Read an Excerpt
Make a fist. This is about the size of your heart. Sixty to one hundred times every minute your heart muscles squeeze together and push blood around your body through tubes called blood vessels.
Try squeezing a rubber ball with your hand. Squeeze it hard once a second. Your hand will get tired in a minute or two. Yet your heart beats every second of every day. In one year your heart beats more than thirty million times. In an average lifetime a heart will beat over 2,000,000,000 (two thousand million) times.
The heart works hard when we relax or sleep and even harder when we work or exercise. It never stops for rest or repair. The heart is a most incredible pump.
Not all animals have hearts. There are tiny creatures in oceans and ponds that take in food and oxygen from the surrounding waters. But in humans and other mammals, most of the cells lie too deep within the body to get food and oxygen directly from the outside.
The human body is made up of hundreds of billions of microscopic cells. Your muscles, nerves, skin, and bones are all made of different kinds of cells. But every cell in your body needs food and oxygen, and your cells also need to be protected against germs that can cause disease.
Your heart, blood vessels, and blood work together to supply each of your cells with all of its needs. Every minute, the heart pushes a pulsing stream of blood through a network of blood vessels to every cell in your body. The constantly moving blood brings food and oxygen to each cell, carries away such wastes as carbon dioxide, and serves as an important component in the body's immune system. The heart, blood, and web of blood vessels make up your circulatory system.
