Anatomy & Physiology I: The Respiratory System – Structure and Function

The Respiratory System

Introduction

The respiratory system is essential for supplying oxygen (O2) for cellular respiration and removing carbon dioxide (CO2). It works closely with the circulatory system to ensure efficient gas exchange and transport throughout the body.

• Pulmonary ventilation: Movement of air into and out of the lungs.

• External respiration: Exchange of gases between blood and alveolar air.

• Transport of respiratory gases: Blood transports gases between lungs and tissues.

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

Functional Anatomy: Upper Respiratory System

1. Nose

The nose serves as the primary entryway for air and plays several roles in conditioning inspired air.

• Functions:

  • Airway for respiration

  • Moistens, warms, filters, and cleans inspired air

  • Resonating chamber for speech

  • Olfactory (smell) receptors

• External nose: Structure varies due to differences in nasal cartilages; skin contains many sebaceous glands.

• Nasal cavity: Divided by midline nasal septum (cartilage and bone); roof formed by ethmoid and sphenoid bones; floor by palate (anterior hard, posterior soft).

• Air enters via external nares (nostrils), passes to internal nares (choanae) into nasopharynx.

• Mucosa: Two types:

  • Olfactory mucosa: Contains smell receptors.

  • Respiratory mucosa: Pseudostratified ciliated columnar epithelium with goblet cells; lamina propria has seromucous glands producing sticky mucus containing lysozyme.

• Ciliated cells: Sweep contaminated mucus toward throat.

• Thin-walled veins: Under nasal epithelium, can lead to nosebleeds.

• Nasal conchae: Three pairs; increase surface area and air turbulence, aiding in warming and filtering air.

• Sensory nerve endings: Trigger sneeze reflex to expel irritants.

2. Paranasal Sinuses

Paranasal sinuses surround the nasal cavity and contribute to respiratory function and cranial structure.

• Located in frontal, sphenoid, ethmoid, and maxillary bones.

• Functions: Lighten skull, produce mucus, warm and moisten air.

• Sinus mucosa is continuous with nasal mucosa, allowing spread of infections.

• Sinus headache: Blocked sinus passageways cause air absorption and partial vacuum.

3. Pharynx

The pharynx is a muscular tube serving as a common pathway for food and air, divided into three regions:

• Nasopharynx: Air passage below sphenoid bone, above soft palate; lined with pseudostratified columnar epithelium; contains pharyngeal tonsils and pharyngotympanic tubes.

• Oropharynx: Connects oral cavity to pharynx; extends from soft palate to epiglottis; lined with stratified squamous epithelium; contains palatine and lingual tonsils.

• Laryngopharynx: From epiglottis to larynx; both food and air pass; lined with stratified squamous epithelium.

Functional Anatomy: Lower Respiratory System

1. Larynx

The larynx is a cartilaginous structure connecting the pharynx to the trachea, involved in airway protection and voice production.

• Located at 4th-6th cervical vertebrae; attached to hyoid bone.

• Functions:

  • Open, two-directional airway

  • Switching: food versus air

  • Voice production

• Framework: Nine hyaline cartilages (except epiglottis, which is elastic cartilage).

• Thyroid cartilage: Large, shield-shaped; laryngeal prominence (Adam's apple).

• Cricoid cartilage: Ring-shaped.

• Arytenoid cartilages: Paired, pyramid-shaped; anchor vocal cords.

• Epiglottis: Anchored to thyroid cartilage; closes respiratory tract during swallowing.

2. Trachea

The trachea is a flexible, mobile tube conducting air to the bronchi.

• Length: 10–12 cm; diameter: ~2.5 cm.

• Wall layers:

  • Mucosa: Ciliated pseudostratified columnar epithelium with goblet cells.

  • Submucosa: Connective tissue with seromucous glands.

  • Adventitia: Outermost connective tissue; reinforced by 16–20 C-shaped cartilage rings.

• Smoking inhibits and destroys cilia, leading to mucus accumulation.

3. Conducting Zone Structures

The conducting zone includes all airways that transport air to the respiratory zone.

• Trachea divides into right and left main bronchi.

• Bronchial tree undergoes 23 orders of branching: bronchi → bronchioles → terminal bronchioles.

• Changes in wall composition:

  • Cartilage: Rings to plates to none.

  • Epithelium: Pseudostratified ciliated columnar to cuboidal; no cilia or mucus cells in bronchioles.

  • Smooth muscle: Increases as passages become smaller.

4. Respiratory Zone Structures

The respiratory zone is the site of gas exchange and includes the smallest airways and alveoli.

• Terminal bronchioles branch into respiratory bronchioles, leading to alveolar ducts and alveolar sacs.

• Alveolar sacs contain clusters of alveoli (~300 million per lung), providing a large surface area for gas exchange.

• Respiratory membrane: Thin barrier (~0.5 μm) of alveolar and capillary walls with fused basement membranes; allows gas exchange by simple diffusion.

• Alveolar walls: Single layer of squamous epithelium (type I alveolar cells); scattered type II alveolar cells secrete surfactant and antimicrobial proteins.

• Alveolar pores: Connect adjacent alveoli, equalize air pressure, and provide alternate routes in case of blockages.

• Alveolar macrophages: Keep alveolar surfaces sterile by phagocytosis.

The Lungs and Pleurae

The lungs are paired organs occupying the thoracic cavity, each in its own pleural cavity.

• Gross anatomy: Apex (top), base (bottom), hilum (entry/exit for vessels and bronchi), cardiac notch (left lung), fissures (oblique and horizontal), lobes (right: 3, left: 2).

• Bronchopulmonary segments: Each lung subdivided into ~10 segments, each with its own artery and vein.

• Lobule: Smallest subdivision visible to the naked eye.

Blood Supply & Innervation

• Pulmonary arteries: Bring deoxygenated blood to lungs.

• Pulmonary veins: Return oxygenated blood to heart.

• Bronchial arteries: Provide systemic blood to lung tissue; bronchial veins drain it.

• Innervation: Both parasympathetic and sympathetic nervous systems.

The Pleurae

The pleurae are thin, double-layered serous membranes dividing the thoracic cavity.

• Parietal pleura: Lines thoracic wall and superior diaphragm.

• Visceral pleura: Covers external lung surface.

• Pleural fluid: Fills pleural cavity, allowing lungs to glide smoothly during breathing; surface tension keeps pleurae together.

• Pleurisy: Inflammation of pleural membranes.

Respiratory Physiology

Pressure Relationships in the Thoracic Cavity

Breathing (pulmonary ventilation) involves inspiration and expiration, driven by pressure changes in the thoracic cavity.

• Atmospheric pressure (Patm): Pressure exerted by air around the body (760 mm Hg at sea level).

• Respiratory pressures: Described relative to atmospheric pressure.

  • Negative respiratory pressure: Less than Patm (e.g., -4 mm Hg: 760 - 4 = 756 mm Hg).

  • Intrapulmonary pressure (Ppul): Pressure within alveoli; rises and falls with breathing, eventually equalizing with atmospheric pressure.

Key Equations

• Transpulmonary pressure: Difference between intrapulmonary and intrapleural pressures. $P_{tp} = P_{pul} - P_{ip}$

Summary Table: Respiratory Pressures

Pressure Type	Definition	Typical Value
Atmospheric Pressure (Patm)	Pressure exerted by air around body	760 mm Hg (sea level)
Intrapulmonary Pressure (Ppul)	Pressure within alveoli	Rises and falls, equalizes with Patm
Intrapleural Pressure (Pip)	Pressure within pleural cavity	~4 mm Hg less than Ppul
Transpulmonary Pressure (Ptp)	Ppul - Pip	Maintains lung inflation

Example: During inspiration, the diaphragm contracts, increasing thoracic volume and decreasing intrapulmonary pressure, causing air to flow into the lungs.

Additional info: These notes cover the essential anatomical and physiological features of the respiratory system, suitable for college-level Anatomy & Physiology students. For further study, review the mechanisms of gas exchange, respiratory volumes, and control of breathing.