Changes

*Micro pathology:  

*Micro pathology:  

**Early cases centred upon the bronchiolar alveolar junction with exudation, which extends into neighbouring alveoli

**Early cases centred upon the bronchiolar alveolar junction with exudation, which extends into neighbouring alveoli

**Exudate contains many [[Neutrophils - WikiBlood|neutrophils]], macrophages and oedema

**Exudate contains many [[[[Neutrophils - WikiBlood|Neutrophils]] - WikiBlood|neutrophils]], macrophages and oedema

**Alveolar capillaries are hyperaemic

**Alveolar capillaries are hyperaemic

**Some haemorrhages in severe cases - later stages contain more cells than fluid

**Some haemorrhages in severe cases - later stages contain more cells than fluid

***Type I alveolar epithelial cells are highly sensitive to injury and cannot proliferate in response to injury

***Type I alveolar epithelial cells are highly sensitive to injury and cannot proliferate in response to injury

***Necrosis and sloughing of injured type I cells, denuding alveolar spaces of lining epithelium

***Necrosis and sloughing of injured type I cells, denuding alveolar spaces of lining epithelium

***Neutrophils begin to enter alveolar spaces distended with inflammatory oedema

***[[Neutrophils - WikiBlood|Neutrophils]] begin to enter alveolar spaces distended with inflammatory oedema

**'''Proliferative phase'''

**'''Proliferative phase'''

***Type II alveolar cells (less sensitive to and can proliferate in response to injury) begin to proliferate within 24 hours and eventually line the alveolar walls denuded of type I cells ***By 6 days cuboidal type II cells can completely line the alveoli

***Type II alveolar cells (less sensitive to and can proliferate in response to injury) begin to proliferate within 24 hours and eventually line the alveolar walls denuded of type I cells ***By 6 days cuboidal type II cells can completely line the alveoli