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BLOOD-AIR BARRIER

The barrier through which gases may pass inexchange between air and blood

Alveolar-capillary membrane In an electronmicrograph, there are 3 layers:

a. Alveolar Epithelium

b. Interstitial space (Zona Diffusa)c. Capillary Endothelium

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Alveolar Epithelium

Gas exchange occurs at:

Greatly artenuated cytoplasm of Type Ialveolar cells (pulmonary epithelial cells) and

its basal lamina

Artenuated cytoplasm of the capillary andcapillary basal lamina

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Zona Diffusa (Interstitial space)

Interposed between the two basal laminae

In some regions, the two basal laminae may

fuse into one and are separated in many otherareas only by a few elastic or reticular fibers

o Alveolar epithelium is separated from the

capillary endothelium by a continuous basallamina

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Capillary Endothelium

Relatively thin, non-fenestrated, withnumerous caveolae

Capillary bed of the lungs: involved withthe metabolism of certain vasoactivesubstances (angiotensin, prostaglandin)

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Alveolar Phagocytes orMacrophages

Free phagocytic cells encountered in the alveoliin practically every section of the lungs containparticles of inhaled dust Dust Cells

Arises from monocytes of the blood marrow

Typical macrophage in the defense of the lung

Continous removal of dust particles

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Alveolar Phagocytes orMacrophages

Heart failure cells/Siderophages

In congestion, the cells are filled with hemosiderinresulting from phagocytosis and degradation ofblood pigment

Numerous when there is stasis of pulmonary blood flow

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Endocrine (Neuroendocrine) Cells

Specialized cells within the epithelial lining of the respiratorysystem with a morphology similar to endocrine cells of thedigestive tract

Common endodermal origin

Converts amine precursors to dopamine or serotonin

Contains peptide hormones (Calcitonin)

May occur singly or in groups

May or may not be associated with nerves

Neuroepithelial Bodies: distinct bodies formed by largeaggregates

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BLOOD VESSELS

2 Sources:

1.Venous Blood purified through the pulmonaryarteries

2.Arterial Blood

for the nutrition of the walls of theconducting system and blood vessels through thebronchial arteries

Returned to systemic circulation through thepulmonaryand bronchial veins

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Pulmonary Blood Vessels

Pulmonary Artery- large and elastic

1. The Pulmonary artery enters the lungs with thecorresponding chief bronchus. It branches withthe bronchial tree to the termination of therespiratory bronchioles.

2. As the alveolar duct is reached, the artery gives

rise to the capillary plexus to the walls of thealveoli. Veins arise from these capillaries.

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Pulmonary Blood Vessels

3. The veins course in the connective tissue septaand later join the bronchioles and is coursed alongto the root of the lung, independently of thearteries to form the pulmonary veins

Arteries: above & behind accompanying bronchial tube

Veins: below & in front of accompanying bronchial tube

4. The pulmonary veins join the bronchioles and

course them to the root of the lung.

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Bronchial Blood Vessels

Much smaller than the pulmonary vessels Accompanies the bronchi

Gives origin to capillaries which supply the walls of thebronchi, arteries, veins, and the peritoneal & septal

connective tissue Bronchial arteries do not extend beyond the respiratory

bronchioles

Capillaries anastomose with the pulmonary capillary

plexuso Part of the blood passes into the pulmonary veins through

this anastomosis, while the remainder returns through thebronchial veins

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LYMPHATICS

2 sets:

1. Superficial Lymph Vessels

set in the pleura

2. Deep Lymph Vessels

set in the substance of the lung

Both drain into the lymph nodes at the hilus ofthe lung

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Superficial (Pleural)Lymph Vessels

Numerous near the juncture of the interlobar septawith the pleura

Outlines the lobules

Has numerous anastomosis between the vessels ofthe superficial vessels: prevents backflow of lymph

Efferents of superficial plexus course first in thepleura then in the CT along the larger bronchi to theroot of the lung

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Lymphatics in the Bronchi

Extend peripherally

No lymphatics beyond the alveolar duct

Pulmonary artery is drained by 2 or 3 main lymphatic

trunks

All lymphatics drain toward the hilus nodes

No valves in the intrapulmonic lymphatics, except in afew vessels in the interlobular connective tissue

Important in the spread of carcinoma in lungs:widespread metastases or secondary growths(e.g. bone)

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NERVES

Pulmonary plexuses are formed by:

o Vagus

o Thoracic sympathetic ganglia

Broncho-constrictor fibers from the X nerve

Bronchodilator fibers from sympatheticnerves (inferior cervical and 1st thoracic ganglia)

Pulmonary vessels are supplied with bothsympatheticand parasympatheticnerve fibers

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PLEXUS

Mesothelial serous membrane which completely lines thepleural cavity

The visceral portion is closely adherent to the lungs,continuous with the parietal pleura at the root of the lungs

Contains a great number of capillaries and lymphaticvessels

Delicate septa extends between the lobules,anastomosing with each other and with the peribronchial

CT

Secretes pleural fluid: permits friction-free movementbetween the parietal and visceral layers