Why does pulmonary oedema give pink frothy sputum?

[scilearner]

Hello everyone,

You usually get pulmonary oedema in conditions such as Left heart failure, where there is increased back pressure in pulmonary vasculature, which in turn increase hydrostatic pressure and then fluid in alveoli giving rise to the condition. Now my question is this fluid in alveoli is a transduate it does not contain blood, however the classic symptom of this condition is pink frothy sputum which indicates blood. Why is this the case. Thanks

[sam1123]

My thinking is it may take on the pink colour because of haemolysis, the change in osmotic pressure in oedema may make red cells lyse (burst) as the osmotic potential changes. Because of the large volume of fluid in relation to the possibly small amount of red cells lysed, it may give it a light pinky-red tinge, as opposed to a deep red when a lot of red blood cells lyse in a relatively small volume of fluid.

 

As the red cells break down into it's constituents, the colour of solutions they are dissolved in change, such as bilirubin which is yellow (a build-up of this this compound it what gives patients with jaundice the yellow appearance of their skin and mucous membranes).

 

A pink colour may be caused by free haemoglobin, which is released from lysed red blood cells.

 

As for the frothy texture, it may be due to abnormal gas exchange in the lungs due to the build-up of fluid. This can happen as your ability to remove CO2 from, and add O2 to, your blood is impaired, due to the accumulation of fluid seen in oedema.

 

I hope that has helped you!