Structure and Functional Dynamics of the Respiratory System

Structural and Functional Divisions

The respiratory system is divided structurally into the Upper Respiratory (nose, nasal cavity, mouth, pharynx) and Lower Respiratory (larynx, trachea, bronchi, alveoli).

Functionally, it is divided into the Conducting Zone (nose, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, terminal bronchioles) and the Respiratory Zone (respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli). Gas exchange occurs only in the respiratory zone.

Epithelial Tissue Types

Epithelial tissue is classified by number of cell layers (simple or stratified) and cell shape (cuboidal or columnar).

Simple Squamous — "fried-egg" appearance; found in alveoli and capillary bed
Simple Cuboidal — cube-shaped; found in respiratory bronchioles
Simple Columnar — found in bronchioles
Stratified — multiple layers; found in oral cavity

Pseudostratified Ciliated Columnar Epithelium (PCCE) is the primary airway epithelium, found in the nasopharynx, larynx, trachea, and bronchi. PCCE features cilia (motile and non-motile) and goblet cells that secrete mucus. The Mucociliary Escalator functions as follows: the gel layer traps particles, the sol layer allows cilia movement, and cilia sweep particles toward the mouth (approximately 200 cilia per cell).

Upper Respiratory Anatomy

External Nose — composed of bone and hyaline cartilage; nares = nostrils

Nasal Cavity — lined with PCCE; warms, moistens, and filters air

Conchae — three bony shelves that create corresponding meatuses

Sinuses — maxillary, frontal, ethmoid, sphenoid; reduce skull weight and modify voice quality

Oral Cavity — contains the hard palate, soft palate, and uvula

Pharynx — three divisions:

Nasopharynx (PCCE, contains Eustachian tube opening)
Oropharynx (stratified squamous epithelium, common passage for air and food)
Laryngopharynx (stratified squamous epithelium)

Lower Respiratory Anatomy

Larynx — composed of 9 cartilages:

Thyroid cartilage — forms the Adam's apple
Epiglottis — leaf-shaped, covers the glottis during swallowing
Cricoid cartilage — complete ring, landmark for tracheotomy
Arytenoids — triangular, responsible for vocal cord movement
Corniculate — horn-shaped
Cuneiform — club-shaped

The vallecula is an important intubation landmark. The glottis contains the vocal cords and mediates the cough reflex.

Trachea — anterior to the esophagus, extends from the larynx to T5, approximately 12 cm long, contains 16–20 C-shaped cartilage rings giving it a D-shaped cross-section

Carina — the point of tracheal bifurcation; triggers cough reflex and is an ET tube landmark

Bronchi:

Right primary bronchus — 1, wider, shorter, more vertical
Left primary bronchus — 1
Secondary (lobar): Right 3, Left 2
Tertiary (segmental): Right 10, Left 10

Cartilage transitions from C-shaped rings to plates to none as airways branch. Bronchioles feature ciliated simple columnar epithelium with smooth muscle. Terminal bronchioles are nonciliated and contain Clara cells. Collateral ventilation is provided by Channels of Martin, Lambert channels, and Pores of Kohn.

Thoracic Cavity and Lungs

Ribs: 1–7 are true ribs, 8–12 are false ribs, 11–12 are floating ribs

Mediastinum — the space between the pleural cavities

The left lung has a cardiac notch and is approximately 10% smaller. The right lung is shorter due to the liver. Each lung has an apex, base, and hilum.

Pleural Membrane:

Parietal pleura — lines the thoracic cavity; pain-sensitive
Visceral pleura — surrounds the lung; pain-insensitive

The pleural cavity contains approximately 18 mL of serous fluid. The costophrenic angle is the recess where the diaphragm meets the ribs. Thoracentesis is the procedure of draining fluid from the pleural space.

Fissures: Right lung has an oblique and a horizontal fissure → 3 lobes; Left lung has an oblique fissure → 2 lobes. There are 10 bronchopulmonary segments in each lung.

Respiratory Zone and Pulmonary Blood Flow

Respiratory bronchioles transition from simple cuboidal to simple squamous epithelium. Alveolar ducts are lined with simple squamous epithelium leading to alveolar sacs.

Alveolar Cell Types:

Type I cells (90–95%) — gas exchange
Type II cells (5–10%) — secrete surfactant
Type III / Macrophages — phagocytes

Surfactant reduces surface tension, increases compliance, decreases elastic recoil, and regulates alveolar size. Surfactant deficiency leads to atelectasis.

The respiratory membrane is only 0.5 microns thick with a surface area of 70–100 m².

Pulmonary Circulation: Right heart → pulmonary arteries → lungs → pulmonary veins → left heart. The bronchial arteries branch from the aorta to supply the lung tissue itself. Hypoxic pulmonary vasoconstriction is unique to the pulmonary circulation (opposite of the systemic response).

Pulmonary Ventilation

Air flows down pressure gradients. Inhalation is active: the diaphragm descends approximately 1 cm during quiet breathing (up to 10 cm during strenuous breathing) and is responsible for 75% of air movement. External intercostals contribute the remaining 25%. Accessory muscles are recruited during forced inhalation.

Exhalation is passive during quiet breathing, driven by elastic recoil from fibers and surface tension. Exhalation becomes active during exercise, using abdominal and internal intercostal muscles.

Intrapleural pressure changes from approximately 756 mmHg to 754 mmHg during inhalation. Loss of negative intrapleural pressure results in pneumothorax.

LaPlace's Law (P ∝ T / r): pressure is proportional to wall tension divided by radius.

Compliance: High compliance means the lung expands easily (emphysema); low compliance means the lung is stiff (ARDS, fibrosis, edema, surfactant deficiency). Restrictive disease = decreased compliance, increased elasticity. Obstructive disease = increased compliance, decreased elasticity.

Airway Resistance (Raw): 80% from airways, 20% from tissue. Poiseuille's Law (Raw = 8nL / πr⁴): halving the radius increases resistance by 16 times. Resistance decreases during inhalation and increases during exhalation. Sympathetic stimulation causes bronchodilation. The majority of Raw occurs in the first 5 generations of airways.

Lung Volumes and Capacities

Tidal Volume (Vt) — volume of air per breath

Minute Ventilation = respiratory rate × Vt

Anatomic dead space = approximately 1 mL per pound of ideal body weight (~30% of Vt)

Alveolar Ventilation = Vt − dead space

Spirometer measures lung volumes.

Lung Volumes:

Tidal Volume (Vt)
Inspiratory Reserve Volume (IRV)
Expiratory Reserve Volume (ERV)
Residual Volume (RV) — cannot be measured by spirometry

Lung Capacities (sum of 2+ volumes):

Inspiratory Capacity (IC) = Vt + IRV
Functional Residual Capacity (FRC) = ERV + RV
Vital Capacity (VC) = IRV + Vt + ERV
Total Lung Capacity (TLC) = Vt + IRV + ERV + RV

Measurements are based on age, height, sex, and race.