heart, 

 organ that serves as a pump to circulate the blood. It may be a straight tube, as

in spiders and annelid worms, or a somewhat more elaborate structure with one or more
receiving chambers (atria) and a main pumping chamber (ventricle), as in mollusks.
In fishes the heart is a folded tube, with three or four enlarged areas that correspond to
the chambers in the mammalian heart. In animals with lungs—amphibians, reptiles,
birds, and mammals—the heart shows various stages of evolution from a single to a
double pump that circulates blood (1) to the lungs and (2) to the body as a whole.

In humans and other mammals and in birds, the heart is a four-chambered double

pump that is the centre of the circulatory system. In humans it is situated between the
two lungs and slightly to the left of centre, behind the breastbone; it rests on
the diaphragm, the muscular partition between the chest and the abdominal cavity.

The heart consists of several layers of a tough muscular wall, the myocardium.

A thin layer of tissue, the pericardium, covers the outside, and another layer,
the endocardium, lines the inside. The heart cavity is divided down the middle
into a right and a left heart, which in turn are subdivided into two chambers.
The upper chamber is called an atrium(or auricle), and the lower chamber is
called a ventricle. The two atria act as receiving chambers for blood entering
the heart; the more muscular ventricles pump the blood out of the heart.
The heart, although a single organ, can be considered as two pumps
that propel blood through two different circuits. The right atrium receives
venous blood from the head, chest, and arms via the large vein called the
superior vena cava and receives blood from the abdomen, pelvic region, and
legs via the inferior vena cava. Blood then passes through the
tricuspid valve to the right ventricle, which propels it through the
pulmonary artery to the lungs. In the lungs venous blood comes in contact
with inhaled air, picks up oxygen, and loses carbon dioxide. Oxygenated blood
is returned to the left atrium through the pulmonary veins. Valves in the heart
allow blood to flow in one direction only and help maintain the pressure
required to pump the blood.

(Source: Britannica)