The Respiratory System

Overview and Major Functions

The respiratory system is essential for gas exchange, supplying oxygen (O2) to the body for cellular respiration and removing carbon dioxide (CO2), a waste product. It also plays roles in olfaction (smell) and speech. The respiratory and circulatory systems are closely linked to ensure efficient transport and exchange of gases.

• Supply of O2: Needed for cellular respiration in tissues.

• Removal of CO2: Prevents toxic buildup from metabolism.

• Other functions: Sense of smell and speech production.

Four Processes of Respiration

Respiration involves four distinct processes, divided between the respiratory and circulatory systems:

1. Pulmonary Ventilation: Movement of air into and out of the lungs (breathing).

2. External Respiration: Exchange of O2 and CO2 between lungs and blood.

3. Transport of Respiratory Gases: Movement of O2 and CO2 in the blood.

4. Internal Respiration: Exchange of O2 and CO2 between blood and tissues.

Functional Anatomy of the Respiratory System

Major Organs

The respiratory system is divided into upper and lower regions, each with specific structures and functions.

• Upper Respiratory: Nose, nasal cavity, paranasal sinuses, pharynx

• Lower Respiratory: Larynx, trachea, bronchi and branches, lungs, alveoli

Upper Respiratory System

The upper respiratory system filters, warms, and moistens incoming air and is involved in olfaction and speech.

The Nose and Paranasal Sinuses

The nose is the only external part of the respiratory system. It provides an airway, moistens and warms air, filters particles, serves as a resonating chamber for speech, and houses olfactory receptors. The nose is divided into the external nose and nasal cavity.

• External Nose: Includes root, bridge, dorsum nasi, apex, and nostrils (nares). Formed by nasal/frontal bones, maxillary bones, and hyaline cartilage.

• Nasal Cavity: Divided by the nasal septum (septal cartilage, vomer, perpendicular plate of ethmoid). Contains nasal vestibule, lined with vibrissae (hairs) and mucous membranes.

• Olfactory mucosa: Contains olfactory epithelium for smell.

• Respiratory mucosa: Pseudostratified ciliated columnar epithelium with goblet cells; seromucous glands secrete mucus and enzymes (lysozyme, defensins).

• Nasal Conchae: Superior, middle, and inferior conchae increase mucosal area and air turbulence, aiding in filtering, heating, and moistening air.

• Paranasal Sinuses: Located in frontal, sphenoid, ethmoid, and maxillary bones; lighten skull, secrete mucus, warm/moisten air.

Clinical Note: Rhinitis and Sinusitis

• Rhinitis: Inflammation of nasal mucosa; can spread to throat, chest, tear ducts, and sinuses, causing sinusitis and headaches.

The Pharynx

The pharynx is a muscular tube connecting the nasal cavity and mouth to the larynx and esophagus. It is divided into three regions:

• Nasopharynx: Air passageway; lined with pseudostratified columnar epithelium; contains pharyngeal tonsils and auditory tubes.

• Oropharynx: Passageway for food and air; lined with stratified squamous epithelium; contains palatine and lingual tonsils.

• Laryngopharynx: Passageway for food and air; lined with stratified squamous epithelium; continuous with esophagus.

Clinical Note: Adenoid Enlargement

• Swollen adenoids can block air passage, forcing mouth breathing and affecting speech and sleep.

Lower Respiratory System

Major Structures and Functions

The Larynx

The larynx (voice box) extends from the 3rd to 6th cervical vertebra and attaches to the hyoid bone. It provides a patent airway, routes air and food, and produces voice. The larynx is composed of nine cartilages (mostly hyaline), including the thyroid, cricoid, arytenoid, cuneiform, corniculate, and epiglottis (elastic cartilage).

• Epiglottis: Covers laryngeal inlet during swallowing; prevents food entry.

• Vocal folds: Vibrate to produce sound; glottis is the opening between folds.

• Vestibular folds: Help close glottis during swallowing; do not produce sound.

• Epithelium: Superior portion is stratified squamous; inferior is pseudostratified ciliated columnar.

Voice Production and Sphincter Function

• Speech: Produced by intermittent release of expired air through glottis.

• Pitch: Determined by length and tension of vocal cords.

• Loudness: Depends on force of air.

• Sphincter function: Vocal folds can act as a sphincter (e.g., Valsalva's maneuver).

Clinical Note: Laryngitis

• Inflammation of vocal folds causes hoarseness; often due to viral infection, overuse, dry air, or irritants.

The Trachea

The trachea (windpipe) extends from the larynx into the mediastinum, dividing into two main bronchi. It is composed of three layers:

• Mucosa: Ciliated pseudostratified epithelium with goblet cells.

• Submucosa: Connective tissue with seromucous glands; supported by 16–20 C-shaped cartilage rings.

• Adventitia: Outermost connective tissue layer.

• Trachealis: Smooth muscle connecting posterior cartilage rings; contracts during coughing.

• Carina: Last tracheal cartilage; highly sensitive, triggers coughing if contacted by foreign objects.

Clinical Note: Smoking and Tracheal Obstruction

• Smoking: Destroys cilia; coughing becomes the only way to clear mucus.

• Tracheal obstruction: Life-threatening; Heimlich maneuver can expel obstructions.

The Bronchi and Subdivisions

The bronchial tree undergoes 23 orders of branching, transitioning from conducting to respiratory zones.

• Conducting zone: Trachea divides into right and left main bronchi, then into lobar (secondary) and segmental (tertiary) bronchi, bronchioles, and terminal bronchioles.

• Respiratory zone: Terminal bronchioles lead to respiratory bronchioles, alveolar ducts, and alveolar sacs (clusters of alveoli).

• Alveoli: Sites of gas exchange; ~300 million per lung.

• Respiratory membrane: Thin barrier for gas exchange; consists of alveolar and capillary walls and fused basement membranes.

• Type I alveolar cells: Squamous epithelium.

• Type II alveolar cells: Secrete surfactant and antimicrobial proteins.

• Alveolar pores: Connect adjacent alveoli; equalize air pressure.

• Alveolar macrophages: Keep surfaces sterile.

Lungs and Pleurae

Gross Anatomy of the Lungs

• Lungs: Occupy thoracic cavity except mediastinum; apex (superior tip), base (rests on diaphragm).

• Hilum: Entry/exit for blood vessels, bronchi, lymphatics, nerves.

• Left lung: Two lobes (superior, inferior); cardiac notch for heart.

• Right lung: Three lobes (superior, middle, inferior); separated by horizontal and oblique fissures.

• Bronchopulmonary segments: 10 on right, 8–10 on left; each served by its own artery, vein, bronchus.

• Lobules: Smallest subdivisions; hexagonal segments served by bronchioles.

• Stroma: Elastic connective tissue; makes lungs elastic and spongy.

Blood Supply and Innervation

• Pulmonary circulation: Pulmonary arteries deliver deoxygenated blood; pulmonary veins return oxygenated blood.

• Bronchial circulation: Bronchial arteries supply lung tissue (except alveoli); bronchial veins anastomose with pulmonary veins.

• Innervation: Parasympathetic (bronchoconstriction), sympathetic (bronchodilation), and visceral sensory fibers.

Pleurae

• Pleurae: Thin, double-layered serosal membrane dividing thoracic cavity.

• Parietal pleura: Lines thoracic wall, diaphragm, heart, and between lungs.

• Visceral pleura: Covers external lung surface.

• Pleural fluid: Lubricates and creates surface tension for lung expansion and recoil.

Clinical Note: Pleurisy and Pleural Effusion

• Pleurisy: Inflammation of pleurae, often from pneumonia; causes pain and friction.

• Pleural effusion: Accumulation of fluid (blood, filtrate) in pleural cavity; hinders breathing.

Summary Table: Upper vs. Lower Respiratory System

Key Terms: Pulmonary ventilation, external respiration, internal respiration, bronchial tree, alveoli, pleurae, surfactant, mucosa, cilia, glottis, epiglottis, laryngeal prominence, bronchopulmonary segments.

Example: During exercise, increased pulmonary ventilation and gas exchange are required to meet the body's oxygen demands and remove excess CO2.

Additional info: The respiratory system's structure and function are closely integrated with the cardiovascular system for efficient gas transport and exchange. Disorders such as rhinitis, sinusitis, laryngitis, pleurisy, and tracheal obstruction highlight the importance of maintaining respiratory health.