Alveolar Sacs

1. Overview

Alveolar sacs are the terminal structures of the respiratory tree where the majority of gas exchange occurs. Each sac is a cluster of alveoli—tiny, balloon-like air spaces—that open into a common chamber. These sacs represent the endpoint of the respiratory zone, which includes respiratory bronchioles and alveolar ducts. Alveolar sacs provide a vast surface area and a thin blood-air barrier critical for efficient oxygen and carbon dioxide exchange.

2. Location

Alveolar sacs are located in the distal portion of the lungs, beyond the alveolar ducts. Specifically:

They are found at the termini of alveolar ducts, deep within the pulmonary lobules.
Each sac consists of a group of alveoli that share a common opening into the duct lumen.
Distributed throughout the lung parenchyma, they cluster around the lung acini, which are the smallest functional respiratory units.

3. Structure

An alveolar sac is made up of multiple alveoli opening into a single, central air space. Structural features include:

Alveoli: Roughly spherical pouches with thin walls.
Epithelium: Composed of two main cell types:
- Type I pneumocytes: Thin squamous epithelial cells forming most of the alveolar surface and facilitating gas exchange.
- Type II pneumocytes: Cuboidal cells that secrete pulmonary surfactant and can differentiate into type I cells for repair.
Alveolar pores (of Kohn): Small openings between adjacent alveoli allowing for collateral ventilation and pressure equalization.
Capillary network: Dense mesh of pulmonary capillaries surrounding each alveolus, separated only by the blood-air barrier (epithelium, basement membrane, and endothelium).

4. Function

The primary function of alveolar sacs is:

Gas exchange: Oxygen diffuses from inhaled air into pulmonary capillary blood, while carbon dioxide diffuses from blood into alveoli to be exhaled.

Secondary functions include:

Surfactant production (by type II cells): Reduces surface tension and prevents alveolar collapse during expiration.
Immunologic defense: Alveolar macrophages patrol the sacs, engulfing inhaled pathogens and particles.

5. Physiological Role(s)

Alveolar sacs play a crucial role in maintaining respiratory efficiency and systemic oxygenation:

High surface area: The lung contains approximately 300 million alveoli, collectively providing a gas exchange area of ~70 m².
Thin diffusion distance: The blood-air barrier is <1 micron thick, enabling rapid diffusion of gases.
Ventilation-perfusion matching: Proper function ensures efficient oxygen delivery relative to blood flow.
Compliance and elasticity: Alveolar sacs expand and recoil with each breath, driven by lung compliance and aided by surfactant.

6. Clinical Significance

Emphysema

A type of chronic obstructive pulmonary disease (COPD) marked by destruction of alveolar walls, leading to:

Decreased surface area for gas exchange
Air trapping and hyperinflation
Loss of elastic recoil and dyspnea

Often caused by smoking or α1-antitrypsin deficiency.

Acute Respiratory Distress Syndrome (ARDS)

In ARDS, alveolar sacs become filled with proteinaceous fluid due to increased capillary permeability, leading to:

Impaired oxygenation
Loss of surfactant
Reduced lung compliance

Treatment requires mechanical ventilation and addressing the underlying cause.

Neonatal Respiratory Distress Syndrome (NRDS)

In preterm infants, insufficient surfactant leads to collapse of alveoli, making breathing difficult. Managed with:

Surfactant replacement therapy
Oxygen support and CPAP

Pulmonary Fibrosis

Fibrotic thickening of the alveolar interstitium impairs diffusion across the blood-air barrier. Causes include:

Idiopathic pulmonary fibrosis
Chronic exposure to dusts or toxins

Results in progressive dyspnea and hypoxia.

Pneumonia

Infection leads to consolidation of alveolar sacs with inflammatory exudate. Common pathogens include Streptococcus pneumoniae, viruses, and fungi. Symptoms:

Fever, cough, chest pain
Crackles on auscultation
Opacity on chest X-ray

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