Heart failure is a mode of death and not, in itself, a natural disease. As such, it must be followed on a Death Certificate by what disease caused it. It merely means that the heart can no longer fulfill its function as well as it should. If it causes death, the failure is obviously profound.
But what does ‘failure’ mean? It doesn’t usually mean the heart stops, although technically it could. It usually means that its pumping action is no longer efficient and the heart’s output is reduced. People can live with cardiac failure for a long time.
Since the heart is in fact 2 pumps, heart failure can affect just the right side, or just the left side, or most commonly both. Because the ventricle(s) is/are no long pushing out the blood as well as it/they should, it/they dilate(s) (because blood is left in them when the heart beats) and there is a rise in upstream pressure. This is why the lungs fill with water in left or biventricular failure (the atria don’t contribute a great deal to the act of pushing blood out of the heart), because the pressure in the veins of the lungs increases. The blood coming out of the lungs is slightly thicker because water has been pushed out of the blood vessel walls.
This is also why ankles swell in biventricular or right side heart failure. it also affects internal organs such as the liver and can be so severe that the liver scars (‘cardiac cirrhosis’) and may even fail itself.
The water causes breathlessness (aka DYSPNOEA). It’s worse if the patient lies flat, which is why people with cardiac failure sleep with multiple pillows propping them up.
And if the heart is not pushing out blood as it should, arteries constrict to keep up blood pressure, and the heart enlarges because blood is not being ejected but it is still coming in from the veins. The kidneys need blood pressure because they are essentially just high pressure filters; they have their own mechanisms for making sure that the pressure of blood remains reasonable.
But why? Why does it fail?
It might be for one or more of a variety of reasons. Intrinsic problems with the heart muscle (reduced blood supply causing death of myocytes and fibrosis of the heart muscle due to coronary atheroma), myocarditis (where the heart muscle is inflamed, perhaps due to an infection), cardiomyopathy (in which the heart muscle is not normal), etc; or extrinisic problems such as hypertension. If the heart has to push blood out against raised blood pressure, it has to do more work, and eventually that tells.