**4.1 Chronic bronchitis**

Chronic bronchitis is a clinical entity defined by a chronic productive cough for three months in each of two successive years in a patient in whom other causes of chronic cough have been excluded. It is characterised by an overlapping pathologic process of bronchial wall inflammation. The submucosal glands show dilated ducts and hypertrophy and hyperplasia. Reid index (the ratio of glandular to bronchial wall thickness) as well as goblet cell frequency and airway smooth muscle thickness is increased in chronic bronchitis(Reid,1960). The airway wall contains inflammatory cells predominated by macrophages and CD8+T lymphocytes. Bronchus-associated lymphoid tissue(BALT) is also present in late GOLD stages(Hogg & Timens,2009 as cited in Shapiro SD,2010). Increased numbers of neutrophils are found in the airway lumen and in the glands during episodes of exacerbations(Saetta et al.,1997;Thompson et al.,1989 as cited in Shapiro SD,2010).

#### **4.2 Emphysema**

Pulmonary emphysema, a pathological entity defined as destruction and enlargement of air spaces distal to the terminal bronchiole involving respiratory bronchioles, alveolar ducts, and alveoli. Cigarette smoking, inhaled irritants, recurrent infections and proteinaseantiproteinase imbalance lead to inflammatory cell recruitment, proteolytic injury to the extracellular matrix (ECM), and cell death. Alveolar walls become perforated and later due to incomplete and disorderly repair, become obliterated with coalescence of small distinct air spaces into abnormal and much larger air spaces, which is the pathological hallmark of emphysema (Shapiro & Ingenito,2005 as cited in Shapiro SD,2010). Emphysema has been classically described with absence of interstitial fibrosis to differentiate from restrictive lung diseases. However, scarring of the small airway subepithelial space and collagen accumulation around larger disrupted air spaces has been noted in emphysema.

Various subtypes of emphysema have been described based on location and distribution of the lesions in the acinus (Pipavath et al.,2009 as cited in Shapiro SD,2010). In most patients, however, the process within the lung will be heterogeneous and in advanced stages, distinction becomes blurred. The following three patterns of emphysema are noted:
