Within a month of conception, the cluster of cells that will, in the course of time, become a human being begins throbbing, signaling the development of Scarcely four weeks more pass before an intricate networkof veins and arteries the size of a pea forms and subdivides into a tiny replicaof the four chambers that will one day make up the adult heart. As the fetusgrows, so does its vitally important circulatory system. Although most of thefunctions of the heart remain dependent upon the mother throughout theentire pregnancy, in the latter stages the organ becomes strong enough to beaton its own. Even so, until birth the baby is cared for and nourished throughthe mother's placenta. An umbilical cord provides a supply line that furnishesfood and oxygen for the baby, and also removes waste. When at last thebirthing moment arrives, the baby emerges a separate individual; almost assoon as its first cries are sounded, its pulmonary and circulatory systemsundergo a change that renders them self-sufficient.How it Works Technically speaking, the circulatory system is a masterpiece of organicactivity. Composed of a network of 60,000 miles of blood vessels and apintsized, powerhouse pump known as the heart, it services more than 2,000gallons of blood per day, feeding and replenishing other organs and makinglife possible.In an adult, the heart is normally an 11 - ounce, fistsized organ that literallypushes blood through arteries, veins and capillaries. It does this by means ofmuscular contractions sparked by electrical impulses from the heart'spacemaker (sinoatrial node). All of the cells within each of the chambersmagically work on cue. First, the right side sends blood to the lungs. There carbon dioxide is removed and oxygen is added, turning the blood abright red color. Then the blood is pumped to the left side of the heart andsent via the aorta to the rest of the body. To survive, each of the body's approximately 1 billion cells must benouris...
