Left-sided failure floods the lungs
The left heart receives oxygen-rich blood from the lungs and drives it out to the body. When the left ventricle fails — left-sided heart failure — pressure backs up into the lungs. The lung capillaries become engorged, and fluid weeps into the air sacs. This is pulmonary edema, and it is why the leading symptom of left-sided failure is breathlessness.
Two clues point straight to the lungs. Orthopnea is breathlessness on lying flat: lie down and the fluid pooled in the legs by day redistributes into the chest, so patients prop themselves up on extra pillows. Paroxysmal nocturnal dyspnea is the alarming cousin — waking an hour or two into sleep gasping for air, forced to sit up or open a window before the breathlessness eases.
Right-sided failure swells the body
The right heart collects blood returning from the body and sends it to the lungs. When the right ventricle fails — right-sided heart failure — pressure backs up into the body's veins instead. Now the swelling is peripheral: ankles and shins puff up as peripheral edema, the liver becomes congested and tender, and the abdomen may fill with fluid. Press a swollen shin and a dimple lingers — that is pitting edema.
A reliable bedside window onto right-sided pressure is the jugular venous pressure — the height of the blood column in the neck veins. When the right heart cannot keep up, that column rises and becomes visible high in the neck. Together, raised neck veins, leg swelling, and a tender liver are the classic triad of right heart fluid overload.