I'm a second year medical student and I've been reading about CHF. I've been trying to answer one simple question, and I've been having a lot of trouble getting a straight answer. My question is this: What causes the backup of fluid at the backdoor to the left heart? I understand that ventricular output is compromised in CHF. I also understand that compensatory mechanisms increase blood volume. However, it seems to me that a reduced cardiac output would result in a lower volume of blood returning to the heart. This would seem to prevent overloading of the left heart in itself, especially when the patient was standing. What's wrong with my reasoning here? Thanks.