General Organization and Functions of the Respiratory System

Overview of Respiratory System Structure

The respiratory system is anatomically divided into the upper and lower respiratory tracts and functionally into conducting and respiratory portions. The conducting portion transports air, while the respiratory portion is where gas exchange occurs.

• Upper respiratory tract: Includes the nose, nasal cavity, paranasal sinuses, and pharynx.

• Lower respiratory tract: Includes the larynx, trachea, bronchi, bronchioles, and lungs.

• Conducting portion: Nose to terminal bronchioles (no gas exchange).

• Respiratory portion: Respiratory bronchioles, alveolar ducts, and alveoli (site of gas exchange).

Functions of the Respiratory System

• Breathing (Pulmonary Ventilation): Cycles of inhalation and exhalation move air in and out of the lungs.

• Gas Exchange:

  • External respiration: Exchange of gases between atmosphere and blood.

  • Internal respiration: Exchange of gases between blood and body cells.

• Gas Conditioning: Inhaled air is warmed, humidified, and cleaned.

• Sound Production: The larynx produces sound for speech.

• Olfaction: The nasal cavity contains olfactory receptors for smell.

• Defense: Protection against pathogens and debris.

Upper Respiratory Tract

Nose and Nasal Cavity

The nose is the main conducting airway for inhaled air. It is supported by nasal bones and various cartilages, and contains the nasal cavity, which is divided by the nasal septum and lined with specialized epithelium.

• Nasal bones: Form the bridge of the nose.

• Cartilages: Septal, lateral, and alar cartilages provide structure and flexibility.

• Vestibule: Anterior region of the nasal cavity, containing hairs (vibrissae) that filter particles.

• Nasal septum: Divides the cavity into right and left portions; has bony and cartilaginous parts.

• Nasal conchae (superior, middle, inferior): Bony projections that increase surface area and condition air; each overlies a nasal meatus (air passage).

• Olfactory epithelium: Located in the superior part, contains olfactory receptor cells for smell.

Paranasal Sinuses

Paranasal sinuses are air-filled spaces within the bones of the skull that lighten bone weight and condition inhaled air. They are named for the bones in which they are located:

• Frontal sinuses

• Ethmoidal sinuses

• Sphenoidal sinuses

• Maxillary sinuses

Pharynx

The pharynx, or throat, is a muscular tube shared by the respiratory and digestive systems. It is divided into three regions, each with distinct structure and function:

• Nasopharynx: Posterior to nasal cavity, superior to soft palate; lined with pseudostratified ciliated columnar epithelium; conducts air only; contains pharyngeal tonsil and openings of auditory tubes.

• Oropharynx: Posterior to oral cavity, between soft palate and hyoid bone; lined with nonkeratinized stratified squamous epithelium; conducts air and food; contains palatine and lingual tonsils.

• Laryngopharynx: Inferior to hyoid bone, extends to esophagus; lined with nonkeratinized stratified squamous epithelium; conducts air and food.

Larynx

Structure and Function of the Larynx

The larynx, or voice box, connects the pharynx to the trachea and is involved in air passage, sound production, and protection of the lower respiratory tract.

• Cartilaginous framework: Nine cartilages, including thyroid (largest, with laryngeal prominence), cricoid (ring-shaped), and epiglottis (spoon-shaped, closes during swallowing).

• Paired cartilages: Arytenoid, corniculate, and cuneiform cartilages assist in sound production.

• Ligaments: Extrinsic ligaments attach larynx to surrounding structures; intrinsic ligaments include vocal and vestibular ligaments.

• Vocal folds: True vocal cords, produce sound when air passes through; vestibular folds (false vocal cords) protect the vocal folds.

• Glottis: The opening between the vocal folds (rima glottidis).

Sound Production

• Vocal range: Determined by length and thickness of vocal folds.

• Pitch: Determined by tension on vocal folds.

• Loudness: Depends on force of air passing across vocal folds.

Lower Respiratory Tract

Trachea

The trachea, or windpipe, is a tubular passageway for air, supported by C-shaped cartilages and lined with pseudostratified ciliated columnar epithelium. It bifurcates into the main bronchi at the carina.

• Tracheal cartilages: C-shaped rings provide support and keep airway open.

• Trachealis muscle: Connects posterior ends of cartilage rings, allows for esophageal expansion during swallowing.

• Mucosa: Contains goblet cells that secrete mucus to trap particles.

Bronchial Tree

The bronchial tree is a branching system of airways that conducts air from the trachea into the lungs. It includes the main bronchi, lobar bronchi, segmental bronchi, and smaller bronchioles.

• Main bronchi: Right and left branches from the trachea to each lung.

• Lobar bronchi: Branch to each lobe of the lung.

• Segmental bronchi: Supply bronchopulmonary segments.

• Bronchioles: Less than 1 mm in diameter, lack cartilage, have smooth muscle for bronchoconstriction and bronchodilation.

• Terminal bronchioles: Final part of the conducting pathway.

Respiratory Bronchioles, Alveolar Ducts, and Alveoli

These structures form the respiratory portion of the system, where gas exchange occurs. Alveoli are tiny air sacs surrounded by capillaries, with thin walls for efficient diffusion of gases.

• Respiratory bronchioles: Branch from terminal bronchioles, first site of gas exchange.

• Alveolar ducts: Lead to alveolar sacs.

• Alveoli: Saccular outpocketings where oxygen and carbon dioxide are exchanged.

• Alveolar pores: Allow collateral ventilation between alveoli.

• Cell types:

  • Type I alveolar cells: Simple squamous, for gas diffusion.

  • Type II alveolar cells: Secrete surfactant to reduce surface tension.

  • Alveolar macrophages: Engulf debris and pathogens.

• Respiratory membrane: Thin barrier for gas exchange, consisting of alveolar and capillary membranes and their fused basement membranes.

Pleura and Pleural Cavities

Pleural Membranes

The lungs are enclosed by pleura, a double-layered serous membrane. The visceral pleura adheres to the lung surface, while the parietal pleura lines the thoracic cavity. The pleural cavity between them contains lubricating fluid to reduce friction during breathing.

Gross Anatomy of the Lungs

Lung Structure

Each lung is conical, with a broad base resting on the diaphragm and an apex above the clavicle. The right lung has three lobes (superior, middle, inferior) separated by horizontal and oblique fissures; the left lung has two lobes (superior, inferior) and a cardiac notch for the heart.

Bronchopulmonary Segments

Each lung is divided into bronchopulmonary segments, each supplied by its own segmental bronchus and branch of the pulmonary artery and vein. These segments are functionally and anatomically distinct.

Blood Supply and Lymphatic Drainage

• Pulmonary circulation: Conducts blood to and from gas exchange surfaces.

• Bronchial circulation: Supplies blood to bronchi and bronchioles.

• Lymphatic drainage: Lymph nodes and vessels drain fluid from the lungs through a series of nodes and trunks.

Pulmonary Ventilation and Mechanics of Breathing

Mechanics of Breathing

Breathing involves changes in thoracic cavity volume and pressure, following Boyle’s law: as volume increases, pressure decreases, and vice versa. Inhalation increases thoracic volume and decreases pressure, drawing air in; exhalation does the opposite.

• Quiet breathing: At rest, involves diaphragm and external intercostals.

• Forced breathing: During exertion, involves additional muscles for deeper inhalation and stronger exhalation.

Skeletal Muscles of Breathing

• Muscles of quiet breathing: Diaphragm, external intercostals.

• Muscles of forced inhalation: Sternocleidomastoid, scalenes, pectoralis minor, serratus posterior superior, erector spinae.

• Muscles of forced exhalation: Internal intercostals, abdominal muscles, transversus thoracis, serratus posterior inferior.

Thoracic Cavity Volume Changes

• Vertical changes: Diaphragm movement.

• Lateral changes: Rib cage elevation or depression.

• Anterior-posterior changes: Sternum movement.

Innervation and Control of Breathing

Nervous Control

The respiratory system is innervated by both somatic and autonomic nerves. The brainstem respiratory centers (medullary and pontine) regulate the rate and depth of breathing, integrating sensory input and coordinating muscle activity.

• Somatic control: Skeletal muscles of respiration and sound production.

• Autonomic control: Larynx, trachea, bronchial tree, and lungs; sympathetic stimulation causes bronchodilation, parasympathetic causes bronchoconstriction.

• Medullary respiratory center: Ventral respiratory group (VRG) controls basic rhythm; dorsal respiratory group (DRG) processes sensory input.

• Pontine respiratory center: Modifies medullary activity for smooth transitions.

Aging and Development of the Respiratory System

Aging

With age, the respiratory system becomes less efficient due to decreased elasticity of connective tissue, leading to reduced ventilation. Chronic diseases and pollutants can further impair function.

Development

The respiratory system develops from the respiratory diverticulum in the embryo, branching into bronchial buds and forming the bronchial tree and alveoli. Surfactant production begins before birth, enabling newborns to breathe.