Respiratory system | Anatomy | Achievable USMLE/1

The respiratory system is divided into a conducting zone and a respiratory zone. Only the respiratory zone participates in gas exchange. The conducting zone is composed of nasal cavities, nasopharynx, oropharynx, larynx, trachea, bronchi, bronchioles and terminal bronchioles. The respiratory zone is composed of respiratory bronchioles, alveolar ducts and alveoli.

The nasal cavity

The nasal septum divides the nasal cavity into right and left halves. The lateral nasal wall shows three bones called superior, middle and inferior conchae which form the three turbinates which border spaces called meatues- superior, middle and inferior. The posterior ethmoid and sphenoid sinuses drain into the superior meatus, the anterior ethmoid, maxillary and frontal sinuses drain into the middle meatus while the nasolacrimal duct drains into the inferior meatus. The nasal mucosa is ciliated, pseudostratified columnar epithelium. It produces IgA and IgE. The nose has a rich blood supply provided by branches of the external and internal carotid artery. The external nose is supplied by the facial artery, infraorbital artery and ophthalmic artery. Kiesselbach’s plexus or Little’s area is present on the anterior and inferior part of the nasal septum. It is the site of anastomoses between four major arteries - anterior ethmoidal (branch of ophthalmic), sphenopalatine (branch of maxillary), greater palatine (branch of maxillary) and superior labial (branch of facial). It is the major site of epistaxis or bleeding from the nose. Posterior bleeding derives primarily from the posterior septal nasal artery (a branch of the sphenopalatine artery), which forms part of the Woodruff plexus. Woodruff plexus is located on the lateral nasal wall, posterior to the inferior turbinate. It is mainly a venous plexus. Posterior nosebleeds are more difficult to control and can lead to aspiration.

The Pharynx

The pharynx is a common passage for the alimentary and respiratory tracts. It is divided into the nasopharynx, oropharynx and laryngopharynx. It receives the auditory or eustachian tube which connects it to the middle ear. Infections can spread from the pharynx to the middle ear by this route. The nasopharynx also has nasopharyngeal tonsils or adenoids, on the posterior wall. The adenoid tonsil is supplied by the ascending palatine branch of the facial artery, ascending pharyngeal artery, pharyngeal branch of the internal maxillary artery, artery of the pterygoid canal and the ascending cervical branch of the thyrocervical trunk.

The oropharynx extends from the nasopharynx to the epiglottis. It has a lymphatic ring of nasopharyngeal, tubal, palatine and lingual tonsils. The palatine tonsil is supplied by the tonsillar and ascending palatine branches of the facial artery, the ascending pharyngeal artery, the dorsal lingual branch of the lingual artery and the palatine branch of the maxillary artery. The laryngopharynx ends at the cricoid cartilage. The wall of the pharynx has an external circular and an internal longitudinal layer of muscles. The external muscles are constrictors. The internal muscles have stylopharyngeus and palatopharyngeus muscles.

The Larynx

It extends from C3 to C6 vertebra. The larynx has three single and three paired cartilages. These are thyroid, cricoid, epiglottic, arytenoid, corniculate and cuneiform cartilages. The thyroid cartilage is superficial and can be seen and palpated as the “Adam’s apple”. The cricoid cartilage is at the level of C6 vertebra. In cricothyrotomy, the cricothyroid membrane is incised to access the larynx ,in an emergency, to establish patency of the airway in cases where endotracheal intubation is not possible.

The Trachea

It begins at the lower edge of the cricoid cartilage and ends at the level of T4 vertebra and sternal angle, at the carina by dividing into right and left main bronchi. The trachea is located anterior to the esophagus. It is incompletely surrounded on the anterior and lateral side by about 15-20 C shaped hyaline cartilages. They give flexibility to the trachea and help to keep it open, preventing collapse of the airway. The trachealis muscle, which is a smooth muscle, runs on the posterior wall. The posterior wall is devoid of cartilage, allowing the esophagus, which is present posteriorly, to expand during the process of swallowing. The trachea is lined by respiratory epithelium which is ciliated, pseudostratified columnar epithelium with goblet cells. Brush cells with numerous microvilli on their surface , endocrine APUD cells which regulate secretion from the goblet cells and mucosal glands and basal cells which are stem cells with high mitotic potential are also seen in the mucosal lining. The lobes of the thyroid gland are anterolateral to the cervical trachea and the thyroid isthmus crosses the trachea anteriorly at the level of second or third tracheal rings.

Bronchi, bronchioles, alveolar ducts and alveoli

There are two main bronchi. The right is wider and more vertical (hence more risk of aspiration), compared to the left which is more horizontal. The main bronchi further divides into lobar bronchi. There are two left lobar and three right lobar bronchi. The lobar bronchi further divides into segmental bronchi which supply the bronchopulmonary segments. The bronchi are lined by respiratory epithelium, as above. They have a prominent layer of circular smooth muscle. Hyaline cartilages are present in the wall.

The bronchioles are lined by simple ciliated columnar epithelium with goblet cells and Clara cells. The terminal and respiratory bronchioles are lined by simple, ciliated, cuboidal epithelium with Clara cells. Cartilage is absent in the respiratory bronchioles. Occasional alveoli open into the terminal and respiratory bronchioles. Alveolar ducts are tubular structures lined by simple squamous epithelium. The alveolar ducts are lined by alveoli. Cilia are absent in the epithelium of the alveolar ducts and alveoli. They are connected to alveolar sacs which give rise to clusters of alveoli. A network of capillaries surround the alveoli. The alveolar lining is made of mainly two types of cells - type I and type II pneumocytes. Type I pneumocytes are flattened, simple squamous cells joined together by tight zonula occludens. Type II pneumocytes are cuboidal shaped cells. They secrete surfactant which is stored as lamellar bodies. They are also stem cells and undergo hyperplasia in response to alveolar injury and repair. Alveolar macrophages are present in the septae. Pores of Kohn are present in the interalveolar septum. Their role is collateral ventilation and equalization of pressure across the alveoli. They form channels for the spread of infections like pneumonia and in cancers. Sometimes, alveolar brush cells are called type III pneumocytes. Their role is ill defined.

Blood air barrier

It is formed by the surfactant layer, type I pneumocyte, basement membrane and capillary endothelial cell. Diffusion occurs across the blood air barrier.

Lungs

Both lungs are divided into lobes by fissures - 3 on the right and 2 on the left. The right lung is further divided into 10 segments of which 3 are in the upper lobe, 2 are in the middle and 5 in the lower lobe. The left lung has 8 segments - 4 in the upper lobe and 4 in the lower lobe. Aspiration pneumonia happens in the apical segment of lower lobes (right more common) and the posterior segment of upper lobes in a supine patient. In erect position, aspiration pneumonia occurs in the basal segment of lower lobe (right more common).

Surface anatomy

It is important in assessing thoracic and abdominal injuries and for doing procedures like thoracotomy. The apices of the lungs extend three cms above the medial third of the clavicle. The inferior margins of the lung are at T6 in the midclavicular line, T8 in the midaxillary line and T10 posteriorly. The parietal pleura is situated about two vertebral levels lower than the lung margins. Upper lobe of the right lung extends up to the fourth rib, the middle lobe extends from fourth to sixth ribs while the lower lobe is below the sixth rib. On the left, the upper lobe extends all the way up to the sixth rib while the lower lobe is below it. Anteriorly, upper and middle lobes of the right lung and upper lobe of the left lung can be auscultated. Posteriorly, lower lobes of right and left lungs are auscultated.

Important receptors of the lung

Pulmonary stretch receptors: They are present in the airway smooth muscle and are stimulated by inflation. They cause bronchodilation and inhibition of inspiration. They are innervated by myelinated vagal fibres.
J receptors: They are present in the alveolar wall and are stimulated by inhalation of irritant gases, pulmonary edema and microembolism. They are responsible for the sensation of dyspnea. They cause rapid, shallow breathing when stimulated and are innervated by unmyelinated vagal fibres.
Irritant receptors: They are present in the epithelial layer, are innervated by myelinated vagal fibres and are stimulated by dust, irritants, pneumothorax, bronchoconstriction, atelectasis etc. They result in hyperventilation, tachypnea and bronchoconstriction.
Cough receptors: These are present in the trachea, large airways, pharynx, pleura, diaphragm, ear canal, pericardium and stomach. They respond to chemical and physical stimuli to initiate the cough reflex. Cough is a protective response and helps in clearing of the airways.