If you have heart failure, your heart has difficulty pumping blood around your body. This can happen for many reasons - commonly because you have (or had) a medical condition which has damaged (e.g. coronary heart disease with a heart attack) or placed extra workload (e.g. high blood pressure) on your heart.
This damage or extra workload can affect the squeezing (contraction) or filling (relaxation) of your heart, and frequently both.
If your heart does not contract as it should, it will not be able to pump sufficient blood in your ventricles out of your heart. If your heart is not able to empty and relax fully, less blood will be able to enter your heart, and therefore less blood is available to pump out of your heart.
Heart failure has two main effects. Firstly, not enough blood is getting to your body, which may cause fatigue. Secondly, blood backs up waiting to enter your heart. This build up of blood causes fluid to leak out of your blood vessels and into the surrounding tissues. This leads to fluid accumulation (usually in your legs and abdomen) and congestion in your lungs.
At first your body will adapt to try and compensate for your weakened heart's poor pumping. However, these adaptations can only compensate for a limited time, and in fact, in the long-term these adaptations can actually make your heart even weaker.
What goes wrong in heart failure