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Transcript
Cells of the Respiratory System Epithelial Cell Types Respiratory epithelial cells include three types: a) ciliated cells, b) goblet cells, and c) basal cells. Ciliated cells have cilia on the surface, Goblet cells, shaped like a wine goblet, secrete mucus. Basal cells, found at the basal region of epithelial sheets, are stem cells used to regenerate and maintain a healthy epithelial cell layer. • Cilia The apical surfaces of ciliated epithelia cells possess specialized structures called cilia. Depending upon the tissue cilia may function in either sensory or mechanical capacities. The cilia in the nasal cavity and bronchial tree sweep mucus toward the pharynx to be swallowed or expectorated. Swallowing delivers the microorganisms and particles that were trapped in the mucus to the stomach where the low pH of the stomach will destroy them. • Macrophages in the lung The respiratory system interacts with the external environment during gas exchange. This interaction can provide contact with pathogens (viruses, bacteria, and other disease causing organisms), atmospheric debris and other particulates. We have already covered how the sticky mucus traps many pathogens and particles and facilitates removal from the body. Yet the mucus does not trap all inhaled particles. Particles that make it to the level of the alveoli are typically removed by alveolar macrophages through the process of phagocytosis. • Alveolar cells In the alveoli there are specialized epithelial cells called alveolar cells. Type I alveolar cells are very thin, simple squamous cells through which gases easily diffuse. These cells are so thin that they can only be seen through the use of an electron microscope. The type I epithelial cells also make angiotensin-converting enzyme (ACE), an important enzyme of the renin-angiotensin system used in the control of blood pressure. Inhibition of this enzyme is one of the methods used to control hypertension. Scattered among the squamous cells are type II alveolar cells. Type II alveolar is cuboidal epithelial cells with microvilli on their apical surface. These cells make and secrete surfactant, which decreases the surface tension on the alveolar surfaces. The alveoli themselves are so thin that without the surfactant the force of surface tension created by the water on the cell surfaces would cause the alveoli to collapse upon exhalation. Mixed in among the types I and II alveolar cells are macrophages that migrate within the sacs and clean up material that has entered this part of the system. These cells may accumulate and sequester non-biodegradable material that persists in the cells. Cells that contain accumulated material are referred to as dust cells. The alveoli are considered to be functionally sterile, due to the combination of the mucus and cilia found through most of the respiratory membranes and the macrophages in the alveoli. Epithelial Types Throughout the Respiratory system In the nasal cavities and upper respiratory tract, epithelial cells are primarily ciliated psuedostratified columnar epithelium. The bronchial tree is lined with pseudostratified, ciliated, columnar epithelia with goblet cells dispersed among the columnar cells. At the terminal bronchioles, epithelial become ciliated cuboidal cells without any goblet cells. Anatomy of the Respiratory-Cardiovascular Junction There is intimate contact between respiratory tissue and the blood supply in the lungs. Alveolar sacs in the lungs are wrapped in capillary beds of the cardiovascular system. At the cellular level, the simple squamous alveolar cells are in close contact with the capillaries, which are also one endothelial cell thick. Oxygen from the inhaled air diffuses from the alveoli to the hemoglobin in the red blood cells. In order to do so, it has to diffuse through the alveolar epithelial cell, the capillary endothelial cell, plasma in the capillary, and into the red blood cell.