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The heart pumps deoxygenated blood from the venous circulation into the lungs, where it is oxygenated. Newly oxygenated blood travels via the pulmonary veins to the left atrium and left ventricle, where it is ejected via the aorta into the arterial circulation to supply oxygenated blood to peripheral tissue. Heart failure arises when structural or functional abnormalities prevent the heart adequately filling with or ejecting blood, resulting in the inability to meet metabolic needs of peripheral tissue. The cardiovascular system has a large reserve capacity, so overt clinical signs are only seen with severe disease when the heart cannot compensate for the decreased function.  

The heart pumps deoxygenated blood from the venous circulation into the lungs, where it is oxygenated. Newly oxygenated blood travels via the pulmonary veins to the left atrium and left ventricle, where it is ejected via the aorta into the arterial circulation to supply oxygenated blood to peripheral tissue. Heart failure arises when structural or functional abnormalities prevent the heart adequately filling with or ejecting blood, resulting in the inability to meet metabolic needs of peripheral tissue. The cardiovascular system has a large reserve capacity, so overt clinical signs are only seen with severe disease when the heart cannot compensate for the decreased function.  

Factors affecting cardiac output include:

The definition of heart failure is: a complex syndrome initiated by an inability of the heart to maintain a normal cardiac output at a normal filling pressure.

'''Preload.''' Preload is the end-diastolic volume and when this increases so does the systolic function of the myocardium based on the '''Frank-Starling Law.'''

Heart failure can be further classified according to the cause, whether it leads predominantly to underperfusion or congestion (forward or backward failure) and whether the right or left side of the circulation is affected to a greater extent (right-sided failure or left-sided failure). In some cases, biventricular failure may occur.  

'''Afterload:''' Refers to the resistance the left ventricle encounters as it ejects blood into the peripheral circulation. Depends upon many variables; for example ventricular volume, arterial tone etc. In the failing heart, in order to maintain blood pressure with reduced cardiac output the peripheral resistance and so the afterload has to increase contractility, heart rate, distensibility and synergy of contraction.

* '''Forward failure''' (cardiogenic shock): underperfusion of the arterial circulation at normal pressure

* '''Backward failure''' (congestive heart failure): adequate output at abnormal pressures, too much fluid in the venous circulation

Heart failure can affect the right ventricle, the left ventricle, or both ventricles (biventricular). Clinical signs are characteristic and can help determine which ventricle is affected.

Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV)

Cardiac Output (CO) = Venous Return (VR)

== Mechanisms of failure  ==

== Mechanisms of failure  ==