Heart and circulation
The heart and blood vessels are among the first organs to start working in a human body. And when they stop working 70, 80 or 90 years later, that marks the end of life. Until then the heart will beat up to three billion times and pump 200 million liters of blood through the blood vessels. The heart and circulation adapt quickly to the body's needs. The larger blood vessels are flexible, muscular tubes that can quickly become wider or narrower.