The element that has the most effect on blood viscosity, or stickiness, however, is the red blood cell. Red cells, which carry the oxygen from the lungs through the heart and into the rest of the body, must be flexible, like very soft rubber, to pass through the smallest blood vessels. They are disc-like, with a diameter of around 7 microns (7 thousandths of a millimeter). The smallest capillaries (the blood vessels from which the oxygen is given up to the tissues) are only 5 to
10 microns across, so the red cells have to fold and bend to pass through them.
If the red cells become less flexible or pile up, almost like a tube of gumdrops, the circulation inside the capillaries becomes much less free, and the pressure needed to force the blood through the circulation has to rise. This, of course, is an extra strain on the heart.
Stiffened, piled up red cells can even lead to blockage of the capillaries, with thrombosis, so that the capillary circulation can even stop completely in places. This process is made worse in some people, in whom the substance fibrinogen, which promotes blood clotting, is present in higher than normal amounts in the blood.
What if the red cells are not only stiffer than normal, but they also don’t carry enough oxygen from the lungs? They are designed to take up oxygen in large amounts, then give it up freely when they reach the tissues. However, the red cell oxygen uptake mechanism works even better for the lethal gas called carbon monoxide. When you breathe in a mixture of carbon monoxide and oxygen, the red cells preferentially take up carbon monoxide, rather than oxygen, and it remains solidly bound to them, so that they can no longer take up oxygen from the lungs.
Cigarettes and barbecues, as well as car exhaust (which has been a favored method of suicide for many years) still contain carbon monoxide. However, you don’t need to breathe in a suicidal dose of carbon monoxide to do yourself harm: if you expose yourself to a small amount, day in and day out, you are giving your heart less oxygen and reducing the supply side of the equation even further.
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