Anatomy of the lower respiratory tract

anatomy relevent for anesthesiologist

1. ANATOMY OF THE LOWER RESPIRATORY TRACT Presented By:- Dr. Bharat Bhushan Shukla , Junior resident Moderator:- Dr. Virendra Kumar (Associate Prof.)

2. CONTENTS Trachea Bronchial Tree Bronchopulmonary Segments Alveoli Blood supply Nerve supply Histology

3. TRACHEA A fibromuscular tube which serves as a conduit for ventilation and clearance of tracheal and bronchial secretions It starts at the level of body of sixth cervical vertebra and ends at level of body of fourth thoracic vertebra Has 20 C-shaped cartilaginous rings which forms its lateral and anterior walls and gives it structural support while posteriorly they are connected by membranous wall of trachea It divides into right and left main bronchus at carina Right at 25 degrees Left at 45 degrees In children below 3 years of age both main stem bronchi take off at equal angle of 55 degrees

4. Important dimensions of Trachea Mean length of trachea Mean Tracheal Diameter Mean Distance Lip to Carina Mean distance from base of nose to carina MALE 12-14 cm 2.5cm 28.5 cm 31.0 cm FEMALE 10-14 cm 2.5 cm 25.2 cm 28.4 cm

5. Bronchial Tree Broadly divided into three functional divisions Conductive ,Transitional and Respiratory Conductive --- the only function is bulk gas movement Main(primary) bronchi Lobar(secondary) bronchi Segmental(tertiary) Bronchi (after multiple divisions)Terminal Bronchioles The bronchioles are typically 1 mm in diameter and lack the fibrocartilaginous framework and secretory gland cells which are present in bronchi and therefore the bronchioles can passively dilate during inspiration

6. Right main bronchus is about 0.5-2.5 cm long. Left main bronchus is about 4-6 cm long. The right upper lobe bronchus divides directly posterior at 90 degrees facilitating aspiration of foreign bodies and fluids into right upper lobe in supine position Transitional --- Function is gas movement and limited gas exchange It includes respiratory bronchioles and alveolar ducts Each respiratory bronchiole aerates a small part of lung called as Pulmonary unit Respiratory --- The only function is gas exchange Includes alveoli and alveolar sacs

7. Bronchopulmonary Segments These are well defined anatomic, functional and surgical sectors of lung It is the smallest functionally independent region of lung and the smallest area of lung that can be isolated and removed without affecting adjacent regions Each segment has a segmental bronchus, segmental artery, autonomic nerves and lymph vessels Each segment is shaped like a irregular cone with its apex at the origin of segmental bronchi and base projected peripherally onto the base of the lung It should be noted that bronchopulmonary segment is not a bronchovascular segment as it does not have its own vein

8. Alveoli It is a thin walled polyhedral sac The number of alveoli increases progressively with age, from about 24 million at birth and reaches the final adult count of 300 million at the age of 8 or 9 years Alveoli are associated with about 250 million precapillaries and 280 billion capillary segments and results in a surface area of about 70 meter square Average diameter of alveolus is about 0.05-0.33 mm In upright position largest alveoli are at apex while smallest are at base but during inspiration the discrepancies in alveolar size diminish The gas exchange is facilitated by pulmonary capillary beds which are the densest capillary network in the body Each alveolus is closely associated with some 1000 short capillary segments

9. The alveolar wall consists of a thin capillary epithelial cell, a basement membrane, a pulmonary capillary endothelial cell and a surfactant lining layer Type I alveolar cells cover 80% of alveolar surface and provide extremely thin cytoplasmic extensions that provide suitable surface for gas exchange Type I cells are highly differentiated but metabolically limited and highly susceptible to injury Type II cells manufacture the surfactant and have high metabolic and enzymatic activity Both type I and II have tight intracellular junctions and provide a relatively impermeable layer to fluids Type III cells are alveolar macrophages and important in immunologic lung defense Other cell types are mast cells, neutrophils , lymphocytes and APUD cells

10. Alveolar-Capillary Interface

11. Arterial supply The bronchial arterial system provides oxygen and nutrition to the conductive tissues and pulmonary vessels On right side single bronchial artery which arises from third right posterior intercostal artery On left side two bronchial arteries and both arise from descending thoracic aorta Branches of bronchial artery supply the wall of the bronchi and follow the airways as far as terminal bronchioles and along their course they anastomose with the pulmonary arterial circulation Except for the main bronchi within the mediastinum almost all the blood carried by the bronchial arteries enter the pulmonary circulation

12. Venous drainage There are two bronchial veins on each side Venous blood from first and second division of bronchi are carried by bronchial veins The right bronchial veins drain into azygous vein and left bronchial veins drain into hemiazygous vein The pulmonary veins do not accompany the bronchi or pulmonary arteries, they run independently in the intersegmental planes Anatomic connections between the bronchial and pulmonary venous circulations create an absolute shunt of about 2-5% of the total cardiac output and represent ‘normal’ shunt

13. Nerve supply Parasympathetic supply is by vagus nerve It provides motor supply to the bronchial vessels and on stimulation causes bronchospasm Provides s ecretomotor supply to the mucous glands of bronchial tree and stimulation causes increased secretions Carries sensory fibres for cough reflex and stretch reflex Sympathetic supply is from 2 nd to 5 th sympathetic ganglia It mediates bronchodilation and decreases gland secretion of bronchial tree via β -2 receptors Both Parasympathetic and sympathetic nerves first form anterior and posterior plexuses in front and behind lung roots

14. Histology of Bronchial tree Intrapulmonary bronchus--- Pseudostatified ciliated columnar epithelium with goblet cells The cilia prevents the accumulation of mucus in the bronchial tree Terminal bronchioles ---simple columnar epithelium All the divisions of bronchial tree beyond terminal bronchiole lacks the cartilage plates and glands Respiratory bronchiole--- cuboidal epithelium Alveoli--- squammous cells

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