Cells must be constantly supplied with the materials necessary for life. The blood in humans is the transport system of these materials and this system must be constantly moving materials to and from the cells.

The heart is a muscular pump and the major organ of the circulatory system. It keeps the blood moving in its’ constant motion.

The human heart is about the size of a clenched fist.

The heart lies in the chest cavity, behind the breast bone and slightly to the left.

The heart is a bundle of cardiac muscles specialized for rhythmic contractions and relaxations, also know as the HEARTBEAT.

The rate of the average heartbeat is 72 beats per minute. Over an average lifetime of about 70 years, the heart beats about 2.5 billion times and pumps 200 million liters of blood.

The heart re-circulates the entire volume of blood, about 5 liters, every minute.

The heart is a strong, muscular organ that has two sides and contains four chambers.

Both sides of the heart beat simultaneously.

The walls are thicker on the one side than on the other.

The upper chambers are called the ATRIA/ARTIUM. The atrium are thin-walled and receive the blood from the veins. They are the receiving chambers of the heart.

The lower chambers are called the VENTRICLES. The ventricles are larger and receive the blood from the atrium above it. They are the pumping chambers of the heart.

Each atrium is separated from the ventricles by a VALVE.

The atrium and ventricles on the right are separated from the left atrium and ventricles by a thick wall of muscle called a SEPTUM.

Blood leaves the ventricles and flows through arteries.

Blood flows one way through the heart with the help of valves. ARTERIES CARRY BLOOD AWAY FROM THE HEART, VEINS CARRY BLOOD TO THE HEART.

PATH OF THE BLOOD
The heart is a double pump.

1. PULMONARY CIRCULATION
– The right ventricle represents the 1st pump. It is the pump for the PULMONARY CIRCULATION.
– Blood that is low in oxygen returns from the upper and lower parts of the body and enters the right atrium of the heart and then the right ventricle.
– The right ventricle pumps the DEOXYGENATED blood into the PULMONARY ARTERIES that bring the blood to the lungs where it receives oxygen and gives up waste products, including carbon dioxide, which is exhaled.
– The blood returns from the lungs by way of the PULMONARY VEINS to the left atrium and the left ventricle of the heart. This begins the SYSTEMIC CIRCULATION, which carries blood from the heart to the rest of the body.

2. SYSTEMIC CIRCULATION
– The left side of the heart is the pump for the systemic circulation.
– Oxygenated blood enters the left atrium and then the left ventricle. The left ventricle contracts and pumps the blood through the largest artery in the body, the AORTA, to all parts of the body.
– The oxygenated blood passes through arteries that lead to smaller arteries, them to arterioles, then to capillaries, where oxygen, nutrients, hormones and other substances move from the blood to the cells.
– It is through the capillary walls that waste products, like carbon dioxide are picked up.
– Other substances such as nutrients and hormones enter the blood through the capillary walls.
– Due to the pumping action of the heart, blood is continuously circulating throughout the body, maintain the constant exchange of materials necessary for life to continue.
– The blood eventually returns to the right atrium of the heart to complete the cycle.

CORONARY CIRCULATION
– The surface of the heart is covered with a number of small arteries and veins.
– Even though the heart has blood flowing through it internally all the time, it is unable to obtain the materials it needs from this blood.
– These small vessels are the CORONARY ARTERIES AND VEINS that carry blood full of oxygen and nutrients to the muscle fibers of the heart and remove waste from the heart tissue.
– These coronary vessels are essential for the health of the heart.
– A heart attack occurs when these vessels become blocked and do not allow the heart tissue to receive oxygen and nutrients.
– This causes death of the heart tissue and the chest pain associated with a heart attack.