THE RHYTHM OF LIFE.
Your heart and blood vessels.
1. The heart is a muscle that can't be beat.
A hollow, muscular organ, the heart is one of your strongest muscles, pumping blood continuously to all parts of your body. Considering how big a job that is, you might expect the heart to be pretty large itself. But it's actually rather small.
In fact, an adult's heart is about the size of a man's clenched fist and is shaped like an egg. Lying in the center of your chest, your heart is protected by the breastbone and the rib cage.
What's your heart made of? It's mostly made of muscle tissue called myocardium. And your heart comes fully lined, too. A paper-thin layer of tissue called endocardium lines the inside of your heart, while an equally thin layer called pericardium surrounds the outside.
2. Day in and day out, the beat goes on and on and on.
With every single beat, your heart expands and contracts. That's around 100,000 times each day-like clockwork. In every young children the heart beats about 100 to 120 times each minute, and in adults it beats about 60 to 100 times a minute.
Of course, there are exceptions. For example, if you're excited or frightened, your hear beats faster than usual. And when you exercise, it beats much faster-and for a very good reason. Your heart has to beat faster and work harder so that your blood can deliver the extra food and oxygen your muscles need during physical activity,
Now, think how tired you'd get if you had to carry 2,000 gallons of fluids to different places today. But that's how many gallons of blood your heart delivers throughout your body every day of your life-about 2 1/2 ounces every time it beats.
It's a big job, but some part of your body's got to do it. And that part is your heart!
part 1- source: American Heart Association