1. Trachea
It is a mobile tube about 13 cm ( 5 inch ) long & 2.5 cm ( 1 inch ) in diameter. It has a fibroelastic wall in which are embedded a U- shaped bars of hyaline cartilage that keep the lumen patent. The posterior free ends of the cartilage are connected by smooth muscle ( trachealis ) .
It lies in the neck below the cricoid cartilage of the larynx at the level of the body of the 6th cervical vertebra. It ends below in the thorax at the level of the sternal angle ( lower border of the 4th thoracic vertebra ) by dividing into the right & left principal ( main ) bronchi. The bifurcation is called the carina. In deep inspiration the carina descends to the level of the 6th thoracic vertebra.

2. Relation of the trachea
Anteriorly: The sternum; thymus; left brachiocephalic vein; the origins of the brachiocephalic & left common carotid arteries and the arch of aorta.
Posteriorly: The esophagus & the left recurrent laryngeal nerve.

3. Right side: The azygos vein ; the right vagus nerve and the pleura.
Left side: The arch of aorta ; the left common carotid & the left subclavian arteries ; the left vagus & the left phrenic nerves and the pleura.
Nerve supply of the trachea are branches of the vagus & recurrent laryngeal nerves and the sympathetic trunks. They are distributed to the trachealis muscle and to the mucous membrane lining the trachea.

4. Principal Bronchi
The main right bronchus is wider ; shorter and more vertical than the left and is about 2.5 cm ( 1 inch ) long.
Before entering the hilum of the right lung , the principal bronchus gives off the superior lobar bronchus.
On entering the hilum, it divides into a middle & an inferior lobar bronchus.
The left main bronchus is narrow , longer and more horizontal than the right & is about 5 cm ( 2 inches ) long.
It passes to the left below the arch of the aorta & in front of the esophagus.
On entering the hilum of the left lung, it divides into a superior & an inferior lobar bronchus.

5. Bronchopulmonary Segments
They are the anatomic, functional and surgical units of the lungs.
Each lobar ( secondary ) bronchus, which passes to a lobe of the lung gives off branches called segmental ( tertiary ) bronchi.

6. Each segmental bronchus passes to a structurally & functionally independent unit of a lung lobe called a bronchopulmonary segment which is surrounded by connective tissue.
The segmental bronchus ( tertiary ) is accompanied by a branch of the pulmonary artery.
But the tributaries of the pulmonary veins run in the connective tissue between adjacent bronchopulmonary segments.
Each segment has its own lymphatic vessels & autonomic nerve supply

7. On entering a bronchopulmonary segment each segmental bronchus divides repeatedly.
As the bronchi become smaller , the U – shaped bars of the cartilage found in the trachea are gradually replaced by irregular plates of cartilage which become smaller & fewer in number.
The smallest bronchi divide & give rise to bronchioles which are less than 1 mm in diameter.
Bronchioles posses no cartilage in their walls & are lined with columnar ciliated epithelium
The submucosa posses a complete layer of circularly arranged smooth muscle fibers.

8. The bronchioles then divided & give rise to terminal bronchioles which show delicate outpouching from their walls.
Gases exchange between blood & air takes place in the walls of these outpouchings which explains the name respiratory bronchiole. The diameter of a respiratory bronchiole is about 0.5 mm.
The respiratory bronchioles end by branching into alveolar ducts which lead into tubular passages with numerous thin–walled outpouching called alveolar sacs.
The alveolar sacs consist of several alveoli opening into single chamber.
Each alveolus is surrounded by a rich network of blood capillaries.
Gases exchange takes place between the air in the alveolar lumen through the alveolar wall into the blood within the surrounding capillaries.
The respiratory zone includes the respiratory bronchioles , alveolar ducts, alveolar sacs & alveoli. It is the only site of gas exchange

9. The main characteristics of a bronchopulmonary segment are as follows:
1- It is a subdivision of a lung lobe.
2- It is pyramid shaped with its apex toward the lung root.
3- It is surrounded by connective tissue.
4- It has a segmental bronchus ; a segmental artery ; lymph vessels and autonomic nerves.
5-The segmental vein lies in the connective tissue between adjacent bronchopulmonary segments.
6- A diseased segment because it is a structural unit can be removed surgically.

10. The main bronchopulmonary segments are as follows:
Right lung
Superior lobe ( upper ) :
Apical; posterior and anterior.
Middle lobe :
Lateral & medial.
Inferior lobe :
Superior ( apical ) ; medial basal ; anterior basal ; lateral basal and posterior basal. .

11. Left lung
Superior lobe :
Apical ; posterior ; anterior ;superior lingular and inferior lingular.
Inferior lobe:
Superior ( apical ) ; medial basal ; anterior basal ; lateral basal and posterior basal.

12. Clinical Notes pleurisy
It is an inflammation of the pleura ( pleuritis ). It is secondary to inflammation of the lung ( pneumonia ).
The pleural surface becomes coated with inflammatory exudate causing the surfaces to be roughened. This roughening produces friction and a pleural rub can be heard with the stethoscope on inspiration and expiration.
Often the exudate becomes invaded by fibroblasts which lay down collagen and bind the visceral pleura to the parietal forming pleural adhesions.

13. Pleural fluid
The pleural space normally contains 5 to 10 ml of clear fluid which lubricates the apposing surface of the visceral & parietal pleurae during respiratory movements.
The formation of the fluid results from hydrostatic and osmotic pressures ( proteinates). Because the hydrostatic pressure ( ionates ) are greater in the capillaries of the parietal pleura than the visceral pleura. The pleural fluid normally absorbed into the capillaries of the visceral pleura.
Any condition that increase the production of the fluid ( inflammation; malignancy and congestive heart failure or impairs the drainage of the fluid results in the abnormal accumulation of fluid ( pleural effusion ) and collapsed lung.
The presence of 300 ml of the fluid in the costodiaphragmatic recess in an adult is sufficient to enable it clinical detection .
The clinical sings include decreased lung expansion on the side of effusion with decreased breath sounds and dullness on percussion over the effusion.
The mediastinum is displaced to the left The right lung is compressed The bronchi are narrowed. Auscultation reveal faint breath sounds over the compressed lung and absent breath sounds over fluid in the pleural cavity.

14. Pneumothorax 1- Spontaneous pneumothorax
It is a condition in which air enters the pleural cavity suddenly without its cause being immediately apparent. After investigation, it is found that air has entered from a diseased lung and a bulla ( bleb ) has ruptured.
2- Open pneumothorax:
Stab or gunshot wounds of the thoracic wall may pierce the parietal pleura so that the pleural cavity is open to the outside air. Each time the patient inspires, it is possible to hear air under atmospheric pressure begin sucked into the pleural cavity.
3- Tension pneumothorax:
The air may pressed on the wounded side and pushes the mediastinum toward the opposite side. In this situation a collapsed lung is on the injured side and the opposite lung is compressed by the deflected mediastinum.

15. If it is associated by pus as pyopneumothorax.
Air in the pleural cavity associated with huge amount of serous fluid is known as hydropneumothorax.
If it is associated by pus as pyopneumothorax.
If is associated with blood as hemopneumothorax blood enters the pleural cavity through stab or bullet wounds to the chest wall or from lacerated lung.
A collection of pus without air is called an empyema. .

16. Fluid ( serous, blood, or pus ) can be drained from the pleural cavity through a wide- bore needle.

17. Inhalation Foreign Bodies
Because the right bronchus is the wider and more direct continuation of the trachea foreign bodies tend to enter the right instead of the left bronchus.
From there, they usually pass to the middle or lower lobe bronchi.
Foreign bodies may be pins; toys; parts of teeth which may be inhaled while a patient is under anesthesia and parts of chicken bones.