The Trachea is also called windpipe and it is a fibrocartilaginious tube that creates the beginning of the lower respiratory tract. 16-20 C-shaped rings of Hyaline cartilage keep its lumen distinct. A band of smooth muscle (Trachealis) and a fibroelastic ligament that bridges the gap between the posterior free ends of C-shaped cartilages, that allow expansion of esophagus during the passage of bolus of the food.
The arrangement of cartilages and elastic tissue in the tracheal wall prevents its kinking and obstruction during the movements of the head and neck.
The trachea extends from the lower border of cricoid cartilage (corresponding to the lower border of C6 vertebra) in the neck to the lower border of T4 vertebra in the thorax. Thus upper half of the trachea lies in the neck (cervical part) and lower half in the superior mediastinum (thoracic part).
The extent of trachea varies as follows:
- C6 to T4 in cadaver placed in supine position.
- C6 to T6 in living individuals in standing position.
- C6 to T3 in newborn.
Length: 10-12 cm.
External diameter: 2 cm in males and 1.5 cm in females. Internal diameter: 12 mm in adult, 3 mm in newborn.
Lumen of trachea:
- The lumen of trachea is smaller in living human beings than in the cadavers.
- It’s 3 mm at 1 year of age; during childhood it corresponds to the age in years (i.e., 5-year-old child will have tracheal diameter of 5 mm) with a maximum of 12 mm in adults. Because of this endotracheal tubes are graduated in mm.
The trachea is the continuation of the larynx and begins at the lower border of the cricoid cartilage in the level of C6 vertebra, about 5 cm above the jugular notch.
It enters the thoracic inlet in the midline and enters downwards and backwards behind the manubrium to terminate by bifurcating into 2 principal bronchi, a little to the right side at the lower border of T4 vertebra corresponding to the sternal angle.
Relations Of The Thoracic Part
- Arch of aorta.
- Brachiocephalic trunk and left common carotid artery.
- Left brachiocephalic vein.
- Superior vena cava (anterolateral).
- Deep cardiac plexus.
- Vertebral column.
- Left recurrent laryngeal nerve (it ascends up between trachea and esophagus).
To The Right:
- Right lung and pleura.
- Azygous vein.
- Right vagus nerve.
To The Left:
- Arch of aorta.
- Left common carotid artery.
- Left subclavian vein.
- Left vagus nerve.
- Left phrenic nerve.
Histologically, tracheal tube from inside outward is created from the following layers:
It is composed of lining epithelium and lamina propria.
- Lining epithelium is pseudostratified ciliated columnar with few goblets cells.
- Lamina propria is composed of longitudinal elastic fibres.
It is composed of loose areolar tissue consisting of large number of serous and mucous glands.
- Cartilage and smooth muscle layer: It’s created from horseshoe-shaped (C-shaped) hyaline cartilaginous rings, that are deficient posteriorly. The posterior gap is filled up chiefly by the smooth muscle (trachealis) and fibroelastic fibres.
- Perichondrium: It encloses the cartilage.
- Fibrous membrane: It’s a layer of dense connective tissue, consisting of neurovascular structure.
There’s no clear difference between lamina propria and submucosa.
Vascular Supply And Lymphatic Drainage
A. Blood supply to the trachea is by inferior thyroid arteries.
B. Venous drainage of the trachea takes place into the left brachiocephalic (innominate) vein.
C. Lymphatic drainage of the trachea is into pretracheal and paratracheal lymph nodes.
Nerve supply takes place by the autonomic nerve fibres:
A. Parasympathetic fibres are sensory and secretomotor to the mucous membrane, and motor to the trachealis muscle.
B. Sympathetic fibres are vasomotor.
Tracheal Shadow In Radiograph
It’s viewed as a vertical translucent shadow in front of cervico-thoracic spine. The translucency is because of the presence of air in the trachea.
Palpation Of Trachea
Medically, trachea is palpated in the suprasternal notch. Normally, it’s median in position but appreciable shift of trachea to left or right side indicates the mediastinal shift.
Importance Of Carina
It’s a keel-like median ridge in the lumen in the bifurcation of trachea. The lowest tracheal ring in the bifurcation of trachea is thick in its central part. From the lower margin of the thick central part a keel-shaped (hook-shaped) process projects downwards and backwards between the left and right principal bronchi. It’s both functional and pathological importance.
Functional importance: The mucosa of trachea over the carina is most sensitive. The cough reflex is generally started here, which helps to clear the sputum.
Pathological importance: It’s visible as a sharp sagittal ridge in the tracheal bifurcation during bronchoscopy, for this reason acts as a useful landmark. It’s located about 25 cm from the incisor teeth and 30 cm from the nostrils. If the tracheobronchial lymph nodes in the angle between the main (principal) bronchi are enlarged because of spread of bronchiogenic carcinoma, the carina becomes distorted and flattened.
Importance Of Mucous Secretion In Tracheal Lumen
It helps to trap the inhaled foreign particles and solid mucous is then expelled during coughing. The cilia of lining epithelium of mucous membrane also beat upwards pushing the mucous upwards. The fibroelastic ligament prevents over distension of tracheal lumen while trachealis muscle reduces the diameter on contraction during coughing which involves increased velocity of expired air required for cleaning the air passages.