The circulatory system includes the heart and all the blood vessels in the body.  The heart acts as a pump, pushing blood throughout the body through the blood vessels.  Arteries carry blood away from the heart to the lungs and body.  Veins carry blood from the body tissues back to the heart.  Red blood cells contain hemoglobin, the major transporter of oxygen.  The heart pumps blood through the lungs where oxygen is attached to hemoglobin. Carbon dioxide is removed and exhaled.  Blood is an "alphabet soup" of nutrients.  The blood vessels deliver these nutrients to the body tissues where they are processed.  Waste products from cells are returned to the blood where they can be processed by the liver and removed by the kidneys.

 

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Heart

The heart is a four-chambered hollow muscle in the center of the chest that acts as the body's blood pump.  The two upper chambers of the heart are called atria and serve as a collection area for blood entering the heart.  Blood from the atria enters the two lower chambers, called ventricles, where it can be forced under pressure into the blood vessels of the body.  Four heart valves prevent blood from flowing backwards through the system.  In an average adult, the heart beats about 60 times per minute.  In a 5 year old child, the heart beats about 80-100 times per minute at rest.  In a newborn infant, the heart rate is normally between 120 and 160 beats per minute.

Arteries

Arteries are muscular blood vessels that carry blood away from the heart to the lungs and body.  The aorta is the artery that receives blood from the heart to deliver to the body.  Arteries pulsate.  That means that you can feel them stretch at the same rate as the heart beats.  This is called a pulse and can be felt on your wrist along with many other places on your body.

Veins

Veins are vessels that carry blood away from the heart.  They typically appear blue or purple when seen close to the skin.  The blood carried by veins has had oxygen removed by the body tissues.  This loss of oxygen makes the blood appear darker, giving veins their darker color.  Veins don't have muscles to help control blood flow like arteries.  They do have valves that helps prevent blood from flowing "backwards."  The contraction of muscles in the legs and arms helps push blood in veins back to the heart.

Capillaries

Capillaries are the small blood vessels that connect veins and arteries.  These tiny blood vessels make it easier for the body tissues to trade oxygen and nutrients with the blood.  Capillaries can change size to increase or decrease blood flow.  When capillaries in the skin dilate, the skin appears reddish or pink.