The heart and circulatory system

Every organ in the body needs to be supplied continuously with blood
carries away unwanted carbon dioxide and other waste products. The blood
to function properly. Fresh blood brings oxygen and food to the tissues and
heart is the pump which pumps it around. It is a 4-chambered pump with a
circulates around the body through a closed system of blood vessels. The
into the arteries. The arteries divide into progressively smaller branches to
one-way valve system. Each contraction, or heart-beat ejects blood forward
supply a microscopic network of capillaries, so distributing the blood to
every part of the body. Blood is collected from the capillaries by the veins,
whose branches join to form progressively larger veins delivering circulated
blood back to the heart. Venous blood fills the heart as i relaxes during the
interval (diastole) between each contraction (systole). This total
circulatory system is called the cardiovascular system. It contains about 8
pints of blood continuously re-circulated by the heart, which beats some
100,000 times and pumps some 5,000 gallons of blood per day.
The heart is, in fact, two separate pumps which work synchronously.
The right heart receives dark blood which has circulated around the body
and pumps this to the lungs, to pick up a fresh supply of oxygen and become
bright red again. The left heart receives fresh blood from the lungs and
pumps it into the arterial system which supplies the rest of the body. Each
side of the heart has a thin-walled collecting reservoir (atrium) which helps
fill the thick-walled major pump (ventricle). The heart wall is made of a
special sort of muscle called the myocardium.
Like every other living tissue, the myocardium itself needs to be
continuously supplied with fresh blood, through arteries that are, in this
case, called the coronary arteries.