Functions and Structure of the Respiratory System

Oxygen (O₂) intake & CO₂ removal

Main function of the respiratory system.

Speech & vocalization

One of the functions of the respiratory system.

Sense of smell

Function of the respiratory system related to olfaction.

Acid-base balance

Regulation of pH in the body by the respiratory system.

Helps circulation

Function of the respiratory system via pressure changes & venous return.

Upper Respiratory Tract (URT)

Includes the nose, nasal cavity, pharynx, and larynx.

Lower Respiratory Tract (LRT)

Includes the trachea, bronchi, bronchioles, and lungs.

Main function of the upper respiratory tract

Filters, humidifies, and warms incoming air.

RIGHT bronchus

More likely to receive an inhaled foreign object due to being wider and more vertical.

Conducting Zone

Moves air without gas exchange.

Respiratory Zone

Area where gas exchange happens.

Conducting zone structures

Includes the nose to terminal bronchioles (trachea, bronchi, bronchioles).

Anatomic dead space

Conducting zone is called this because no gas exchange happens here—just airflow!

Epithelium lining the conducting zone

Pseudostratified ciliated epithelium (mucociliary escalator).

Respiratory zone structures

Includes respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Primary function of the respiratory zone

Gas exchange (O₂ in, CO₂ out) through alveoli.

Type I pneumocytes

Alveolar cell type responsible for gas exchange.

Type II pneumocytes

Alveolar cell type that produces surfactant.

Inhalation process

Diaphragm contracts & moves DOWN → thoracic volume ↑ → air in.

Exhalation process

Diaphragm relaxes & moves UP → thoracic volume ↓ → air out.

Boyle's Law

Explains why air moves into the lungs during inhalation (↑ volume = ↓ pressure).

Muscles for forced exhalation

Internal intercostals & abdominal muscles.

Surfactant

A substance made by Type II alveolar cells that reduces surface tension.

Need for surfactant

Prevents alveoli from collapsing by reducing surface tension.

Laplace's Law

Explains alveolar collapse: P = 2T/r (smaller alveoli collapse easier without surfactant).

Neonatal Respiratory Distress Syndrome (NRDS)

Condition in premature babies who lack surfactant.

Lung elasticity

How well lungs return to their original shape after stretching.

Lung compliance

How easily the lungs stretch when filled with air.

Emphysema

Disease causing high compliance but low elasticity (lungs become too floppy).

Pulmonary fibrosis

Disease causing low compliance (stiff lungs).

Residual Volume (RV)

Lung volume that CANNOT be measured by spirometry.

Tidal Volume (TV)

Amount of air breathed in or out normally (~500mL).

Total Lung Capacity (TLC)

Max air lungs can hold (VC + RV).

Obstructive lung disease

Trouble exhaling due to narrowed airways.

Examples of obstructive lung diseases

Asthma, COPD (Emphysema + Chronic Bronchitis).

Lung volumes in obstructive lung disease

RV ↑ (air trapping), FEV1 ↓↓↓, TLC ↑.

Restrictive lung disease

Lungs are stiff & can't expand properly.

Examples of restrictive lung diseases

Pulmonary fibrosis, scoliosis, TB.

Lung volumes in restrictive lung disease

TLC ↓, RV ↓, FEV1/FVC ↑ or normal.

Obstructive lung disease flow-volume loop

Scooped-out shape (airflow slows down when exhaling).

Restrictive lung disease flow-volume loop

Narrower loop (small lung volumes, but fast exhalation).

Lung function with age

Lung compliance ↑ (lungs too stretchy, less recoil) and Residual Volume (RV) ↑ (more air trapped in lungs).