Leslie Van Frank, age 12, of Salt Lake City, Utah, for her question:
Does gravity have anything to do with blood circulation?
The force of the earth's gravity exerts itself on every object in the world. Our bodies were designed to cope with it. Our bones are strong enough to stand up¬right and built in gadgets in our ears signal for adjustments to keep us in balance. The circulating blood also has built in gadgets to counteract the downward pull of gravity.
Life, of course, depends upon many factors and one vital factor is the constant circulation of about 10 quarts of blood through the human body. This circulatory system extends from the toes to the scalp and hence about half of the trip is uphill. This part of the operation is in opposition to the force of gravity, which exerts a downward pull on the blood stream. Nevertheless, the blood does complete the uphill and downhill circuit continuously. The body, then, must have its own built in ways of counteracting the pull of gravity.
The branching blood vessels are about 60,000 miles long and all this tubing is linked together in an unbroken, closed plumbing system. Such a system needs a pump to keep the fluid flowing through the tubes. And the force of a pump can push a liquid uphill against the pull of gravity. The body's pump, of course, is the beat¬ing heart. The tubes nearest the pump feel most of its strength. Blood in the large aorta leaving the heart spurts forth with enough force to squirt a pulsing fountain six feet high. This force diminishes as the blood stream flows through smaller and still smaller branching vessels.
The heart pumps blood from the main arteries to smaller arterioles and on to fine networks of branching capillaries. Most of this part of the circuit is downhill and gravity tens to help the circulation on its way. All this arterial blood is loaded with fresh oxygen to fuel the busy tissues. The fine walled capillaries de¬liver the oxygen and also absorb waste carbon dioxide from the cells. At this point the life giving arterial blood becomes a waste disposal circuit of venous blood. The fine networks become venules that gather into larger and. still larger veins. This part of the circuit carries used blood back to the heart and lungs to be replen¬ished with fresh oxygen for the next trip around.
The veins have most of the uphill part of the circuit, moving against the pull of gravity. So the body must provide extra assistance. The linings of most veins are fitted with small valves to keep the blood from flowing back downhill. These valves are flaps like little pockets open at the top. Most of them are in the veins of the lower body where they are needed most. As each pulse pumps the blood upward through the circuit, the valves are pushed flat. As the pump force slackens, the blood is caught in the valve pockets and pushed up higher with the next pulse.
The blood vessels leaving the heart feel most of its surging beat. They must be able to stretch and give with each forceful pulse. For this reason, the walls of the main arteries are lined with strong, stretchable muscles. The veins do not need this elasticity, though they must be strong enough to enclose the blood stream. The walls of the veins tend to be thinner and more rigid. And many of them are fitted with valves to counteract the pull of gravity.