What primarily causes fluid retention in heart failure?

Fluid retention in heart failure is primarily caused by decreased cardiac output. When the heart's ability to pump blood effectively diminishes, there is a reduction in the amount of blood that reaches the organs and tissues. This decrease triggers compensatory mechanisms in the body to maintain blood pressure and blood flow. One key response is the activation of neurohormonal systems, including the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system.

The activation of these systems leads to increased sodium and water retention by the kidneys, as the body tries to compensate for the perceived low blood volume and maintain adequate circulation. As a result, fluid accumulates in the body, leading to symptoms such as edema and pulmonary congestion. Thus, the fundamental issue in heart failure that leads to fluid retention is the diminished ability of the heart to pump effectively, resulting in decreased cardiac output.