RespiratoryAnatomy-Histology Correlate

By: Michael Lu, Class of ‘07

NOTE:

- Visceral pleura is in direct contact with the lungs, while parietal pleura is everything else.

- 4 sections of parietal pleura: cupola, costal, diaphragmatic, and mediastinal.

- Surface projections of parietal pleura:

- Potential spaces of the costomediastinal and costodiaphragmatic recesses, where there is no lung tissue.

- The right lung contains 3 lobes – superior, middle, and inferior – separated by horizontal and oblique fissures.

- The left lung only contains superior and inferior lobes. In addition, the cardiac notch is an indent in the superior lobe, forming the lingula that wraps around the apex of the heart.

- Review the locations of the great vessels – aorta, pulmonary trunk, pulmonary arteries and veins, superior and inferior vena cava, azygos vein, thoracic duct, etc.

Landmark Inferior border of lung Pleural reflection

Midclavicular 6th rib 8th rib

Midaxillary 8th rib 10th rib

Scapular 10th rib 12th rib

- With the lungs removed, we see a better view of the mediastinum. Notice on both sides the phrenic nerves passing anterior to the root of the lungs, vagus nerves passing posterior, and the sympathetic trunk running along the vertebrae. In the thorax, the sympathetic trunk gives off both white and grey rami communicantes posteriorly and the greater splanchnic nerve anteriorly.

- Note the left vagus nerve giving off the recurrent laryngeal nerve wrapping under the aortic arch. The arch of the aorta is also attached to the left pulmonary artery via the ligamentum arteriosum.

- Note on the right side the azygos vein running posteriorly and draining into the vena cava.

- Note on the left side the thoracic duct that drains all the lymph in the body below the diaphragm and on the left side above the diaphragm.

- Note the relative locations of the pulmonary arteries and veins and bronchi, and the differences between the right and left sides. These will be looked more closely in the next slide.

NOTE:

- The left lung contains 2 lobes – an upper and a lower lobe – separated by an oblique fissure. The right lung contains 3 lobes – an upper, a middle, and a lower lobe – separated by transverse and oblique fissures. The corresponding segments are detailed in the next slide.

- Both lungs have grooves for the subclavian arteries, brachiocephalic veins, 1st rib, esophagus, and cardiac impression. The left lung has a groove for the aorta, while the right lung has one for the azygos vein.

- Since the apex of the heart is situated left of midline, the left lung also has a cardiac notch and lingula that wraps around the apex.

- The reflection of the parietal pleura forming the pulmonary ligament.

- On both sides, the bronchus is posterior and the pulmonary veins are anterior and inferior.

- On the left side, the pulmonary arteries are the most superior, even above the bronchus.

- On the right side, the pulmonary arteries lie anterior to the bronchus.

Right lung Left lung

SUPERIOR LOBE

Apical, anterior, and posterior segments Apicoposterior, anterior, superior lingular,

and inferior lingular segments

MIDDLE LOBE

Medial and lateral segments N/A

INFERIOR LOBE

Superior, medial basal, lateral basal,

anterior basal, and posterior basal segments

Superior, medial basal, lateral basal,

anterior basal, and posterior basal segments

- The respiratory system is divided into the conducting portion and the respiratory portion. The conducting portion includes the nasal cavities, pharynx, larynx, trachea, and bronchi; the respiratory system starts from the first branches of the respiratory bronchioles, alveolar ducts, and alveoli. The conducting portion warms, humidifies, and cleans the air as it passes down to the respiratory portion for actual gas exchange.

- As shown above, the conducting portion is characterized by pseudostratified columnar epithelium with cilia, goblet cells, gland tissue, and hyaline cartilage. The respiratory portion begins with the first branches of the respiratory bronchioles; essentially where hyaline cartilage ends and there is abundant smooth muscle, elastic fibers, reticular fibers, and epithelium transition from cuboidal to simple squamous.

- We will start at the beginning of the respiratory tract and work our way down.

- Pay attention to the superior, middle, and inferior nasal conchae and meatuses. The following histology slides will show the abundance of blood vessels near the surface to warm the air, and ciliated cells along the epithelium to clean it. In addition, the conchae and meatuses create turbulence within the nasal cavity to slow down the inspired air and to help warm and humidify it.

- Note the various sinuses and their openings: frontal sinus, sphenoidal sinus, maxillary sinus, ethmoidal cells, ethmoidal bulla, semilunar hiatus, frontonasal duct, and nasolacrimal duct.

- Nasal concha and its mucosa are shown in the histological slides here.

- The top panel emphasizes the warming function of the nasal mucosa. The large number of dilated veins transfer heat from the blood to the inspired air. This appearance of thin-walled veins immediately adjacent to the mucosa is typical of the nasal walls and cavity.

- The lower panel is a close-up of the nasal mucosa. Note the respiratory epithelium, which is mainly composed of pseudostratified, ciliated columnar epithelium with goblet cells. The other structures include mixed muco-serous glands, nerves, normal arteries and veins, and thin-walled dilated veins.

- The goblet cells and glands provide mucus to humidify the inspired air and trap dust, particles, and bacteria.

- The ciliated cells beat and help to clear the nasal cavity of these foreign particles and mucus and clean the inspired air.

- The olfactory mucosa is located at the superior part of the nasal cavity. Respiratory epithelium lines the remainder of the walls of the nasal cavity.

- Olfactory epithelium:

- thicker than respiratory epithelium

- Bowman’s glands, which are pure serous glands, are found below the epithelial surface

- abundant nerve fibers originating from olfactory receptor cells converge and give rise to the olfactory tract, providing the special sense of smell

- absence of goblet cells

- Respiratory epithelium:

- thinner than olfactory epithelium

- many mucus-secreting glands

- no nerve fibers

- abundance of goblet cells

- After the nose and nasal cavity, the air travels down the pharynx, which is divided into the nasopharynx, oropharynx, and laryngopharynx.

- In the side panel, the pharyngeal mucosa is detailed – note the non-keratinized stratified squamous epithelium, connective tissue, mucous glands, and underlying elastic layer.

- Note the following: nasal septum, hard and soft palate, hyoid bone, epiglottis, thyroid cartilage, cricoid cartilage, vocal fold, thyroid gland, pharyngeal and lingual tonsils, pharyngeal constrictor muscles, and retropharyngeal space.

- The pharynx connects the nasal and oral cavities superiorly with the larynx and esophagus inferiorly. It sorts food, water, and air to arrive at their destinations.

- In the pharynx, the paths of food and air cross. Food travels from the mouth (anterior) to the esophagus (posterior). Air travels from the choanae (posterior) to the trachea (anterior).

- The pharynx contains 2 layers of muscles – outer circular and inner longitudinal.

- The outer circular muscles include the superior, middle, and inferior pharyngeal constrictor muscles. One easy landmark to identify them is the tip of the greater horn of the hyoid bone, to which the middle pharyngeal constrictor attaches. The 3 muscles contract serially to push a bolus down the esophagus.

- The inner longitudinal muscles include the stylopharyngeus, salpingopharyngeus, and palatopharyngeus muscles, which elevate and widen the pharynx to accommodate a bolus when swallowing.

- The levator veli palatini and tensor veli palatini (not shown here) muscles elevate the soft palate to seal off the nasopharynx when swallowing. The epiglottis closes off the larynx and trachea.

- The interior fascia is the pharyngobasilar fascia, an area which does not have any muscle tissue.

- The pharyngeal mucosa is covered by non-keratinized stratified squamous epithelium, with an underlying dense layer of elastic tissue (blue brackets).

- The larynx connects the nasopharynx with the trachea. It specializes in voice production. Note the abundance of cartilages – 3 paired and 3 unpaired.

- The thyroid cartilage is the largest cartilage but does not continue posteriorly. Anteriorly, it forms the laryngeal prominence (Adam’s apple) with the superior thyroid notch.

- The cricoid cartilage is the only complete ring of cartilage in the larynx.

- The epiglottis protects the airway when swallowing.

- The 3 paired arytenoid, corniculate, and cuneiform (not shown here) cartilages participate in voice production.

- The intrinsic muscles of the larynx play a crucial role in voice production.

- Cricothyroid muscles – tense the vocal folds to control pitch.

- Posterior cricoarytenoid muscles – the ONLY abductors of vocal folds.

- Lateral cricoaryteniod muscles – adduct vocal folds.

- Transverse arytenoid muscle – adducts vocal folds.

- Thyroarytenoid muscles – relaxes vocal folds.

- Vocalis muscles – contraction affects frequency of vibrations and controls pitch.

- The vagus nerve is the major motor innervation, via the superior and inferior laryngeal nerves and recurrent laryngeal nerves.

-The panel on the left shows the relative positions of the true vocal cords (vocal fold) and false vocal folds (ventricular or vestibular folds). The panel below shows a magnified view, with the true vocal cord closer to the cricoid cartilage and the false vocal fold near the thyroid cartilage.

- The true vocal cords contain underlying skeletal muscle called the vocalis muscles. Under the false vocal folds, the connective tissue is filled with glands that secrete mostly mucus.

- The vocal cord is covered by stratified squamous non-keratinized epithelium.

- The false vocal cord is covered by respiratory epithelium.

- As we continue down the respiratory tract, we enter the trachea. Shown on the left panel is the trachea and the major bronchi, which branch into segmental bronchi and determine the bronchopulmonary segments.

- Histological slides of the trachea are shown below. The bottom left panel exhibits the characteristic C-shaped rings of hyaline cartilage (C). The rings are joined posteriorly by bands of smooth muscle known as trachealis muscle (T). Tracheal mucosa (M) and some strands of longitudinal muscle (L) are also shown.

- The trachea is lined with respiratory epithelium sitting on a thick basement membrane. The elastic layer contains many longitudinally oriented elastic fibers. The submucosa contains loose connective tissue and mixed muco-serous glands.

-Continuing down the respiratory tract, the trachea bifurcates into two main or primary bronchi. Within the tracheal bifurcation is a keel-shaped cartilage known as the carina.

- The right main bronchus is wider, shorter, and runs more vertically than the left main bronchus. The left main bronchus is longer and passes inferior to the arch of the aorta and anterior to the esophagus and thoracic aorta.

- The main bronchi later divide into segmental bronchi. A characteristic component that allows us to identify bronchi is the presence of cartilage that appear as chips and not semi-circular as in the trachea.

- Bronchi are lined with the pseudostratified ciliated columnar epithelium (respiratory epithelium) with glands within the submucosa.

- Note the presence of bronchial arteries, which are distinct from pulmonary arteries and veins. This will be explained in a later slide.

- The bronchi further separate into bronchioles. The defining feature of a bronchiole versus a bronchus is the absence of cartilage.

- Notice the abundance of smooth muscle within the bronchiole wall. The epithelium is frequently folded due to contraction of the smooth muscle.

- The epithelium goes through a transition from the pseudostratified ciliated columnar respiratory epithelium to cuboidal ciliated epithelium, as shown magnified in the lower panel.

- The lower panel also compares the relative size of the bronchiole to a typical pulmonary artery.

- There are progressively fewer goblet cells. They are replaced by Clara cells. Within the magnified inset of the lower panel, the Clara cells are cuboidal but do not have cilia. They secrete a more watery substance than mucus and the fluid continues to moisten, warm, and clean the air.

- Terminal bronchioles are the smallest and last branches of the conducting portion and ciliated respiratory epithelium is replaced by non-ciliated and ciliated cuboidal cells.

- Terminal bronchioles continue as respiratory bronchioles, which then open into alveolar ducts and individual alveoli.

- This is the respiratory portion of the respiratory system, where the actual gas exchange occurs.

- Note the walls are composed of squamous epithelium, containing both type I and II pneumocytes. Type I pneumocytes or alveolar cells are very thin and provide support to the alveoli. The type II pneumocytes synthesize and secrete surfactant – reducing surface tension and allowing the alveoli to remain open.

- The arrows in the lower right panel indicate pulmonary macrophages in the alveoli, but not within the walls.

-There are two different circulations within the lung.

- The low pressure, high volume circulation flows to the lungs from the right heart in order to be oxygenated. The pulmonary arteries and veins are both thin-walled vessels in this circulation.

- The high pressure, small volume circulation provides oxygenated blood primarily from the left heart and aorta to the conducting portion of the respiratory system. It includes the bronchial arteries that have thicker walls to carry the high pressure blood flow.

- There are 4 layers that exist between the blood and inspired air in the blood-air barrier:

- 1) capillary endothelium (continuous, no fenestrations)

- 2) fused basal lamina

- 3) alveolar epithelium (type I pneumocyte)

- 4) surfactant

- Note the proximity of the red blood cell to the inhaled air within the alveoli.