1. The heart is the first organ to develop and function after fertilization of a human egg and sperm. It is visibly beating by the fifth week of pregnancy. 
2. The heart sends five quarts of blood each minute into the circulatory system to nourish every cell in the human body.
3. The heart beats more than 2 billion times during a person’s lifetime (2.5 billion in the life of a seventy-five year old).
4. The heart is the size of a man’s fist.
5. The heart consists of two pumps: a low pressure pump to send deoxygenated blood to the lungs, and a high pressure pump to send oxygenated blood into the circulation.
6. The heart fills and empties in a second-long heartbeat of rhythmic contraction and relaxation.
7. A pacemaker, originating at the tiny bundle of nerve tissue, the sinus node at the top of the heart, ignites a series of electrical impulses that causes the heart muscle to contract and beat.
8. Paper-thin valves that are as sturdy as iron hinges, open and close with each heartbeat to control the passage of blood through the heart.
9. The heart is nourished by two encircling coronary arteries that draw blood from the aorta and span out into a network of coronary tributaries that sink into the heart’s muscle wall and deliver oxygen to each cell in the heart.
10. Once thought of as only a pump, it is now known that the heart also has intrinsic endocrine (hormonal) and neural functions.

Reference: The Incredible Machine, National Geographic Society, 1986, Washington, D.C.