Like the respiratory system to which it is so intimately tied, the circulatory system of a fetus undergoes a dramatic transition at the time of birth. Separation from the placenta triggers a sudden change in blood pressure in the newborn's heart chambers, causing circulation to shift and closing key valves. The heart pumps blood to the lungs and the baby breathes on her own.

A newborn's heart beats as fast as the heart of a strenuously exercising adult, about 120 times a minute, because the infant's rapidly growing tissues demand oxygen-rich blood. The child's heart rate gradually slows and by late teens, it's around 70 beats a minute. That's still about 100,000 heartbeats each day, pumping blood through a system of arteries, veins, and capillaries that would circle the world three to four times or some 60,000 miles, if stretched out end to end, according to writer Barry Werth. All but 1 mile of this extensive network of vessels is invisible to the human eye, however. These tiny vessels, or capillaries (from the Latin for "hairlike"), are so small that blood cells must pass through in single file.

Many different nutrients, including protein, lipids, vitamins, and minerals, are essential for the development, growth, and functioning of the circulatory system. Iron and magnesium, for example, are essential for the production of trillions of red blood cells that pick up oxygen and carry it to cells throughout the body. Copper is also required for hemoglobin synthesis as well as for the production of collagen in capillary walls. Vitamins A, C, and E, riboflavin, folate, choline, and calcium are also required. And while anemia caused by insufficient iron is the most common nutritional deficiency, a great many other nutrients are essential for optimum cardiovascular health.

Healthy kidneys are essential not just for waste elimination but also to the function of the circulatory system. Kidneys filter the body's entire blood supply several times a day. They also play a critical role in the creation of red blood cells by producing the hormone erythropoietin, which signals bone marrow to produce more of the cells. Red blood cells live about 100 days and, according to Werth, die in the body at the stunning rate of about 2 million per second.

Kidneys also require nutrient building blocks and energy to develop and grow. In the womb, the placenta handles the waste removal functions that will later be taken over by the lungs and kidney. At birth, the kidneys are still immature, but must assume responsibility for the control of fluid and electrolyte balance as well as excretion of metabolic wastes. Renal function continues to mature over the first two years.