This figure shows an islet of Langerhans (IL) among the far more numerous acini. (Islets are most numerous in the tail of the pancreas and least numerous in the head). Cells within the islets are arranged as irregular cords. In routine preparations, it is not possible to identify the various cell types within the islets. Note, however, that B cells are the most numerous; these produce insulin. The next most numerous are A cells; these produce glucagon. The inset also shows numerous capillaries (arrows). The labels A and B are not intended to identify specific cells but rather to show those parts of the islets where A and B cells are found in greatest number.
ducts in the pancreas.) Leading to the intralobular duct is an intercalated duct (ID), which is seen in cross section at the furthest distance from the intralobular duct and then, in longitudinal section, in the center of the illustration as it travels toward the intralobular duct. The lumen is evident where the intercalated duct is seen in cross section but is not evident where it is seen in longitudinal section. This is because the plane of section cuts chiefly through the cells rather than the lumen. As a consequence, this figure provides a good view of the nuclei of the duct cells. They are elongate, with their long axis oriented in the direction of the duct. In addition, they display a staining pattern similar to that of centroacinar cells and different from that of nuclei of the parenchymal cells.
Once the cells of the intercalated duct have been identified in one part of the section, their staining characteristics and location can be used to identify the intercalated ducts in other parts of the lobule, several of which are marked (ID).
KEY A, region with most A cells B, region with most B cells BV, blood vessels CC, centroacinar cells CT, connective tissue Er, ergastoplasm ID, intercalated ducts IL, islets of Langerhans InD, intralobular duct arrows, capillaries asterisks, lumen of acini
OVERVIEW OF THE RESPIRATORY SYSTEM / 664 NASAL CAVITIES / 665 Vestibule of the Nasal Cavity / 665 Respiratory Region of the Nasal Cavity / 666 Olfactory Region of the Nasal Cavity / 667 Paranasal Sinuses / 670 PHARYNX / 670 LARYNX / 670 TRACHEA / 670 Tracheal Epithelium / 672 Basement Membrane and Lamina Propria / 673 BRONCHI / 676 BRONCHIOLES / 677 Bronchiolar Structure / 677 Bronchiolar Function / 678 ALVEOLI / 678 BLOOD SUPPLY / 687 LYMPHATIC VESSELS / 687 NERVES / 687 Folder 19.1 Clinical Correlations: Squamous Metaplasia in the Respiratory Tract / 672 Folder 19.2 Clinical Correlations: Cystic Fibrosis / 685 Folder 19.3 Clinical Correlations: Emphysema and Pneumonia / 686
The respiratory system consists of the paired lungs and a series of air passages that lead to and from the lungs. Within the lung, the air passages branch into increasingly smaller tubes until the very smallest air spaces, called alveoli, are reached (Fig. 19.1).
Three principal functions are performed by this system: air conduction, air filtration, and gas exchange (respiration). The latter occurs in the alveoli. In addition, air passing through the larynx is used to produce speech, and air passing over the olfactory mucosa in the nasal cavities carries stimuli for the sense of smell. The respiratory system also participates to a lesser degree in endocrine functions (hormone production and secretion), as well as regulation of immune responses to inhaled antigens.