately into contact with mucosal surfaces,which only thinly separate the air from these blood vessels. Deposition of toxic chemicals in the upperairway system can therefore result in both toxicity to the mucosal tissue and absorption of the agentinto the systemic circulation by way of these capillaries.Sinus CavitiesThere are four pairs of hollow cavities within the skull that are lined with a mucosal lining that issimilar to the lining of the nasopharyngeal region. In order to view these sinuses from different angles,Figure 9.1 shows a frontal view of the skull, while Figure 9.2 represents a sagittal view. Since thesinuses are connected to the nasopharyngeal airways through a number of small openings, inhaled airalso enters the sinuses. Acute sinusitis can occur when inhaled airborne toxins irritate the surfaces ofsinus mucosa. As in other parts of the respiratory system, irritation of the mucosal lining leads to aninflammatory response in these tissues. As a result of the inflammation, there is an accumulation ofFigure 9.1 Frontal view of the skull, showing frontal, maxillary, and ethmoid sinuses. (Reproduced withpermission from W. O. Fenn and H. Rahn, Handbook of Physiology. American Physiology Society, Washington,DC, 1964.)