Underlying heart failure is an impairment of the pumping ability of the heart. The nature of the impairment is determined largely by the side of the heart involved. When the right side of the heart fails, blood accumulates in the venous circulation, causing organ congestion (liver, gastrointestinal tract, spleen) and peripheral tissue oedema. When the left side fails, blood accumulates in the pulmonary circulation, resulting in pulmonary congestion and fluid in the lungs. The ejection of blood from the left ventricle is impaired, so that cardiac output is diminished greatly, often to levels lower than the venous return to the right atrium. Indeed, a low cardiac output reduces blood pressure and decreases blood flow to all the major organs – the brain, the kidneys and the heart itself. As the blood flow to the kidneys decreases, urine production dwindles, culminating in fluid retention and more peripheral oedema.