Lungs And Pleura

Pleura 

• It is the outer covering of the lungs.
• It develops from the primitive coelom.
• It has two layers:
• Parietal layer
• Visceral layer
• The parietal layer attached to the chest wall and it is pain sensitive.
• The visceral layer is attached to the lungs, and it is pain insensitive.

• There is a potential space between the pleura, that is known as the pleural space/ cavity.
• This space contains nearly a few ml of fluid, called pleural fluid.
• This fluid helps in the lubrication between the pleura, during the respiration.
• Collection of excessive pleural fluid is called as the pleural effusion.
• The pressure is negative inside the pleural cavity.
• There are two most dependent parts of the pleura, they are called pleural recess,
• Costodiaphragmatic recess
• Costomediastinal recess.

• The parietal pleura is not attached with the lungs, but there is space between the lungs and the pleura.
• Pleural reflections:
• Pleura are attached to the chest wall and are reflected to them.

Nerve supply: 

• Intercoastal nerves
• Phrenic nerves 

Trachea

• 10-12 cm long
• 1.5-2.5 cm wide
• Extends to second rib anteriorly and T4-T5 posteriorly
• 15-20 C shaped rings, and the posterior surface has a muscle called as trachealis.
• The first generation is zero (0), the trachea itself, then it divides upto 23 generations  to become the alveoli.
• Bifurcates at the carina.

Main Stem Bronchi

Right bronchus

• Wider
• More vertical
• 5 cm shorter
• Supported by C shaped cartilages
• 20-30 degree angle
• First generation

Left bronchus

• Narrower
• More angular
• Longer
• Supported by C shaped cartilages
• 40-60 degree angle
• First generation

Lobar Bronchi

R main stem divides into:

• Upper lobar bronchus
• Middle lobar bronchus
• Lower lobar bronchus

L main stem divides into:

• Upper lobar bronchus
• Lower lobar bronchus

Segmental Bronchi,3rd generation

R lobar divides into

• Segmental bronchi
• 10 segments on right

L lobar divides into

• Segmental bronchi
• 8 segments on left

Subsegmental Bronchi

• 4th to 9th generations
• Progressively smaller airways
• 1-4 mm diameter
• At 1 mm diameter connective tissue sheath disappears

Noncartilagenous Airways

Bronchioles

• 10th to 15th generation
• Cartilage is absent
• Lamina propria is directly connected with lung parenchyma
• Surrounded by spiral muscle fibers
• Epithelial cells are cuboidal
• Less goblet cells and cilia
• With no cartilage, airway remains open due to pressure gradients

Terminal Bronchioles

• 16th to 19th generation
• Average diameter is 0.5 mm
• Cilia and mucous glands begin to disappear totally
• End of the conducting airway
• Canals of Lambert-interconnect this generation, provide collateral ventilation

Gas exchange zone

• Respiratory bronchioles
• Acinus (aka primary acinus; aka primary lobule)—respiratory bronchioles to the alveoli
• Ducts, sacs, alveolar
• Squamous epithelium

Lungs 

• Human lungs are located in two cavities on either side of the heart.
• Each lung is divided into the different lobes with the help of the fissures:
• Right- three lobes, upper, middle and lower lobes
• Left- two lobes, upper and lower lobes.

The right lung has two fissures:

• Oblique fissure
• Transverse fissure

The left lung has only one fissure.

• Oblique fissure.

• Paired, cone-shaped organs in thoracic cavity
• Separated by heart and other mediastinal structures
• Covered by pleura 
• Extend from diaphragm inferiorly to just above clavicles superiorly
• Lies against thoracic cage (pleura, muscles, ribs) anteriorly, laterally and posteriorly

Parts-

• Apex
• Base
• Costal surface
• Mediastinal surface and hilum.

Each lung is further divided into the small independent respiratory units called as the bronchopulmonary segments.

There are 10 segments in the right lung and 8 in the left one.
Right:

• Upper lobe- apical, anterior, posterior
• Middle lobe- medial and lateral
• Lower lobe-  superior, anterior basal, posterior basal, lateral basal and medial basal

Left lung

• Superior lobe- apical, anterior, posterior
• Lower lobe-
• Superior and inferior lingular
• Anteromedial basal, lateral basal and posterior basal.

They are the segment of the lung, that are supplied by the segmental bronchi, and a single branch of the pulmonary artery.

Hilum 

• It is on the medial surface of the lung.
• It is also called as the root of the lung. 

It has the following structures:

• Pulmonary artery
• Pulmonary veins
• Bronchus
• Lymph nodes
• Bronchial vessels
• Pleural folds

Blood supply
Dual Supply

• Bronchial Supply: arises from superior thoracic aorta or the aortic arch. 2 % supply.
• Supply bronchi, airway airway walls and pleura
• Pulmonary Supply (98% supply)
• Pulmonary arteries enter at hila and branch with airways
• Venous drainage accompany the arteries.

Lymphatic drainage follows vessels

• Parabronchial (peribronchial) lymphatics and nodes  
• hilar nodes  (bronchopulmonary nodes)
• bronchomediastinal nodes   
• Thoracic and right lymphatic duct and to the venous system.
• Occasionally some lymphatics may reach up to pre- and para-tracheal nodes  and  supraclavicular nodes

Functional Units of Gas Exchange

• Three generations of respiratory bronchioles
• Three generations of alveolar ducts
• 15-20 clusters--sacs

Gas exchange terminology

All of the structures arising from a single terminal bronchiole are called

• Primary lobule
• Acinus
• Terminal respiratory unit
• Lung parenchyma
• Functional units

Acinus/Primary lobule

• Respiratory bronchioles with some alveoli arising from their walls
• Alveolar ducts arise from respiratory bronchioles--alveoli whose septal wall contain smooth muscle

Alveoli

• contains 300 million alveoli
• Between 75 µ  to 300 µ in diameter
• Most gas exchange takes place at alveolar-capillary membrane

Anatomic Arrangement of Alveoli

• 85-95% of alveoli covered by small pulmonary capillaires
• The cross-sectional area or surface area is approximately 70m2

Acinus or Lobule

• Each acinus (unit) is approximately 3.5 mm in diameter
• Each contains about 2000 aveloli
• Approximately 130,000 primary lobules in the lung

Alveolar epithelium
Two principle cell types:

• Type I cell, squamous pneumocyte
• Type II cell, granular pneumocyte

Type I Cell

• 95% of the alveolar surface is made up of squamous pneumocyte cells
• Between 0.1 µ and 0.5µ thick
• Major site of gas exchange

Type II Cell

• 5% of the surface of alveoli composed of granular pneumocyte cells
• Cuboidal in shape with microvilli
• Primary source of pulmonary surfactant
• Involved with reabsorption of fluids in the dry, alveolar spaces

Pore of Khon

• Small holes in the walls of adjoining alveoli (alveaolar septa)
• Between 3 to 13 µ in diameter

Formation of pores may be due to:

• Desquamation due to disease
• Normal degeneration due to aging
• Movement of macrophages leaving holes

Canals of Lambert/Pores of Kohn

• Provide for collateral ventilation of difference acinii or primary lobules
• Additional ventilation of blocked units
• May explain why diseases spread so quickly at the lung tissue (paremchymal) level

Alveolar macrophages

• So-called Type III cell
• Remove bacteria and foreign particles
• May originate as
• Stem cells precursors in bone marro
• Migrate as monocytes through the blood and into the lungs

Intersitium/interstial space

• Surround, supports, and shapes the alveoli and capillaries
• Composed of a gel like substance and collagen fibers
• Contains tight space and loose space areas

Functions of lungs:

• Respiration
• Angiotensin converting enzyme production
• Protection
• Metabolism of certain chemicals and drugs
• Excretion of volatile chemicals like ethanol