Can you describe the anatomy of the lungs, including lobes, fissures, bronchopulmonary segments, airway branching, pulmonary vasculature, pleural coverings, and diaphragmatic attachment?

Anatomy of the Lung

The lungs are paired respiratory organs with a hierarchical branching architecture that connects a small tracheal entrance (cross-section 2.5 cm²) to a massive alveolar gas exchange surface of approximately 100 m² in humans through sequential, irregular dichotomous branching 1.

Lobar Structure and Fissures

The right lung contains three lobes (upper, middle, and lower) divided by the oblique and horizontal fissures, while the left lung has two lobes (upper and lower) separated by the oblique fissure alone 2. However, anatomical variations are common in clinical practice:

• Incomplete fissures occur frequently, with studies showing incomplete oblique fissures in approximately 28-33% of right lungs and 32.5% of left lungs 3, 4
• Absent fissures are documented in 7.5-11.4% of specimens 3, 4
• Accessory fissures appear in 11-14% of right lungs and up to 37.5% of left lungs 5, 3, 4

These variations are clinically significant for surgical planning and radiological interpretation, as they can affect lobectomy procedures and be mistaken for pathology on imaging 6, 5.

Bronchopulmonary Segments

The lungs contain 19 functional bronchopulmonary segments total: 10 in the right lung (3 upper, 2 middle, 5 lower) and 9 in the left lung (5 upper, 4 lower) 2. This segmental anatomy is critical for:

• Calculating predicted postoperative lung function using the formula: PPO FEV₁ = preoperative FEV₁ × (1 - y/z), where y = segments removed and z = total functional segments 2
• Planning segmental resections as alternatives to lobectomy
• Understanding regional disease distribution

Airway Branching Architecture

The airways follow a dichotomous branching pattern where the number of branches doubles with each generation (2^Z branches at generation Z, with trachea = generation 0) 1. Two functionally distinct regions exist:

Conducting Airways

• Multilayered wall structure with mucous membrane, smooth muscle, and cartilage
• Extend from trachea through approximately 16 generations
• Follow the form of the lobes they serve, with "monopodial" branching in elongated lobes 1

Acinar Airways

• Intimately associated with gas-exchanging alveoli
• Form a sleeve of alveoli over approximately 8 generations of the most distal airways 1
• Comprise the gas exchange apparatus

Pulmonary Vasculature

Pulmonary Arteries

Pulmonary arteries follow airways in a similar branching pattern but branch over approximately 5 more generations than airways before reaching capillaries 1. Key features include:

• Small "supernumerary" branches arise at nearly all levels to perfuse nearby parenchyma 1
• Order numbers of arteries and airways are not congruent due to these supernumerary branches 1

Pulmonary Veins

Pulmonary veins course independently of airways in an intermediate position related to interlobular septa, converging on the left atrium in four main stems 1. Important variations include:

• Single unilateral pulmonary vein documented in 5.7% of right lungs and 35% of left lungs 4
• Accessory veins present in 20% of right lungs and 5% of left lungs 4
• Overall venous drainage variations observed in 52% of specimens 4

These venous variations are critical for cardiothoracic surgical planning and must be identified preoperatively 4.

Pleural Coverings and Parenchymal Structure

The lung parenchyma comprises:

• Numerous air-containing passages (alveolar ductal lumens and alveoli)
• Alveolar septa and small pulmonary vessels occupying approximately 10% of total parenchymal volume 7
• Visceral pleura covering the lung surface, separated from parietal pleura by the pleural cavity

Clinical Pitfalls

Common errors in anatomical assessment include:

1. Misinterpreting incomplete or accessory fissures as pathology on imaging - these are normal variants present in up to one-third of patients 5, 3, 4

2. Assuming standard pulmonary venous anatomy - with 52% variation rate, preoperative imaging must specifically identify venous drainage patterns before thoracic surgery 4

3. Using standard segment counts without accounting for obstructed segments - when calculating PPO lung function, only functional/unobstructed segments should be counted 2

4. Failing to recognize that airway obstruction from endobronchial lesions may produce baseline testing abnormalities that improve postoperatively - this affects surgical risk stratification 2