Lung Histology

the respiratory system is subdivided into which two parts?

conducting portion: responsible for moving air from outside the body into the lungs

respiratory portion: performs gaseous exchange in the lungs

conducting portion of the respiratory system is divided into?

extrapulmonary tissues - those outside the lungs

intrapulmonary tissues - those that lie inside the lungs

extrapulmonary conducting system

includes the nasal cavities and the trachea

- surfaces are lined by respiratory epithelium

respiratory epithelium

lines the extrapulmonary conducting system

- pseudostratified columnar, with cilia

- contains Goblet cells

function: warms, moistens and filters the air as it moves over these chambers

respiratory epithelium is underlain by?

lamina propria, a loose connective tissue layer that contains blood vessels and seromoucous glands that extend into the epithelial surface

histological appearance of respiratory epithelium

- numerous layers of nuclei (hence the label pseudostratified)

contains:

- columnar cells

- basal cells

- goblet cells

- columnar cells of the respiratory epithelium

- compose a majority of the epithelium

- span from the basal to the apical surfaces

- cilia on their apical surfaces

basal cells of the respiratory epithelium

- stem cells that replenish ciliated columnar cells that have died

- sit on the basal surface of the epithelium but do not extend to the apical surface of the epithelium

goblet cells of the respiratory epithelium

- embedded among the ciliated cells

- located towards the apical surface of the epithelium

- secrete mucous that lines the respiratory epithelial surface

the cilia of respiratory epithelium help to move

- air towards the lungs

- trapped particles towards the pharynx

nasal cavities

- present a large surface area for contact with the air

- surface area is maximized by a turbinate bone system (nasal conchae) that protrudes into the nasal cavities and paranasal sinuses

what are the supporting components of the nasal cavities?

- supported by a cartilaginous plate: the nasal septum

- lined by the respiratory epithelium

- underlain by lamina propria

the lamina propria underlying the nasal cavities contains?

a large number of lymphocytes that mediate immune reactions against air borne pathogens

layers of the trachea

1. mucosa

2. submucosa

3. adventitia

tracheal mucosa

contains the respiratory epithelium underlain by a lamina propria

tracheal submucosa

- dense irregular fibroelastic connective tissue

- contains seromucous glands, lymphoid elements, and blood vessels

tracheal adventitia

- contains a C-ring of cartilage that structurally supports the mucosa/submucosa of the trachea (that otherwise would be very floppy)

- ends of the C-ring of cartilage are fused together by a small smooth muscle called the trachealis

trachealis

muscle that constricts the trachea to increase air pressure (such as during coughing)

bronchial tree: conducting portion

composed of extra- and intra-pulmonary bronchi, bronchioles and terminal bronchioles.

- begins with the bifurcation of the trachea into the left and right primary bronchi

- ronchi then enter the lungs and form more branches of decreased diameter

what are the changes in the respiratory epithelium and supportive tissues as one progresses through the bronchial tree?

as airways become smaller:

1. epithelia:

- become less tall

- contain fewer Goblet cells

- contain fewer seromucous glands

2. walls:

- contain less cartilage

- more smooth muscle and elastic tissue

what is the effect of the change in smaller airway walls?

- decreases the movement of the air in the lungs

- increase in smooth muscle in smaller airways helps move air through these small passages

the cross section of an individual airway ____ down the bronchial tree, but the net cross-sectional area for air passage ____, due to the fact that?

decreases; increases; there are so many smaller bronchioles

intrapulmonary bronchi

- each enters a lobe of the lung

layers include:

respiratory epi

mucosa

adventitia

intrapulmonary bronchi respiratory epithelium

- shorter than the tracheal epi

- still pseudostratified

intrapulmonary bronchi mucosa

- contains several layers of smooth muscle, specifically located between the mucosa and submucosa

intrapulmonary bronchi adventitia

- contains discrete plates of cartilage (rather than a single C-ring)

- contains elastic fibers that connect to elastic fibers arising from other parts of the bronchial tree

Kartagener's syndrome

- results from a defect in dynein, a motor protein normally found in cilia in the bronchi.

- results in cilia that cannot move and an accumulation of irritants in the bronchi

- net result is that the bronchi become permanently dilated and the smooth muscle layer is destroyed, leading to bronchitis and other respiratory diseases

bronchioles

- airways of less than 1 mm in diameter

- contain no submucosal connective tissue

bronchiole epithelium

- simple columnar/cuboidal ciliated

- no Goblet Cells

- thin lamina propria underlain by a thick smooth muscle layer

bronchiole adventitia

lacks cartilage plates but does contain elastic fibers

terminal bronchioles

- smallest bronchioles of the conducting portion of the bronchial tree

- will give rise to respiratory bronchioles

- adventitia is similar to that of bronchioles (elastin)

terminal bronchioles epithelium

- simple cuboidal (with cilia)

- contains nonciliated Clara cells.

- underlain by a thin lamina propria with two layers of smooth muscle.

clara cells

several functions, including secretion of a surfactant like material that protects the bronchial epithelium from surface tension distortion

bronchial tree: respiratory portion

mediates the exchange of O2 in the incoming air with CO2 in the blood

what are the structural and functional units for gas exchange in the pulmonary system?

alveoli

Respiratory bronchioles

- tend to be very short

- terminate into alveolar ducts, linear arrangements of alveoli

- walls contain little sacs or alveoli (at least one)

main difference between respiratory bronchioles and terminal bronchioles

- respiratory bronchioles contain alveoli, small outpouchings/sacs that mediate the exchange of gases

Both terminal bronchioles and respiratory bronchioles have?

- lined by a simple cuboidal epithelium containing Clara Cells

- have smooth muscle beneath their lining epithelium

Alveolar Ducts/Sacs

Ducts:

- do not have their own walls

- walls are created by the septa of the alveoli that line them

- eventually terminate into alveolar sacs

Sacs:

- circular sacs with continuous alveoli lining them

- lumen of an alveolar duct or sac also often contains one or few smooth muscle cells

Alveolar Ducts/Sacs are surrounded by?

(the alveoli are surrounded by) a network of elastic fibers that connects to those of other parts of the lung

- maintains the elasticity of the alveoli during inspiration and expiration

- is the point of contact between the septa of an alveolus

alveoli

- structural and functional units of respiration

- very small outpouchings

- lined by a simple epithelium that mediates the exchange of gas across the blood-air barrier

Alveolar walls consist of

contain two types of epithelial cells

1. type I pneumocytes

2. Type II pneumocytes

And Macrophages (Dust Cells)

type I pneumocytes

- only 5 % of the population of pneuomocytes, yet they contribute to 95% of the surface area of the alveolar wall

- very squamous type of epithelium, with a flattened, stretched morphology

- mediate gas exchange with adjacent capillaries

Type II pneumocytes

- cuboidal epithelial cells with very bulging nuclei

- mediate the synthesis of surfactant, which lines the aloveolar lumen, and maintains surface tension

Macrophages (Dust Cells)

- also present in the walls and the lumens of alveoli

- play a role in some of the diseases that affect the respiratory system such as asthma and emphysema

surfactant

- a lattice like structure that coats the alveolar wall and lines the aloveolar lumen

- maintains surface tension

- important at birth when the lungs suddenly fill with air, and the alveoli suddenly inflate, or when two adjacent alveoli contact each other

capillary-type I pneumocyte boundary

- where exchange of gas occurs between air and blood

- region is defined by a confluent basal lamina secreted by both the type I pneumocytes, and the endothelial cells lining the continuous capillary on the other side

- gas moves across this dual basal lamina

Type II pneumocytes morphology

- very large rounded cells with a large cell body

- contain a distinct subcellular structure called the lamellar body (ovoid shaped)

lamellar body

- ovoid shaped

- associated with type II synthesis and secretion of a phospholipid involved in surfactant synthesis

- exocytose from the type II pneumocyte cells during secretion of the surfactant associated phospholipids

- phospholipids then combine with other secretory products to make surfactant

Pulmonary vessels

- involved in exchange of gases

1. pulmonary artery

2. pulmonary vein

pulmonary artery

- transports deoxygenated blood to the lungs

- enters the lung with the main bronchi, branches with the bronchioles and terminates at the respiratory bronchioles

- it then gives rise to the extensive capillary network surrounding the alveoli

pulmonary vein

- collects oxygenated blood from the alveolar capillaries

- sends that blood back to the heart for subsequent distribution to the tissues of the body

Bronchial arteries

- supply the cells and tissues of the bronchial tree and surrounding connective tissue.

- they also follow the bronchial tree path

- in some cases, they intertwine with the pulmonary arteries

- transport oxygenated blood to the lungs

pleura

- connective tissue coverings over the two lung lobules

- consist of fibroelastic connective tissue with a simple squamous epithelium lining

types of pleura

1. parietal pleura

2. visceral pleura

- space and pressure between these two pleura changes during different stages of exhalation and inhalation

parietal pleura

- lines the thoracic cavity

- folds in on itself to form the visceral pleura

visceral pleura

- covers the lungs

- epithelium is called a mesothelium

- similar to the covering of the heart, it secretes a serous fluid that minimizes friction between the two pleural layers