HIstology

• The respiratory system consists of two major components- the conductive and the respiratory parts
• Conductive portion

  Includes all the structures from the nares to the terminal bronchioles
• Respiratory portion

  Starts from the respiratory bronchiole in the alveoli
• Regions of the nasal cavity

  • Vestibular
  • Respiratory
  • Olfactory
• Vestibular region

  • Point of reflection of the skin as the mucous membrane
  • Lamina propria-t. submucosa is coarse areolar connective tissue
  • Hairs (vibrissae), sweat glands, sebaceous glands occur
  • Branched tubuloalveolar serous and mixed glands may also be present
• Respiratory region

  • Lining epithelium is pseudostratified ciliated with goblet cells
  • Muscularis mucosae is absent
  • Lamina propria-t.submucosa is areolar connective tissue
  • Glandular tissue humidifies the air
  • Engorged erectile tissue cools or warms the air
• Olfactory region

  • Lining epithelium is pseudostratified columnar nonciliated
  • Sustentacular cells are tall with broad apices and narrow bases
  • Basal cells are cuboidal
  • Olfactory cells are modified neurons
• The nasopharynx connects the nasal cavity with the oropharynx
• Nasopharynx

  • Lining epithelium is pseudostratified columnar ciliated
  • No muscularis mucosae
  • Lamina propria-t. submucosa consist of loose connective tissue with numerous diffuse and nodular lymphatic tissue and tonsils
  • T. muscularis is skeletal muscle in various orientation
• Larynx

  • Lining epithelium is either stratified squamous or pseudostratified columnar ciliated
  • No muscularis mucosae
  • Lamina propria-t. submucosa is areolar connective tissue with diffuse and nodular lymphatic tissue and branched tubuloalveolar mucous, serous and mixed glands
  • T. muscularis consist of striated muscle and cartilage
• Trachea

  • Lining epithelium is pseudostratified columnar ciliated with goblet cells, basal cells, brush cells, serous cells, Clara and Kultschitzky or neuroendocrine (APUD) cells
  • No muscularis mucosae
  • Lamina propria-t. submucosa is areolar c.t. and presents branched coiled tubuloalveolar mucous glands
  • T. muscularis is reduced to a transversely oriented mass of smooth muscle (trachealis muscle)
• Extrapulmonary bronchi are structurally similar to the trachea
• Lung

  Structurally is considered as a compound tubuloalveolar gland
• Types of Lung

  • Type I (Ruminants and pigs)
  • Type II (Dog, cat and monkey)
  • Type III (Horse and human)
• Type I Lung

  • With well-developed secondary lobules
  • Thick interlobular septa and thick pleura
  • Alveoli between lobules are without interalveolar pores of Kohn
  • Complete interlobular septa which extends from connective tissue of visceral pleura to c.t. surrounding major bronchi and blood vessels
• Type II Lung

  • Without secondary lobules
  • With poorly developed intraparenchymal supportive tissue strands
  • With thin visceral pleura
• Type III Lung

  • Incompletely developed secondary lobules
  • Well-defined but disorderly arrangement of interlobular septa
  • Thick visceral pleura
• Structures found in the lung

  • Intrapulmonary bronchi
  • Bronchioles
  • Respiratory bronchioles
  • Terminal bronchiole
  • Alveolar ducts
  • Alveoli
• Intrapulmonary bronchi


  • Lining epithelium is pseudostratified columnar ciliated with goblet cells
  • Lamina propria is areolar
  • Muscularis mucosae is present
  • T. submucosa is areolar with branched, coiled tubuloalveolar mucous glands
• Bronchioles


  • Lining epithelium is simple columnar or simple cuboidal cells and is devoid of goblet cells
  • Lamina propria is fine collagenous and elastic fiber
  • Muscularis mucosae is present and continuous
  • Cartillage is absent
• Respiratory bronchioles


  • Lining epithelium is simple cuboidal, some which are ciliated
  • Lamina propia is indistinct
  • Muscularis mucosae is present but loosely organized
• Terminal bronchiole


  The most distal conducting airway free of alveoli
• Clara cells


  Bronchiolar exocrine cells with secretory function and are also capable of metabolizing xenobiotic compounds
• Alveolar ducts


  • Completely lined by alveoli
  • Smooth muscle may be present along the luminal border at the apices between adjacent alveoli
• Alveoli


  • Lining epithelium consists of Type I pneumocytes (for gaseous exchange) and Type II pneumocytes (producer of alveolar fluid with pulmonary surfactant)
  • Other cells: alveolar macrophages and alveolar brush cells
• Interalveolar Septa


  • Thin sheets of tissue containing a capillary plexus and covered on both sides by a layer of epithelium (consisting of Type I & II pneumocytes) and capillary endothelial cells that are attached to a continuous basal lamina
• Arterial systems of the lung

  • Pulmonary artery and its peripheral subdivisions
  • Bronchial arteries
• Avian respiratory system


  • Upper respiratory tract includes the nasal cavity, trachea, primary bronchi and syrinx
  • Lungs are extremely small compared to the size of the thoracic cavity and are not expandable
  • Bronchial tree is distinctly different from mammals with primary bronchi, vestibulum, mesobronchus, secondary bronchi and parabronchi
  • Air sacs aid in the movement of air through the lung
• Mural elements of the avian respiratory system


  • Intrapulmonary bronchi
  • Vestibulum and mesobronchus
  • Secondary bronchi
  • Parabronchi
  • Air vesicles
  • Air capillaries
• Secondary bronchi


  • Lining epithelium is simple columnar or simple cubiodal devoid of goblet cells
  • Lamina propria is loose connective tissue and usually devoid of lymphatic tissue
  • Muscularis mucosae is interrupted due to the presence of parabronchi and in some areas, air vesicles
• Parabronchi


  • Lining epithelium is interrupted by laterally projecting air vesicles
  • Between interruptions the parabronchi are lined by simple cuboidal or simple columnar epithelium
• Air vesicles and air capillaries

  • Lined by simple squamous epithelium
• Cortex


  • Granular appearance due to the presence of many renal corpuscles
• Medulla


  • Striated appearance due to the presence of many ducts and tubules
• Regions of the cortex

  • Cortex proper or cortical labyrinth
  • Medullary ray
• Cortex proper or cortical labyrinth

  Contains renal corpuscles, proximal and distal convoluted tubules
• Medullary ray


  Composed of the descending and ascending limbs of the loops of Henle and straight collecting tubules
• Regions of the medulla

  • Outer zone
  • Inner zone
• Outer zone of the medulla

  Contains the loops of Henle of short nephrons and straight collecting tubules
• Inner zone of the medulla

  Consists of loops of Henle of long neprons, straight collecting ducts and papillary ducts