Meredith Grey (narration): On average, a healthy heart beats 115,000 times per day. When excited, the heart rate can double. The heart pumps 2,000 gallons of blood through your entire body 24 hours a day. It never rests. The heart is the hardest-working muscle in your body. But when it's damaged, it's just like skin. It scars, and scar tissue can be very dangerous in a heart. It weakens it. And eventually, a heart full of scars stops working. Scarred hearts don't heal, but over time, the scars can change. They can become smoother, softer. And some scars can even fade away.