Respiratory System

Resources

Respiratory System

AtlasWheater’s (5th ed) Chapter 12 Respiratory System
TextRoss and Pawlina (6th ed) Chapter 19 Respiratory System

OBJECTIVES:

  1. Know the basic components of the conducting and respiratory portions of the system and describe distinctive structural features of each component related to particular functions in respiration.
  2. Know the types of cells present in the respiratory epithelium and their functions in respiration.
  3. Be able to identify the trachea, bronchi, terminal bronchioles, respiratory bronchioles, alveolar ducts and alveoli of the respiratory tract on the basis of:
    1. epithelial cell types present, and
    2. relative amounts of glands, cartilage, smooth muscles and connective tissue fibers present in the wall of the tubes.
  4. Be able to name the cellular and structural elements that form the blood-air barrier.
  5. Know the vascular supply of lungs and be able to identify the pulmonary artery, pulmonary vein, and the bronchial artery on the basis of location, structure, and tissues supplied.

For descriptive purposes the respiratory system is divided into (a) conducting and (b) respiratory portions. The conducting part consists of passageways which carry air to the respiratory portion. The respiratory portion begins at the level where alveoli first appear in the final branches of the bronchioles. Respiration (gaseous exchange between blood and air) occurs across the walls of the alveoli. The conducting portion includes parts which are outside the lung or extrapulmonary (nasal cavities, nasal pharynx, larynx, trachea and primary bronchi), and those which are inside the lung or intrapulmonary (rest of the bronchi and bronchioles). All of the respiratory portion is, of course, intrapulmonary and consists of the respiratory bronchioles, alveolar ducts, and alveoli). Click here to see a diagram summarizing the histological organization of the respiratory system.

I. Extrapulmonary Conducting Passageways

A. Nasal Cavity (W pg 225, 12.2-3)
Slide 124 (nasal cavity, frontal sect., monkey, H&E) WebScope ImageScope
Slide 124F (nasal vestibule, frontal sect., monkey, H&E) WebScope ImageScope
Slide 124P (post. nasal cavity, frontal sect., monkey, H&E) WebScope ImageScope
Slide 124-O (post. nasal cavity, frontal sect., monkey, H&E) WebScope ImageScope
Slide 115 (fetal palate, sag. sect, H&E) WebScope  ImageScope

We will look at a frontal section of the nose (slide #124). Identify the palate, nasal septum and nasal cavities. The epithelial lining at the entrance (vestibule) to the nasal cavity exhibits a gradual change from keratinized stratified squamous epithelium of the skin in the nasal vestibule (shown in slide #124F) [example] , to the pseudostratified columnar ciliated epithelium that is characteristic of the nasal mucosa posterior to the vestibule (shown in slides #124 and #124P) [example] and much of the rest of the respiratory system. Therefore, the epithelium is called respiratory epithelium.

Note, slides 124F, 124, and 124P were cut in the FRONTAL (coronal) plane, so you will NOT be able to see the actual transition from squamous to respiratory epithelium in the nasal cavity (to see this transition, you would need a sagittal section). Slide 124F is the anteriormost of these sections (the teeth that can be seen below the nasal cavity are incisors) and lining of the nasal cavity is entirely stratified squamous epithelium. Slides 124 and 124P are more posterior sections (the teeth in these slides are molars), so the lining present in these slides is pretty much entirely respiratory epithelium.

Olfactory epithelium, specialized for smell, lines the roof of the nasal cavity and can be seen very well in slide 124-O [example] and somewhat in slide 124 [example] . This region may be recognized by the dramatically thickened epithelium which LACKS goblet cells.  Beneath the olfactory epithelium are numerous nerve fibers which are branches of the olfactotry nerve as well as specialized olfactory glands (Bowman’s glands), the secretions of which are “serous” (mostly water and some protein) rather than mucous.  The watery secretions dissolve odorant molecules to facilitate their detection and then quickly wash the odorants away so that new scents can be detected.

The nasal mucosa lining the bulk of the nasal cavity is made up of respiratory epithelium and an underlying layer of connective tissue [example] . The connective tissue contains many glands (and associated ducts) and a rich vascular plexus characterized by many dilated, thin walled veins (sometimes called “venous spaces”) which act as heat exchangers to warm and humidify the air entering the nasal cavity.

Now look at a sagittal section of the palate (slide 115) and compare respiratory epithelium of the nasal passage [example] to the stratified squamous epithelium of the oral cavity.

B. Larynx (W pg 237, 12.5)
Slide 125-1 (larynx, sag. sect., H&E) WebScope ImageScope
Slide 125-2 (larynx, sag. sect., H&E) WebScope ImageScope
Slide 125-3 (larynx, sag. sect., H&E) WebScope  ImageScope

The mucosal surface of the rest of the conducting portion is lined by respiratory epithelium, except the true vocal fold (vocal cord or vocal ligament) [example] in the larynx, which is lined by a stratified (sometimes keratinized) squamous epithelium. The upper false vocal fold (vestibular or ventricular fold) [example] is covered by respiratory epithelium, which makes an abrupt transition to a stratified squamous epithelium over the true vocal fold. The stratified epithelium serves to protect the vocal cords from abrasion caused by vibrations of the true vocal folds during phonation. Also, note that the cores of the upper (false) folds contain numerous muco-serous secretory glands (the serous components stain evenly whereas the mucous cells have a bubbly appearance) [example] , while the cores of the lower (true) focal folds contain elastic fibers and skeletal muscle (vocalis muscle) [example] cut in cross section (the elastic fibers will appear as dots in the extracellular matrix).

C. Trachea (W pgs 226-7)
Slide 40 (trachea, H&E) WebScope ImageScope
Slide 126 (trachea & esophagus, cross sect, H&E) WebScope ImageScope
Slide 127 (trachea, cross sect., trichrome) WebScope  ImageScope

The epithelium lining the trachea is typical respiratory epithelium (ciliated pseudostratified columnar) [example] , which, like the nasal epithelium, contains numerous goblet cells. This epithelium has an unusually thick basement membrane, which you can see as a narrow pink-staining region immediately basal to the epithelium. This epithelium plus its underlying layer of loose connective tissue (the lamina propria) make up the tracheal mucosa. The layer under the mucosa is the submucosa wherein you’ll find numerous seromucous glands [example] . The mucosa is separated from the submucosa by a layer of longitudinal elastic fibers. This elastic layer is often not obvious in your sections, so we do not require you to decide where the division occurs. However, as you may recall from the connective tissue lab session, slide #40 happens to show these fibers quite well (cut in cross section, so they appear as eosinophilic, glass-like dots) [example]  .

Outside the connective tissue layers, observe the C-shaped rings of hyaline cartilage [example] which help to keep the lumen of the trachea from collapsing. It is unlikely that your histological section will follow the same C-shaped ring all the way around the trachea; instead, it will probably pass in and out of two or more rings. The ends of the rings are bridged by horizontally oriented smooth muscle (the trachealis muscle) [example] , which can act to adjust the diameter of the trachea. This muscle is seen especially well in those examples of slide #127 that are stained with Masson’s trichrome [example] . Outermost is a layer of connective tissue, the adventitia.

II. Intrapulmonary Conducting Passageways

Slide 129_20x (lung, H&E) WebScope ImageScope
Slide 130_20x (lung, H&E) WebScope ImageScope
Slide 130-1_40x (lung, H&E) WebScope ImageScope
Slide 130-2_40x (lung, H&E) WebScope ImageScope
Slide 132_20x (lung, H&E) WebScope ImageScope
Slide 132_40x (lung, H&E) WebScope  ImageScope

A. Bronchi (W pgs 227-8)

The trachea bifurcates into two primary bronchi, which enter the lung and then branch several times to give rise to smaller secondary and tertiary bronchi [example] . Bronchi differ from the trachea in having plates rather than rings of cartilage, and in having a layer of smooth muscle between the lamina propria and submucosa. In smaller branches, the amount of cartilage decreases, whereas the amount of smooth muscle increases. Also, the number of glands and goblet cells decreases. Don’t worry about trying to distinguish among primary, secondary, and tertiary bronchi, but you should be able to distiguish bronchi in general from the trachea and bronchioles (discussed below).

B. Bronchioles (W pg 229, 12.10-11) Bronchioles [example] are smaller branches of the bronchi, and are distinguished from them by the absence of cartilage and glands. In larger bronchioles, the epithelium is still ciliated, but is now usually simple columnar, whereas in the smallest bronchioles, the epithelium will be simple cuboidal (mostly Clara cells) and lack cilia altogether. The smooth muscle layer is generally quite prominent in these structures as demonstrated in slide 132-2 [example] where the bronchiole was cut in a grazing longitudinal section allowing you to see the circularly arranged bundles of smooth muscle in the bronchiolar wall. As mentioned above, the smallest conducting bronchioles consist of a simple cuboidal (or perhaps “low columnar”) epithelium of mostly Clara cells, a few ciliated cells, and NO goblet cells, and are called terminal bronchioles [example]  .

III. Respiratory Portion of the Lung

Slide 129_20x (lung, H&E) WebScope ImageScope
Slide 130_20x (lung, H&E) WebScope ImageScope
Slide 130-1_40x (lung, H&E) WebScope ImageScope
Slide 130-2_40x (lung, H&E) WebScope ImageScope
Slide 132_20x (lung, H&E) WebScope ImageScope
Slide 132_40x (lung, H&E) WebScope  ImageScope

A. Respiratory Bronchioles (W pg 230, 12.12)
You might see short, transitional regions of bronchioles which have a few individual alveoli in their walls. These bronchioles with individual alveoli in their walls are called respiratory bronchioles [example] . They characteristically exhibit a progressive reduction in height of the epithelium. The walls of respiratory bronchioles might have small knobs of smooth muscle, collagen and elastic fibers between the openings of adjacent alveoli. You can spot the knobs, but shouldn’t try to distinguish the constituents, which are covered by a squamous epithelium too thin to see with the light microscope

B. Alveolar ducts (W pg 230) The walls of alveolar ducts [example] are lined by alveoli and alveolar sacs (clusters of alveoli).

C. Alveolus (W pg 229, 12.13; pgs 231-235) The walls of these structures are covered on both sides by squamous epithelium (too thin to see) of Type I cells lining adjacent alveolar lumens. Within the walls is an extensive capillary network. You may see the space within these capillaries, or they may be filled with RBCs. The Type II pneumocytes [example] , which secrete surfactant, have large, rounded nuclei and vacuolated cytoplasm and are often difficult to identify in the light microscope (the “vacuoles” are actually granules of phospolipids that, unfortunately, are often extracted during tissue processing). In the lumen of some alveoli, you will see macrophages, called alveolar phagocytes or dust cells [example] (W pg 235, 12.18).

While components of the alveolus may be difficult to see in normal lung tissue, pathological changes that occur in the lung as the result of congestive heart failure shown here in slide 42 from your histopathology collection ImageScope WebScope exaggerate many of these features making them a bit easier to see:

  • Poor venous return causes dilation of the alveolar capillaries.
  • Type II pneumocytes with large round nuclei and clear cytoplasm are much more numerous and can be easily seen in the alveolar walls because they proliferate in an attempt to repair damaged alveoli.
  • Alveolar macrophages characteristic of this condition (also known as “heart failure” cells) become laden with brownish-black hemosiderin pigment resulting from the breakdown of erythrocytes leaking from the engorged capillaries.

IV. Blood Supply

Slide 129_20x (lung, H&E) WebScope ImageScope
Slide 130_20x (lung, H&E) WebScope ImageScope
Slide 130-1_40x (lung, H&E) WebScope ImageScope
Slide 130-2_40x (lung, H&E) WebScope ImageScope
Slide 132_20x (lung, H&E) WebScope ImageScope
Slide 132_40x (lung, H&E) WebScope  ImageScope

Although it is almost impossible to appreciate in these adult tissue sections, the lung is divided into lobules with a bronchiole at the center of each lobule. The blood supply is organized according to the lobule:

A. Large, thin-walled pulmonary arteries [example] run alongside the bronchi and bronchioles. These arteries are the main blood supply to the lungs. They carry large volumes of deoxygenated blood at low pressure from the right side of the heart to pulmonary capillaries in the alveoli.

B. Smaller bronchial arteries also accompany the bronchi and bronchioles. These vessels arise from the thoracic descending aorta and carry oxygenated blood at systemic pressure. They supply blood to the tissue comprising the bronchi and bronchioles and thus may be seen as part of a neurovascular bundle (artery, vein, and nerve) either in cross section [example] or longitudinal section [example] in close association with the bronchial walls. These arteries are quite a bit smaller than pulmonary arteries and for their size, have relatively thick walls.

C. Large thin-walled pulmonary veins [example] are found at the periphery of the lobules, at some distance from bronchi or bronchioles. These veins drain the intrapulmonary circulation and carry oxygenated blood (also at rather low pressure).