Your heart beats about 100,000 times every day. It pushes blood to every part of your body. But what happens when a heart gets too sick to keep working? On May 3, 1968, a surgeon named Dr. Denton Cooley performed the first successful heart transplant in the United States. A transplant means taking an organ from one person and putting it into another. The patient was a man whose heart was failing. Dr. Cooley and his team of surgeons at a hospital in Houston, Texas, worked for hours during the operation. They carefully removed the damaged heart and replaced it with a healthy heart from a donor who had recently died. The biggest challenge was not the surgery itself. The human body treats a new organ as a foreign invader, and the immune system attacks it. Doctors had to give the patient special medicines to prevent his body from rejecting the new heart. Sadly, this first patient survived only a few days. But Dr. Cooley proved the operation was possible. Over the following decades, transplant medicine improved dramatically. Today, about 3,500 heart transplants happen in the United States each year.