The Alveolus and Alveolar Gas Exchange

Vocabulary flashcards covering the structural anatomy, cellular components, physiological functions, collateral pathways, and clinical pathologies of the alveolus based on the provided respiratory lecture notes.

Alveolus

The functional unit of gas exchange in the lung. Human lungs contain approximately $480\text{ million}$ alveoli, creating a surface area of $70\text{ - }100\text{ m}^2$ with a thin barrier of $0.2\text{ - }0.6\text{ }\text{μm}$.

Type I Pneumocytes

Flat, squamous epithelial cells that cover roughly $93\text{\%}$ of the alveolar surface and provide a thin barrier for gas diffusion.

Type II Pneumocytes

Cuboidal cells located at alveolar corners covering about $7\text{\%}$ of the surface; they produce surfactant to lower surface tension and act as stem cells to regenerate Type I cells after injury.

Alveolar Macrophages (Dust Cells)

Immune defense cells that roam inside alveoli to ingest debris, dust, and bacteria.

Interstitial Space (Alveolar–Capillary Interstitium)

The connective tissue layer located between the alveolar epithelium and capillary endothelium that structurally supports alveoli and maintains gas exchange.

Elastic Fibers

Interstitial connective tissue fibers that stretch during inspiration and recoil during exhalation to maintain alveolar recoil.

Reticular Fibers

Fine collagen fibers in the interstitium forming a scaffolding that prevents alveolar over-distension during deep breaths.

Air–Blood Barrier

The structure where $O_2$ enters blood and $CO_2$ exits, consisting of Type I pneumocytes, fused basement membrane, and capillary endothelium.

Basement Membrane

A fused membrane shared by the alveolar epithelium and capillary endothelium that acts as a structural support and selective filter, allowing gases to cross freely while blocking fluid and proteins.

Hydrostatic Pressure

The force exerted by fluid due to its weight or volume that pushes outward inside pulmonary capillaries.

Acinus

The smallest functional gas exchange unit in the lung, located distal to the terminal bronchiole.

Alveolar Ducts

Corridors lined almost entirely with alveoli and surrounded by elastic and smooth muscle fibers that funnel air evenly into alveoli.

Pores of Kohn

Collateral ventilation pathways connecting an alveolus directly to an adjacent alveolus (Alveolus ↔ Alveolus), allowing pressure equalization but providing a path for infection to spread.

Canals of Lambert

Collateral ventilation pathways connecting a bronchiole directly to an alveolus (Bronchiole ↔ Alveolus), providing alternate airflow in COPD.

Channels of Martin

Collateral ventilation pathways connecting one bronchiole directly to another bronchiole (Bronchiole ↔ Bronchiole) to allow cross-ventilation.

Surfactant

A fluid secreted by Type II pneumocytes that lowers alveolar surface tension and prevents alveolar collapse, particularly during exhalation.

Alveolar Interdependence

A structural dynamic where adjacent alveoli are tethered together, helping them collectively resist collapse.

Emphysema

An alveolar disease characterized by the destruction of alveolar walls and elastic fibers, resulting in loss of elastic recoil, air trapping, hyperinflation, and bullae formation.

Pulmonary Edema

A condition in which elevated hydrostatic pressure (such as in CHF) pushes fluid across the intact barrier into alveoli, often presenting with crackles and pink frothy sputum.

Acute Respiratory Distress Syndrome (ARDS)

A severe pathology involving breakdown of the air–blood barrier, leakage of protein-rich fluid into alveoli, and surfactant inactivation, causing widespread alveolar collapse.

Pneumonia

An infectious alveolar disease in which alveoli fill with pus and inflammatory fluid, spreading to adjacent alveoli via the Pores of Kohn and causing lobar consolidation.