Respiratory System Shiping Ding ( 丁世萍 ), Ph. D School of Medicine, Zhejiang University

Learning Objectives To be familiar with the trachea. To be familiar with the type of bronchiole. To distinguish with Blood-air barrier.

Consists of the: –nasal cavities –Pharynx –Larynx –Trachea –lung The respiratory system is divided into two major components: the air- conducting components and the respiratory or gas- exchange components

Trachea (the windpipe) Descends: larynx through neck into mediastinum Divides in thorax into two main (primary) bronchi C-shaped rings of hyaline cartilage joined by fibroelastic connective tissue Flexible for bending but stays open despite pressure changes during breathing

Posterior open parts of tracheal cartilage abut esophagus Trachealis muscle can decrease diameter of trachea –Esophagus can expand when food swallowed –Food can be forcibly expelled Wall of trachea has layers common to many tubular organs – filters, warms and moistens incoming air –Mucous membrane (pseudostratified epithelium with cilia and lamina propria with sheet of elastin) –Submucosa ( with seromucous glands) –Adventitia - connective tissue which contains the tracheal cartilages)

A. Mucosa: 1. Epithelium: Pseudostratified ciliated columnar epithelium with much thicker basement membrane.

1. Ciliated cell: The cilia beat simultaneously towards larynx. 2. Goblet cell: The secretion covers the epithelium surface. 3. Basal cell: Stem cell. Separated from the epithelium surface.

Cilia on the surface of trachea Ciliated cell Goblet cell

4. Brush cell: Sensory cell. Columnar in shape and has longer microvilli. The epitheliodendritic synapses are formed when their basal surfaces are connected to nerve endings. 5. Diffuse neuroendocrine cell: Also called small granule cell. Contains many secretory granules in which exist many kinds of bioactive substances. Belongs to DNES.

2. Lamina propria: LCT in which the elastic fibers predominate. B. Submucosa: LCT containing mixed glands and few lymphatic tissue. IgA, which is secreted by plasma cell, will combine with secretory protein produced by glands and epithelium to form SIgA, an antibody released to inner surface of respiratory track. C. Adventitia: LCT containing "C" shaped hyaline cartilage whose crevice is linked by smooth muscles. Provides support.

functions: -----Filters, moistens, and warms air before it enters lungs Olfactory functions to examine air and to protect against breathing in harmful substances Mucous tends to trap foreign substances and then the cilia (of pseudostratified ciliated columnar epithelium) clear the foreign substances from the trachea and bronchi of lungs. ----Cartilages found in trachea and bronchi play an important role in keeping airway open.

Lung Bronchial tree bifurcation –Right main bronchus –Left main bronchus Each main or primary bronchus runs into hilus of lung Main=primary bronchi divide into secondary=lobar bronchi, each supplies one lobe –3 on the right –2 on the left Lobar bronchi branch into tertiary = segmental bronchi Continues dividing: about 24 times Tubes smaller than 1 mm called bronchioles Smallest, terminal bronchioles, are less the 0.5 mm diameter Tissue changes as becomes smaller

End-point of respiratory tree Structures that contain air-exchange chambers are called alveoli Respiratory bronchioles lead into alveolar ducts: walls consist of alveoli Ducts lead into terminal clusters called alveolar sacs – are microscopic chambers There are 3 million alveoli!

Lung Covered by mesothelium and accommodates air tracks that gradually appear as simpler structure, thinner wall, smaller lumen but much more in number, with the ramifications going on. The root-like tracks, the parenchyma, are surrounded by LCT containing blood vessels. etc.

Air channels in lung: A. Conducting portion: Conducts air flow. 1. Primary bronchi: The branches of trachea. Give rise to 2~3 lobar bronchi, each of which supplies one pulmonary lobe. Lamina propria contains many lymphocytes. Decrease in glands. Hyaline cartilages gradually become irregular and appear as plates. Smooth muscles are relatively prominent.

2. Bronchioles: The branches of small bronchus and has a diameter about 1 mm. Each bronchiole supplies one pulmonary lobule.

a. ciliated pseudostratified columnar epithelium with few goblet cells gradually turn into simple ciliated columnar epithelium. b. hyaline cartilage plates and glands gradually disappear. Smooth muscle fibers gradually predominate. Has irregular inner surface.

3. Terminal bronchiole: Lined by simple ciliated columnar cells and surrounded by smooth muscle fibers. No goblet cell, gland and cartilage can be found. Has irregular inner surface. Clara cells, a kind of stem cells that are found among ciliated columnar cells, are non-ciliated and contain rich secretory granules, and are responsible for mucus clotting clearance. E/M: dome-shaped apical/SER/secreting granules:contains proteolyase and oxidase Function: -dissolve the mucus, secrete surfactant. -biological transformation -undifferentiated cell→ciliated cell

Lobar bronchi Bronchiole Terminal bronchiole

d. the unexpected contraction of smooth muscles in bronchiole and terminal bronchiole may result in asthma.

B. Respiratory portion: Capable of air exchange. 1. Respiratory bronchiole: a. the wall is populated with more and more alveoli. b. lined by cuboidal cells (with/without cilia) and Clara cells. c. surrounded by thin and discontinuous layer of LCT and few smooth muscle fibers. 2. Alveolar duct: a. the wall is difficult to see due to numerous opening to alveolar sacs and alveoli. b. knobs appear in the wall because of the contraction of smooth muscles.

3. Alveolar sac: The shared opening of several surrounding alveoli. 4. Alveoli: 200μm in diameter, 3~400 million and 70~80 m 2. Terminal blind ends of the track. The main place where the air exchanges occur.

Lined by 2 types of simple epithelium. Surrounded by rich continuous capillaries.

a. type I cells (squamous alveolar cells): squamous, cover 95% of the alveolar surface. 0.1~0.2 μm thick. Organelles are grouped around the nuclei. Pinocytotic vesicles are responsible for inner surface clearance, and tight junctions among them prevent the leakage of tissue fluid into the alveolar air space.

b. type II cells (Giant alveolar cells): Interspersed among the type I cells. Often found at the angle of alveolar surface. More than type I cells.

* cuboidal with microvilli. Have tight junctions and desmosomes with type I cells. Have acidophilic and foamy cytoplasm. * contain many membrane limited concentric or parallel lamellae, the lamellar bodies, which are osmiophilic and capable to release the pulmonary surfactant to cover the alveolar surface to lower the surface tension. Can differentiate into Type I cells. Become mature in 7- month-old fetus.

neonatal respiratory distress syndrome

C. Interalveolar septum: Thin layer of LCT between adjacent alveoli. Several structures exist in Interalveolar septum: 1. Continuous Caps: Branches of pulmonary artery. The richest Cap network in the body. 2. Abundant elastic and reticular fibers: Function when alveoli are enlarged. 3. Alveolar macrophages: Also called dust cells or heart failure cells. 4. Alveolar pores: Windows between the adjacent alveoli that can equalize the air pressure.

D. Blood-air barrier: The structure that the gas has to penetrate during the air exchanges. Composed of type I cells covered by surfactant, (LCT), fused basement membrane and endothelium.

Thank you for your attention!