Comprehensive Anatomy and Physiology of the Respiratory System
Structural Organization of the Respiratory System
The respiratory system is anatomically and functionally divided into upper and lower respiratory tracts. The upper respiratory tract comprises the nose, nasal cavity, laryngopharynx, and the larynx. The lower respiratory tract includes the trachea, the primary bronchi, the extensive bronchial tree, and the lungs themselves.
Anatomy and Regions of the Nose
The nose is composed of the external nose and the internal nasal cavity. These two regions are bisected into right and left halves by a structure known as the nasal septum. The external nose contains two elliptical orifices referred to as the nostrils or nares. The lateral margin of these nostrils is called the ala nasi, which is characterized by its rounded shape and mobility. Structurally, the nose is composed of both bone and cartilage.
Internal to the nostrils is the nasal cavity, which extends from the nostrils in the front to the posterior nasal apertures, also known as the choanae, at the rear. It is at these choanae that the nasal cavity opens into the nasopharynx. The nasal cavity is further divided into specific regions: the nasal vestibule, located just inside the nostrils, and the olfactory area. The roof of the nasal cavity is formed by the frontal and nasal bones, the body of the sphenoid, and the cribriform plate of the ethmoid bone.
Vasculature and Clinical Pathologies of the Nasal Region
The nasal region receives its blood supply from several key arteries, including the superior labial artery, the anterior ethmoid artery, the posterior ethmoid artery, the sphenopalatine artery, and the greater palatine artery. A significant vascular landmark is Kiesselbach's plexus, located in an area known as Little's area, which is a frequent site for clinical issues.
Common clinical conditions associated with the nasal region include Epistaxis (nosebleed), Rhinorrhea (nasal discharge or a runny nose), and Rhinitis (inflammation of the nasal mucous membrane).
The Paranasal Sinuses
The paranasal sinuses are air-filled cavities located within the bones of the skull surrounding the nose. These sinuses open into the lateral wall of the nasal cavity. There are four primary sets of paranasal sinuses, named according to the bones in which they reside: the frontal air sinuses, the ethmoidal air sinuses, the sphenoidal air sinuses, and the maxillary air sinuses.
Structural Divisions of the Pharynx
The pharynx is a muscular tube approximately in length, situated behind the oral, nasal, and laryngeal cavities. It extends from the base of the skull down to the lower border of the cricoid cartilage, which corresponds to the level of the lower border of the cervical vertebra (), where it becomes continuous with the esophagus. The pharynx is divided into three distinct parts: the nasopharynx, the oropharynx, and the laryngopharynx.
The nasopharynx is the uppermost portion, located behind the nasal cavity. The auditory tubes open into the lateral walls of the nasopharynx. The oropharynx lies behind the oral cavity, joined with the nasopharynx at the oropharyngeal orifice or isthmus. The lateral walls of the oropharynx house the palatine tonsils. The laryngopharynx is the lowermost part, positioned behind the larynx and continuing anteriorly with the inlet of the larynx. On either side of the laryngeal inlet, the laryngopharynx contains a dead space known as the piriform recess. This recess is a common site for the stagnation of food and foreign bodies. Inferiorly, the laryngopharynx is continuous with the esophagus.
The Larynx and Phonation
The larynx serves as both a phonation box and an air passage, located in the middle anterior region of the neck. It is constructed from various cartilages held together by a network of membranes, ligaments, and muscles. The cavity of the larynx extends from the laryngeal inlet to the beginning of the trachea. The central, middle portion of the laryngeal cavity contains two pairs of vocal folds or cords used for sound production.
The Trachea and Bronchial Tree
The trachea begins at the laryngeal outlet and extends downward for approximately . It terminates at the level of the sternal angle, where it divides into the right and left primary (principle) bronchi. This bifurcation point is known as the carina. The wall of the trachea is fibromuscular and is structurally supported by a series of C-shaped hyaline cartilages. The open end of these cartilages is bridged by the trachealis muscle.
The right and left principle bronchi originate at the level of the sternal angle. As they enter the lungs, they branch further into tertiary bronchi, which correspond to specific bronchopulmonary segments.
Gross Anatomy and Characteristics of the Lungs
The thoracic cavity contains a pair of lungs, which are protected by the thoracic skeleton and encased in a serous sac called the pleura. Each lung features an apex at the top, a broad base or inferior surface at the bottom, a sharp anterior border, and a sharp inferior border. There are three primary surfaces on each lung: the sterno-costal surface, the diaphragmatic surface, and the mediastinal surface.
The right lung and left lung exhibit distinct anatomical differences. The right lung is shorter and broader because the right lobe of the liver pushes it upward. It is divided into three lobes—the upper, middle, and lower lobes—by two fissures: the horizontal (transverse) fissure and the oblique fissure. It typically weighs about and has a straighter anterior border.
In contrast, the left lung is longer and narrower but smaller and lighter, weighing approximately . It consists of only two lobes—the upper and lower lobes—separated by a single oblique fissure. The anterior border of the left lung is interrupted by the cardiac notch to accommodate the heart.
Lung Surfaces and the Hilum
The costal surface of the lung is large, smooth, and convex, related directly to the costal pleura that separates it from the ribs, costal cartilages, and innermost intercostal muscles. The mediastinal surface is concave because it is related to the middle mediastinum, which contains the heart and pericardium. This surface features the hilum, a wedge-shaped area where the structures forming the root of the lung (such as bronchi and blood vessels) enter or exit the lung. The diaphragmatic surface, also concave, forms the lung's base and rests upon the dome of the diaphragm.
The Pleura and Respiratory Mechanics
The pleura is a serous sac that invests the lungs. it consists of two layers: an outer parietal layer and an inner visceral layer. The small space between these layers is the pleural cavity. Clinical conditions related to the pleura include Pleuritis (inflammation), Pneumothorax (air in the pleural cavity), and Haemothorax (blood in the pleural cavity).
Breathing (ventilation) involves the expansion and contraction of the chest. During inhalation (breathing in), the chest expands and the diaphragm contracts and moves downward. During exhalation (breathing out), the chest contracts and the diaphragm relaxes and moves upward.
The Diaphragm and Intercostal Muscles
The diaphragm is a musculotendinous structure with two main parts: a fixed muscular part and a movable central part. The muscular fibers radiate toward the center to form a central tendon, which has no bony attachments. The diaphragm contains three major openings to allow structures to pass between the thorax and abdomen: the Caval opening for the Inferior Vena Cava (IVC) at the level of , the Esophageal hiatus for the esophagus at the level of , and the Aortic hiatus for the aorta at the level of .
The intercostal muscles fill the spaces between the ribs and are categorized into three layers: the external intercostal muscle, the internal intercostal muscle, and the innermost intercostal muscle. The innermost layer is further subdivided into the intercostalis intima, the subcostalis, and the sternocostalis (also known as the transversus thoracis).
Questions & Discussion
The presentation concluded with a designated period for student questions regarding the anatomy and physiology of the respiratory system.