PULMONARY EMPHYSEMA.
This usually develops as a result of long- term inflammatory conditions of irritation of the airways, eg in smokers or coal miners. Occasionally,it may be due to a genetic deficiency in the lung of an antiproteolytoc enzyme,anti-trypsin. These conditions lead to progressive destruction of supporting elastic tissue in the lung,and the lungs progressively expand. There is irreversible distension of the respiratory bronchioles,alveolar ducts and alveoli,reducing the surface area for the exchange of gases.There are two main types and both are usually present:
A) Panacinar emphysema.
The walls between adjacent alveoli break down,the alveolar ducts dilate and interstitial elastic tissue is lost. The lungs become distended and their capacity is increased. Because the volume of air in each breath remains unchanged,it constitutes a smaller proportion of the total volume of air in the distended alveoli, reducing the partial pressure of oxygen.
B) Centrilobular emphysema.
In this form there is irreversible dilation of the respiratory bronchioles in the centre of lobules. When inspired air reaches the dilated area the pressure falls,leading to a reduction in alveolar air pressure,reduced ventilation efficiency and reduced partial pressure of oxygen. As the disease progresses the resultant hypoxia leads to pulmonary hypertension and right-sided heart failure.
