How does CHF cause pleural effusion?

Leaky lungs. The pulmonary lymphatic system can hold about 4 liters of fluid per lung (8 liters total) before they start to leak. In heart failure, there is back pressure from the left side of the heart into the lungs; if there is enough pressure to result in more than 4 liters backing up on one side, the lungs will leak into the pleural space leading to an effusion.
Complex mechanism. When both ventricles fail (& it does require both), there is hydrostatic back pressure in the pleural and pulmonary vascular beds causing transudation of fluid from the vascular compartment to the interstitial space. Moreover, hormones are secreted such as aldosterone which act on the kidneys to retain fluid and engorge the central circulation: nature's way of attempting to improve cardiac output.

Related Questions

If an elderly person has renal impairment, CHF & hypernatremia (sodium presently 157 mmol/L) would it seem to be appropriate for him to be on a NORMAL SALINE IV fluid? Other conditions are COPD, pleural effusion, T2 diabetes, delirium, mild hepatic dysfun

May be not!!!! Fluid management in a complex patient is a very complex issue and a strait forward yes or no is not the answer. Amount of fluid administered along with medications being given make it further complex. There is no harm bringing up this question with patient's current physicians.
Multiple med problem. I would consider 1/4 N.S. a more appropriate fluid. Sounds multiple system are failing and he has a very high mortality. If one can correct the CHF and CRF then the other factors may correct themselves, but doubt that Hemodialysis is indicated in a 91 y/o.